Reviewer #2:
In this manuscript, the authors described a hydrophile/hydrophobe amphipathic Janus nanofibers aerogel for constructing the solar steam generator. The water evaporation rate can reach 2.944 kg m-2 h-1 with an energy efficiency of 91.05% under 1 sun. The flipping issue is also involved, resulting in a quick recovery of the water evaporation rate. Overall, this article is well-organized but the novelty and some crucial point of this work needs further refinement. Thus, I do not think that this work qualified enough to be published on Advanced Functional Material:
Comment 1: The authors mentioned that: ”hydrophobic or hydrophilic material alone hardly achieves adequate property for constructing solar steam generators.” However, many works using hydrophilic materials exhibit completive, even higher evaporation rates compared to that proposed in this manuscript (2.944 kg·m-2·h-1 under 1 kW m-2 irradiation).
Response 1: We acknowledge the respected reviewer’s observation that hydrophilic materials can provide high water evaporation rates. Nevertheless, the solar steam generators consisting only of hydrophilic materials hardly independently float on water surface, and additional supporting materials are necessary. Besides, hydrophilic materials are not effective in preventing the shuttle of salt ions. As for the sentence mentioned by the respected reviewer, what we wanted to express is “hydrophilic or hydrophobic materials alone are insufficient in achieving necessary characteristics for constructing high-quality solar steam generators with good comprehensive properties”, high water evaporation rate is just one of important characteristics for solar steam generators, and the properties such as self-floating, salt resistance, and stability are also crucial factors. In order to avoid misunderstanding, we have modified the relevant statement.
Comment 2: According to the SEM images (Figure 1) of nanofibers aerogel and solar steam generator, the surface and the pores of aerogel are filled by photothermal materials. The hydrophilicity/hydrophobicity of nanofiber still plays a key role in water evaporation. The floating feature is due to the hydrophobic/hydrophilic Janus nanofiber or just because of the density change? Will the blended single nanofiber show a similar property? Blended electrospinning instead of parallel spinning is suggested to fabricate non-Janus structural nanofiber for comparison. The merits of hydrophile/hydrophobe amphipathic Janus nanofiber structure should be highlighted and completely clarified since it is the prominent description in the Title.
Response 2: Thanks a lot for the respected reviewer’s good suggestions! Accordingly, CA-PVB blended nanofibers aerogel is fabricated by single-nozzle electrospinning for comparison. It is found that the solar steam generator based on CA-PVB blended nanofibers aerogel shows a very poor water evaporation rate, and relevant discussion is provided in the revised manuscript. Besides, the cycling performance of the solar steam generator based on CA nanofibers aerogel is added in the revised manuscript. The merits of the amphipathic Janus nanofibers aerogel can be highlighted and completely clarified based on these supplemental experimental results.
Comment 3: All figures should be mentioned in the text in numerical or alphabetical order. Namely, Figure 1a should be organized and mentioned before Figure 1b-e.
Response 3: Figure 1 has been modified as requested.
Comment 4: The authors investigated the influence of water states (BW, IW and FW) on evaporation rate due to the silica NPs in detail. But the nanofibers aerogel evaporator is a 3D evaporator, the environmental enhancement on evaporation should be considered as well.
Response 4: There are indeed many other reports on 3D evaporators that have investigated environmental enhancement effects such as airflow, irradiation angle, and cold evaporation. However, these environmental enhancement effects are significant only when the evaporators are exposed above the water surface at considerable heights. That is, the exposed side area is large enough. Although our products are also 3D evaporators, their height above the water surface is less than 1 cm and can be considered negligible in terms of environmental enhancement. In addition, because there is no unified standard of experimental parameters for environmental enhancement in this field, it is hard to select widely approved experimental conditions. Based on the above reasons, the ambient temperature, humidity and irradiation angle are constant in this work, and no airflow is engaged. Thanks a lot for your understanding in advance!
Comment 5: The authors claimed that ”the flipping recovery of the solar steam generator is an important issue in practical outdoor applications.” and flipping experiments of a cubic solar steam generator were carefully conducted. On the other hand, purified water collection is still a key process for solar desalination from the practical view, which the authors also present in the manuscript. So, is the flipping recovery experiments carried out in the evaporation/collection system more meaningful rather than the cubic evaporator?
Response 5: Due to the lack of a standardized preparation protocol for water collection systems in this field, the performance of solar steam generators in different collection systems is influenced by factors such as material, shape, and size of collection systems. To eliminate these factors on experimental results, it is common practice in this industry to evaluate the performance of solar steam generators outside of the collection system whenever possible, and only desalination and contaminant removal properties are tested within the collection system. We conducted experimental design in accordance with the methodology employed in relevant literature.
