Freshwater production via efficient oil-water separation and solar-assisted water evaporation using black titanium oxide nanoparticles
Graphical abstract
Superhydrophobic surface containing black TiO2 nanoparticles for efficient oil-water emulsion separation and solar evaporation in presence of natural sunlight light from sea water.
Introduction
Freshwater is a major global issue and a significant factor for humankind's survival as well as important to the economic development of any nation. Water is the most abundant compound on earth, and about 71% of the earth's surface is covered with water consists of mostly oceans, seas, and rivers, which is not suitable to drink without prior treatment. Therefore, it is necessary to develop an efficient and affordable water purification technique. Traditional technologies include reverse osmosis, and thermal-based designs are inaccessible for the off-grid villages and remote regions due to high cost and energy consumption [1], [2]. Solar desalination processes, an evaporation process using sunlight as an only energy source, is evolving as a promising environment-friendly solution [2], [3]. Solar steam generation and sterilization of waste are addressing two significant concerns for the survival of humankind, i.e., energy and water [4]. Due to poor solar absorption of water and heat losses, the traditional solar evaporation efficiency is too low to generate a large amount of freshwater.
A recently developed technique called air-water interfaces solar heating (AWISH) is one of the most attractive methods for water desalination technology [2], [5], [6]. In comparison to conventional bulk heating, AWISH can localize the heat at the water-air interface to increase the temperature leading to maximum utilization of heat, in energy-efficient, as well as cost-effective path [2]. For the purpose, a floatable superhydrophobic porous sheet consists of a material that is capable of solar light absorption, and it’s conversion to thermal energy could be an excellent choice. In this method, high solar light-absorbing black materials are coated on floating support such as a sponge, mesh, gauze, and cotton fabric, etc. Black materials such as Fe3O4/C carbon nanoparticles, black gold, polypyrrole, MoS2 nanosheets, aluminum nanoparticles have been used due to their solar irradiance absorbance ability [2], [7], [8]. Last few decades, researchers have been put tremendous effort to developed plenty of sunlight harvesting materials.[[9], [10], [13], [14], [12], [11]] In this concern, black titanium oxide (TiO2) could have been a suitable candidate as it possessing sufficient solar light absorption and the ability to convert the light to heat [15].
Black or reduced or defect-induced TiO2 nanoparticles are one of the limelight materials in recent material research because of its ability to absorb massive ultraviolet-visible (UV-VIS) light and converting into chemical or electrical energy, for enhanced photocatalytic activity in artificial photosynthesis and pollution removal [16], [17], [18], [19], [20], [21]. The black TiO2 nanoparticles are mainly obtained by introducing oxygen vacancies, H, or Ti3+ doping into TiO2 lattice during synthesis via reduction or oxidation [16].
For the floating purpose, porous sheets, cotton fabrics, mesh, and gauzes are excellent choices. But due to hydrophilic nature, these substrates tend to sink under the water. Superhydrophobicity plays a vital role in resolving this obstacle. Superhydrophobic or super anti-wetting surfaces, i.e., possessing a water contact angle of >150° and sliding angle of <10° provides excellent floating properties along with other benefits such as self-cleaning, friction reduction, anti-icing/fogging, anti-bio adhesion, water/fog-collection, micro-template for patterning, microdroplet manipulation and oil-water separation [22], [23]. The researcher invented several methods and materials to create the superhydrophobic surface, and the use of inorganic nanomaterials and organosilane is one of the most promising routes to achieve a higher water contact angle. Inorganic material like silica, silver, alumina, zinc oxide, iron oxide, and copper nanoparticles, is being used with silanes like perfluorooctyltrichlorosilane, polydimethylsiloxane, hexadecyltrimethoxysilane, and many more silanes has been reported [24], [25], [26], [27], [28], [29], [30], [31], [32]. Among the variety of nanomaterials, TiO2 nanoparticles are used to fabricate superhydrophobic surfaces for applications like self-cleaning, antibacterial, due to their physicochemical properties, thermostability, and photocatalytic properties [[33], [34], [35], [37], [38], [42], [39], [36], [40], [41], [43]].
Studies have also been done for solar evaporation using TiO2 coated superhydrophobic surfaces. Huang et al. fabricated an Au-TiO2 coated film [44]; Shi et al. synthesized Fe3O4-TiO2 nanoparticle [45]; Li et al. made an Ag-TiO2 coated membrane [46]; Gao et al. fabricated a superhydrophobic copper mesh [47]; Zhang et al. made a superhydrophobic steel mesh [48].
