3D microflowers CuS/Sn₂S₃ heterostructure for highly efficient solar steam generation and water purification

Highlights

• Microflowers composed of vertically aligned CuS/Sn₂S₃ nanosheets were prepared.

• The resultant microflowers displayed a high solar-to-thermal conversion efficiency.

• The solar evaporator showed a remarkable solar water evaporation rate.

• Excellent solar desalination and wastewater purification performances are achieved.

Abstract

Solar-driven interfacial steam generation is a promising method to produce potable water using renewable energy and help solve global clean water scarcity problems. However, the design of photothermal materials (PTMs) with excellent light absorption that can localize heat at the air/water interface, and facilitate water vapor generation remains a key challenge for its practical implementation. In this work, we demonstrate the synthesis of heterostructure microflowers composed of vertically aligned CuS/Sn₂S₃ nanosheets (3D CSS-NS MF) using a single-step solvothermal method for solar steam generation application. The microflower structures and the abundant nanocavities between the vertically aligned nanosheets resulted in significant sunlight harvesting over the solar spectrum, excellent heat localization through trapping and re-absorbing the heat, and fast escape of water vapor. Under 1 sun (1 kW m^-2) illumination, a high water evaporation rate of 1.42 kg m^-2 h^-1, corresponding to an efficiency of 82.93% was obtained. The 3D CSS-NS MF based solar evaporator exhibited remarkable salt ions rejection efficiency and good reusability over 10 cycles. Furthermore, efficient removal of organic dyes was observed in application geared towards wastewater treatment with a rejection ∼99.9%. Our work demonstrates the potential of using novel semiconductor-based nanocomposites as effective photothermal materials for high-performance solar steam generation in water desalination and wastewater treatment applications.

Introduction

Safe drinking water is essential for the human population and has remained a significant challenge. The World Health Organization estimated that more than one billion people suffer from a lack of clean drinking water [1]. Unsafe drinking water can cause serious water-borne diseases such as diarrhea, respiratory infections, and cholera [[1], [2], [3], [4]]. These issues are often more severe in remote or rural areas where it is difficult to implement traditional clean water production systems and processes [5].

Renewable energy-driven solar-steam generation (SSG) has attracted tremendous attention as a promising technology to produce safe drinking water [6]. Solar steam generation devices can be considered as efficient, clean water production systems as they require minimal capital cost, simple setup, and can be driven using solar energy to treat a range of saline and contaminated water sources. Previous work on SSG technology relied on heating bulk water by dispersing PTMs on the water [7,8]. However, these systems exhibited poor performance with very low water evaporation efficiency due to insufficient light absorption and underwater heat loss issues [9]. Recently, interfacial SSG has attracted significant attention as an effective strategy to overcome the above-mentioned issues [10,11]. In this system, the solar thermal conversion via PTM occurs at the air/water interface; hence high water evaporation and less energy loss is observed when compared to the dispersed system. A typical interfacial solar evaporator consists of PTMs or (solar absorber) that absorb sunlight across the solar light spectrum placed at the air-water interface. A thermal insulator is placed beneath the solar absorber to suppress the converted heat and minimize its loss to the bulk water. Meanwhile, water is continuously supplied via capillary channels that link the bulk feedwater to the solar absorber region [10,[12], [13], [14], [15], [16], [17]]. The interfacial design thus enables high light-to-vapor conversion efficiencies.

There are three key points for designing PTMs with high solar-to-vapor conversion efficiency, namely: (1) it requires high solar light absorption with excellent light-to-heat conversion efficiency, (2) morphology and structure should enable effective heat localization and recovery of emitted heat, and (3) the internal structure facilitates water transport and allows vapor release [18]. However, it remains a significant challenge to synthesize PTMs that satisfy all of the criteria. Previous research demonstrated the use of various classes of PTMs for high efficiency SSG, such as plasmonic metals, carbon materials, conjugated polymers, and semiconductor materials. Among them, nano/microstructured semiconductor materials display great potential as PTMs. This is due to the relatively low cost, ease of fabrication, excellent photothermal conversion efficiency, and the range of possible morphologies and structures (e.g., crystal structure, dimension, and porosity) [19]. To exploit such advantages of nanostructured semiconductor-based PTMs, manipulation of the PTMs’ morphologies or intrinsic properties should be examined to improve the SSG device performances. For example, Lu et al. demonstrated the synthesis of 1D-O doped MoS_2-x ultrathin nanosheets that generated nano-confined channels for effective water evaporation. Such confined nanochannels effectively reduced the vaporization enthalpy and improved the water evaporation performance [20]. A remarkable water evaporation rate of 2.50 kg m^-2 h^-1 under one-sun irradiation was reported, corresponding to solar-to-vapor efficiency of 89.6%.

Copper sulfide (CuS) has attracted attention as a promising PTM for high-efficiency SSG devices due to its low-cost, strong light absorption in the near-infrared (NIR) arising from a tunable, localized surface plasmon resonance (LSPR), and excellent photothermal conversion efficiency [21,22]. Various forms of copper sulfide nano/microstructures have been reported, such as nanoparticles [23], nanorods [24], nanosheets [25]), shapes [26], and phases [24]. To utilize effective PTMs, solar seawater desalination and wastewater treatment applications have been considered for practical applications. During seawater desalination, salt accumulation on the surface of the evaporator is a common issue. Several strategies have been reported to resolve this problem or at least to minimize the salt formation, such as the Janus absorber [27,28], hydrophobic absorber [29], polymer-based salt-resistant [30,31], and hydrogel structure [32,33].

In this work, we report a single-step solvothermal synthesis of 3D microflowers CuS/Sn₂S₃ heterostructure (3D CSS-NS MF) consisting of vertically aligned ultrathin nanosheet assemblies. The 3D CSS-NS MF PTMs were deposited on a hydrophilic mixed cellulose ester (MCE) membrane and coated with hydrophobic silane layer at the top to reduce the salt formation. Our prepared 3D CSS-NS MF/MCE absorber exhibited high photothermal conversion that led to high performance in solar desalination and organic dye purification.