Comment 6: What’s the purified water collection rate in the acrylic tank?
Response 6: The purified water collection rate in the used acrylic tank is provided in the revised manuscript. Please note that the water collection rate is influenced not only by the performance of the solar steam generator itself, but also by the material, shape and size of the collection system. Therefore, this value should be used as a reference only.
We have tried our best to modify the manuscript in order to meet the requirements of reviewers, and we sincerely look forward to receiving your positive responses!
Thanks a lot for the time and efforts you have spent on our paper!
Sincerely yours,
Prof. Qianli Ma (Corresponding author)
Article category: Full Paper
Subcategory: Solar steam generation
Amphipathic Janus nanofibers aerogel for efficient solar steam generation
Rui Wang, Jinshuo Deng, Ping Wu, Qianli Ma*, Xiangting Dong*, Wensheng Yu, Guixia Liu, Jinxian Wang, and Lei Liu
Rui Wang, Jinshuo Deng, Ping Wu, Qianli Ma*
Chongqing Research Institute, Changchun University of Science and Technology, Chongqing 401135, China E-mail: maqianli@cust.edu.cn
Xiangting Dong*, Wensheng Yu, Guixia Liu, Jinxian Wang, Lei Liu Key Laboratory of Applied Chemistry and Nanotechnology at Universities of Jilin Province, Changchun University of Science and Technology, Changchun 130022, China
E-mail: xtdong@cust.edu.cn
Keywords: solar steam generation, photothermal materials, electrospinning, aerogels, solar desalination
Abstract: Solar steam generation is a promising water purification technology due to its low-cost and environmentally friendly applications in water purification and desalination. However, hydrophilic or hydrophobic materials alone are insufficient in achieving necessary characteristics for constructing high-quality solar steam generators with good comprehensive properties. Herein, novel hydrophile/hydrophobe amphipathic Janus nanofibers aerogel is designed and used as a host material for preparing solar steam generators. The product consists of an internal cubic aerogel and an external layer of photothermal materials. The internal aerogel is composed of electrospun amphipathic Janus nanofibers. Owing to the unique composition and structure, the prepared solar steam generator integrates the features of high water evaporation rate (2.944 kg·m-2·h-1under 1 kW m-2 irradiation), self-floating, salt-resisting, and fast performance recovery after flipping. Moreover, the product also exhibits excellent properties on desalination and removal of organic pollutants. Compared with traditional hydrophilic aerogel host material, the amphipathic Janus nanofibers aerogel brings much higher water evaporation rate and salt resistance.
1. Introduction
Solar steam generation (SSG) is an emerging water purification technology and has become a research hotspot in the field of water treatment in recent years.[1-3] Its working principle is to utilize solar energy to convert water from waste water or sea water into water steam which is then collected and condensed into fresh water. This technique can separate water from contaminants and other impurities in a process similar to the water cycle in the natural environment. Hence, it is an eco-friendly and low-cost water purification strategy that does not require extra energy.
However, due to the low light absorption and photothermal conversion efficiency of water, an efficient SSG process requires placing the materials with the function of photothermal conversion on the surface of water to realize interfacial heating, and these materials are usually called solar steam generators or solar absorbers.[4-6] To date, many types of solar steam generators, such as photothermal aerogels[7], hydrogels[8], foams[9], membranes[10], have been fabricated. Macroscopically, solar steam generators can be classified into one-dimensional (1D), 2D and 3D materials. Among them, 3D solar steam generators are considered to be the most promising materials owing to their high water evaporation rates and versatility.[11-13] In general, 3D solar steam generators have large thicknesses, which is conductive to reduce thermal dissipation from evaporation surface to bulk water and also enhance the energy efficiency by allowing multiple reflections of light inside the 3D solar steam generators. Moreover, the 3D structure provides more feasibility for structural design. By far, many different shapes of 3D solar steam generators have been prepared to endow the materials with the features such as self-floating[14], salt-resisting[15], self-cleaning[16], and cold evaporation[17].
