Abstract
Solar-driven interfacial evaporation is a sustainable and economical technology for fresh water generation. Structural design of photothermal material is an effective strategy to improve the evaporation performance but usually bothered by complicated processes and non-adjustability. Herein, magnetic nanoparticles assembled photothermal evaporator was developed, which showed an adjustable spinal array surface under uniform magnetic field induction. By regulating position in the magnetic field, the desirable surface structures could be uniform at relatively low load density of magnetic nanoparticles to improve light absorption via multiple reflection. Magnetic field induced evaporator could accelerate evaporation to over 1.39 kg m-3 h-1under 1-sun illumination, which was 2.8 times that of natural evaporation. After coated by carbon layer, magnetic nanoparticles could overcome the oxidation to realize stable evaporation in long-term desalination. The facile strategy to optimize the surface structure via magnetic field is appropriate for various fields with special requirements on surface structure.
1. Introduction
With serious water pollution and the energy-saving demands for water purification, solar-driven evaporation has received extensive attention due to its potential in terms of water, energy, and environmental sustainability,1-4 which is widely recognized as a novel desalination technology for solving water resource scarcity issues.5-7 To realize a desirable evaporation efficiency, various kinds of photothermal materials, have been employed to improve the light application thanks to their high absorption of broad-spectrum solar radiation and light-to-heat conversion.8-10 Besides the filtering of ideal materials inside the range of dark matter, the logically designed surface structure is also critical for high performance on solar absorption.11,12 The rough photothermal surface with a suitable structure can induce the light transfer on microscale to dramatically reduce the diffuse reflection that is the primary type of light loss.13,14 For the classic super-black material, thanks to the vertically aligned array structures, carbon nanotube (CNT) achieved approximate complete light absorption in the whole solar spectrum with an absorbance up to 99.97%.15Similarly, confining the plasmonic nanoparticles inside the sub-micron hole channels formed a broadband absorber enabling an absorbance around 99% across the wavelengths from 400 nm to 10 μm.16
Actually, the superior photothermal performance tends to be revealed by a delicate surface structure that is constructed via complicated processing and rigorous preparation conditions, no matter based on top-down etching or bottom-up assembly.17-19 Besides the high production costs, fragility of the fine structure is another factor to limit the application of efficient light absorbers in photothermal evaporation. To simplify the structure construction, external field effects are introduced to induce the controllable assembly of nanoparticles to form the proposed structure.20-24 In particular, magnetic field is considered an ideal tool for applications in structure management,25-27 especially in the construction of functional structures with reconfigurable, recoverable and dynamically adjustable.28-30 In Jeong’s work, Fe3O4 nanoparticles were sprayed onto a Si substrate with patterned nickel islands, and attracted by the external magnetic field to form magnetic micropillars.31 Besides, some conical micro-tip patterns could be molded with the aid of a magnetic field.32 In Song’s work, the mixture of polydimethylsiloxane (PDMS) polymer with magnetic particles was coated onto the substrate and prepared Janus membranes with cacti-spine arrays by magnetic field-assisted molding.33
In our previous work, magnetic Fe3O4nanoparticles were directly used to assemble the spine arrays as photothermal surface following the effect of magnetic field, which revealed an obvious improvement in light harvesting.34However, Fe3O4 tends to be oxidized in high temperature and high humidity conditions during evaporation. The generated Fe2O3 shows a wider band gap indicating the weakening of photothermal performance.35 Additionally, non-uniform magnetic field distribution on the magnet surface induced the formation of an uneven spiny structure, meanwhile, resulting in a relatively high loading density of magnetic nanoparticles to ensure an integrity photothermal layer. Therefore, it is necessary to alleviate the performance degradation caused by the oxidation of Fe3O4 and explore the assembly behavior of magnetic nanoparticles at various relative distances.
Based on the above discussion, in this work, we fabricated a magnetic field induced photothermal evaporator assembled by carbon-coated Fe3O4(Fe3O4@C) nanoparticles, which revealed the adjustable surface structure corresponding to the relative distance to magnet. To improve the stability of magnetic nanoparticles, we synthesized Fe3O4@C by a simple one-step method of coating Fe3O4 nanoparticles with a layer of carbon using glucose as the carbon source (Figure 1). The outer carbon layer of Fe3O4@C nanoparticles could provide effective protection for the Fe3O4 and maintain a stable photothermal performance after the oxidation of magnetic core. At an appropriate relative distance to the magnet, Fe3O4@C nanoparticles assembled into a complete and well-distributed photothermal layer with spine arrays at a low load density of 1 mg mm-2 (which was 3 mg mm-2 in our previous work) (Figure S1). With the optimized surface structure, the Fe3O4@C assembled evaporator obviously improved the evaporation rate up to 1.39 kg m-2h-1 for water and 1.31 kg m-2h-1 for seawater under 1-sun illumination. During 8 hours of continuous evaporation for seawater and 4 times regeneration, there was less than 4% performance degradation for Fe3O4@C assembled evaporator indicating desirable stability. Magnetic field induced adjustable surface structure with spiny arrays is suitable for efficient solar-driven desalination, as well as a universal method for the structure control to satisfy the requirements of other applications in water treatment.