Plant leaves inspired sunlight-driven purifier for high-efficiency clean water production

Natural vascular plants leaves rely on differences in osmotic pressure, transpiration and guttation to produce tons of clean water, powered by sunlight. Inspired by this, we report a sunlight-driven purifier for high-efficiency water purification and production. This sunlight-driven purifier is characterized by a negative temperature response poly(N-isopropylacrylamide) hydrogel (PN) anchored onto a superhydrophilic melamine foam skeleton, and a layer of PNIPAm modified graphene (PG) filter membrane coated outside. Molecular dynamics simulation and experimental results show that the superhydrophilicity of the relatively rigid melamine skeleton significantly accelerates the swelling/deswelling rate of the PNPG-F purifier. Under one sun, this rational engineered structure offers a collection of 4.2 kg m−2 h−1 and an ionic rejection of > 99% for a single PNPG-F from brine feed via the cooperation of transpiration and guttation. We envision that such a high-efficiency sunlight driven system could have great potential applications in diverse water treatments.

moistened filter paper. The weight of the hydrogel was real-time recorded by an analytical balance. Water retention was calculated by the following formula: Water retention = 100 ( − ) / (2) Where is the weight of the hydrogel at regular time intervals, is the dry weight of the hydrogel and is the weight of the hydrogel balanced in pure water.

Reswelling Kinetics Measurement
The reswelling kinetics of the irradiated hydrogels determined gravimetrically by dipping the samples in deioned water and wiping off the excess water on the surface.
The weight was recorded at predetermined dipping time intervals. Water uptake was defined by the following equation: Water uptake = 100 ( − ) / All the symbols were defined the same as the above.

Characterization.
The porous structure and surface topography of melamine before and after PN growth was observed by using a Sirion-200 scanning electron microscope (FEI, USA).
The absorbance spectra of thepurifier were measured using a varian UV-vis spectrophotometer (Cary 5000, USA), coupled with an Agilent integrating sphere. An IR camera (Fluke) were used to measure the temperature increase under solar irradiation. The wettability change of thepurifier after solar irradiation was measured by the contact angle analysis system with a 3.0 μL water droplet. Raman spectra were obtained by using LabRAM HR Evolution (HORIBA Jobin Yvon, France) Raman microscope with a 514 nm laser. Elemental analysis was finished using XPS spectra tanken out by an ESCALAB 250XI photoelectron spectrometer (ThermoFisher Scientific, USA). X-ray diffractions (XRD) were carried out using a D8 Advance Xray diffractometer with Cu Kα radiation (λ=0.15418 nm, Bruker, Germany). The transpiration and guttation experiment experiments were conducted in the lab using a solar simulator with an optical filter for the standard AM 1.5 G spectrum. The optical concentration of one sun and two sun is 100 mW cm −2 and 200 mW cm −2 , respectively.
The contact angle was measured using a Dataphysics OCA 15pro CA measuring instrument (DataPhysics Instruments GHPH, Filderstadt).

Part Ⅱ: Theoretical analysis and the discussion of the mechanism Simulation details.
The OPLS-AA + SPC/E force fields 4,5 were used in MD simulations which have been performed by using LAMMPS. 6 The initial molecular configuration was constructed as follows: In the first step of the structure generation, a monomer of the NIPAM molecule was generated and placed in a simulation cell. The adjacent two monomers were placed with a bond-distance of backbone atoms, which was repeated for obtaining a single 30- of anchored and free chains (PN system) were continued for 100 ns in the NVT ensemble at temperatures at 280 K, and then they were further relaxed at temperature heating to 340 K for another 100 ns. This simulation time was enough to make sure that the stretched or the collapsed conformation were reached. 6 The Lennard-Jones (LJ) and point charge parameters of the PNIPAM force fields were used after the previous work. 5,6 Also, the geometric mean mixing rule was applied for describing intramolecular and intermolecular interactions. All interactions were controlled within a cutoff radius of rc = 1.2 nm. For the long-ranged electrostatic and van der Waals interactions, the particle-particle particle-mesh (PPPM) method was used. The SHAKE algorithm was used to constrain the bond lengths and the angles of water molecules.
All simulations were performed using a time step of 2 fs.