The solar steam generators based on 3D aerogels are hot spot of current researches.[18-20] As a commonly used method for preparing 3D aerogels, electrospinning combined with freeze drying has been developed for several years.[21-24] In addition to their applications in SSG, electrospun 3D aerogels also exhibit other application prospects in thermal insulation[25], tissue engineering[26], photocatalysis[27], oil/water separation[28] and etc . However in the existing reports, without taking into account the post-processing procedures, the aerogels prepared by direct electrospinning are either completely hydrophilic or hydrophobic, which causes some issues in the application of SSG. Although hydrophilic aerogels can pump water to the evaporation surface, the excessive water inside the aerogels acts as a ”thermal bridge” which results in serious thermal dissipation from evaporation surface to bulk water. For another, because hydrophilic aerogels cannot float on the water surface, additional supporting materials are usually necessary. By contrast, completely hydrophobic aerogels are hardly suitable for preparing solar steam generators due to the incapability of pumping water to the evaporation surface. Therefore, it is rational that proper combination of hydrophilic and hydrophobic materials should be an ideal solution for constructing solar steam generators. To date, there have been many reports on macroscopical combination of hydrophilic and hydrophobic materials for SSG[29-32], that is, hydrophilic and hydrophobic materials are separated in different regions of the reported solar steam generators, and some reported solar steam generators possess Janus structure[32-37]. The term “Janus structure” in these works refers to bilayer structure at the macro level, where the hydrophilic materials acts as a water pump, while the hydrophobic materials enable the solar steam generators to float on water surface. However, no research on microscopical combination of hydrophilic and hydrophobic materials can be found. Therefore, it is meaningful to construct a solar steam generator composed of microscopically bonded hydrophilic and hydrophobic materials and discover the advantages of this design philosophy.
Herein, novel hydrophile/hydrophobe amphipathic Janus nanofibers aerogel is first proposed and used as a host material for constructing the solar steam generator. Every amphipathic Janus nanofiber in the aerogel is composed of a hydrophilic cellulose acetate (CA) side and a hydrophobic polyvinyl butyral (PVB) side, forming a “microscopically” Janus structure, as depicted in Figure 1 a. Such peculiar architecture and composition endow the aerogel with water-pumping, self-floating, heat-insulating, salt-resisting properties and excellent continuous working stability which are crucial characteristics for solar steam generators. To obtain the solar steam generator, the amphipathic Janus nanofibers aerogel is coated by photothermal materials consisted of carbon nanotubes (CNTs), silicon dioxide nanoparticles (SiO2 NPs) and polydopamine (PDA), in which CNTs play a prominent role in photothermal conversion, SiO2 NPs can regulate the water state, and PDA is the adhesive and contributes to photothermal conversion as well. The prepared solar steam generator exhibits excellent water evaporation, desalination and removal of organic pollutants performances. Another advantage of the solar steam generator is that its water evaporation performance can quickly recover after it is flipped on the water surface, which is a meaningful feature for dealing with dynamic water environment.
2. Results and Discussion
2.1. Structural Characterization
The microstructures of CA nanofibers, PVB nanofibers, CA//PVB Janus nanofibers, CA//PVB Janus nanofibers aerogel and the surface of the solar steam generator are determined by SEM observation. As shown in Figure 1b and c, the prepared CA nanofibers and PVB nanofibers have similar morphologies, and their diameters are about 600 nm. Figure 1d reveals that every CA//PVB Janus nanofiber is composed of two tightly adjacent nanofibers whose diameters are both about 600 nm, indicating a typical Janus nanofiber structure. Although it is hard to determine the difference in the chemical compositions of the two adjacent nanofibers by existing material characterization techniques because CA and PVB both consist of elemental C, H and O, it is still safe to conclude that the two adjacent nanofibers are respectively comprised of CA and PVB according to the existing reports on fabrications of Janus nanofibersvia parallel electrospinning. In these reports, it has been proved that the two adjacent nanofibers of a Janus nanofiber are respectively derived from two spinning solutions for parallel electrospinning.[38-40] Figure 1e manifests that the Janus nanofibers in the aerogel are successfully crosslinked, which reinforces the shape stability of the aerogel. The crosslinked Janus nanofibers have slightly larger sizes than those before crosslinking, which can be due to the swelling of polymer nanofibers in the solution containing crosslinking agent and volume expansion during thermal crosslinking. Furthermore, it can be noticed that micron-sized irregular channels exist among the Janus nanofibers and can allow water transfer. Figure 1f-h are SEM imagery of the surface of solar steam generator at different levels of magnification. The CNTs, with the diameters ofca. 40 nm, are evenly distributed on the product surface, and SiO2 NPs are attached to the CNTs. Hierarchical pores are formed among the CNTs and SiO2 NPs, which can facilitate water transfer and steam escape.