A few articles found to use black TiO2 for solar evaporation using a surface coating on different substrates. Xue et al. fabricated a hydrophobic Titania mesh with black TiO2 for solar evaporation [49]. Ye et al. reported a superhydrophobic stainless steel mesh using Mg reduced TiO2 for the solar evaporation process [7]. Liu et al. reported excellent photothermal property of black titania/graphene oxide nanocomposite films for the solar steam generation [50].
Besides solar evaporation, superhydrophobicity also has potential applications in oil–water separation [51], [52], [53]. Many studies have been done on oil-water separation using a superhydrophobic surface containing TiO2 nanoparticles. For example, Li et al. reported the facile preparation of underwater superoleophobic TiO2 coated mesh for efficient oil-water separation [54]. Dong et al. also reported a TiO2 coated underwater superoleophobic mesh for oil-water separation in a complex environment with self-cleaning ability [55]. Shi et al. used a TiO2 coated superhydrophilic PVDF membrane [56]. Kang et al. made an anti-corrosive superoleophobic titania mesh membrane and used for oil-water separation [57]. Yuan et al. fabricated and used a TiO2/CuO coated copper mesh [58]. Wang et al. fabricated WO3-TiO2 coated stainless steel superhydrophilic and underwater superoleophobic membrane for oil-water emulsion separation [59]. Deng et al. prepared a TiO2 coated superoleophobic mesh via biomineralization for efficient oil-water separation [60]. Till now, only white TiO2 has been used for superhydrophobic surfaces. There is an opportunity to investigate the proper implementation of reduced or black TiO2 in the field of superhydrophobicity and its extensive applications. In view of creating a superhydrophobic surface, the primary role of nanoparticles is to provide a rough surface on the substrate; the phenomena may suggest a similar role of white and black TiO2 nanoparticles. However, the surface of black and white TiO2 nanoparticles is different enough to produce the different extent of silane functionalization, which may affect the stability of the superhydrophobic surface. The other significant difference is the solar light absorption capacity of the black and white nanoparticles due to their bandgap differences.
Herein, superhydrophobic galvanized steel mesh was fabricated by immersion technique using perfluorodecyltriethoxy-silane (PFDTS) and black TiO2 prepared by lithium reduction for the first time. Surface morphology, wettability, and functionality of the prepared coated meshes have been discussed. Further, the mechanical, chemical and thermal stability of superhydrophobic GS mesh has been examined. Also, its stability and durability were studied at different conditions. The fabricated mesh exhibited excellent self-cleaning ability and successfully used for air-water interface solar evaporation for plain and seawater and oil-water separation for mixture and emulsion.
Section snippets
Materials
PFDTS (molecular weight: 610.38, purity: 97%) was purchased from Sigma-Aldrich Co., USA. Lithium (Li) was purchased from Sigma-Aldrich Co., USA. TiO2 nanoparticles (Molecular weight:79.87, purity: 98%, average particle size: 50 nm) was purchased from SRL Chem., India. Commercial galvanized steel (GS) mesh (pore size: 201 µm, width of wire: 95 µm) was used as the substrate. The mesh mentioned above was cut into 5 × 2 cm dimensions. 35% (v/v) HCl, toluene, and n-hexane were purchased from Rankem,
Results and discussions
Black TiO2 nanoparticles were prepared from white TiO2 nanoparticles by the lithium reduction process [61]. White and black TiO2 nanoparticles both can be visually differentiated by their color, as shown in Fig. 1. The enhancement in light absorption ability in black TiO2 was analyzed by a spectrophotometer. The absorption spectra of white and black TiO2 presented in Fig. 2a indicate the enormous enhancement of absorption in the UV-VIS light region for black TiO2 compared to white TiO2, which
Oil-water separation
The oil-water separation was performed by mixing 1:1 ratio of toluene and distilled water. The optical image of oil-water separation is presented in Fig. S8, SI. The prepared samples are superhydrophobic and superoleophilic in nature, which facilitates the separation of water and oil. The separation efficiency of oil by superhydrophobic mesh is presented in Fig. 6a. The separation efficiency of oil by black TiO2 coated mesh repeatedly checked and a ~99% separation efficiency of oil observed up
Conclusions
In this work, a superhydrophobic GS mesh fabricated using reduced black TiO2 nanoparticles and PFDTS via simple immersion coating method exhibited maximum water contact angle of 157 ± 2° and a tilting angle of 5 ± 1°. The formation of black TiO2 well evidenced by different characterizations like XRD, Raman, XPS, and UV-VIS spectroscopy analysis. There was enough improvement in light absorption in the solar light region compare to white TiO2. Surface morphology of the fabricated mesh had been
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
AS acknowledge DST-INSPIRE (DST, India) Faculty program for funding.
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