Atomic layer deposition of TiO2 on carbon-nanotube membranes for enhanced capacitive deionization
Graphical abstract
Introduction
Under the situation of worldwide increasing demands and decreasing supply of freshwater, extensive attention having being paid on advanced desalination technologies. As a promising alternative to desalination processes, capacitive deionization (CDI) is an environment-friendly and energy-efficient desalination technique in comparison with other desalination techniques which always suffer from the drawbacks such as fouling, water electrolysis and high energy consumption [1], [2], [3], [4], [5], [6], [7], [8], [9]. CDI based on the electrical double-layer capacitor (EDLC) theory is an electrochemical water purification method and capable of reducing the salt concentration of brackish and seawater by electrostatic adsorption of ions on porous electrodes [10], [11], [12]. The electrosorption behavior relies significantly on the electrical conductivity, wettability and internal structures of the CDI electrode materials [13]. Owing to their high surface area, good flexibility and low electrical resistivity, carbon-based materials such as graphene, activated carbon (AC), carbon aerogels (CAs), carbon nanotubes (CNTs), and their composites have been widely investigated for the application in CDI electrode materials over the past years [14], [15], [16], [17], [18], [19]. However, the tedious treatments for the synthesis and/or modification to the carbon building blocks, easy aggregation of powders, and binder addition often complicate the preparation process of the electrodes on one hand [20], [21], [22], and sometimes it cannot obtain acceptable electrosorption capacity on the other [14]. Therefore, advanced electrode materials with good CDI performances which simultaneously have the advantages of simplifying preparation process, avoiding the aggregation, eliminating the blocking caused by binders and can be directly used as electrodes are urgently needed.
Incorporating pristine or modified CNTs with binders, then combining them with polymers or other porous carbon-based materials, and finally depositing them onto a current collector is the most common way to fabricate CDI composite electrodes. The use of CNTs is expected to increase the specific surface area and enhance the electrical conductivity of composite materials, thus improving CDI performance [23], [24], [25], [26], [27], [28], [29]. However, these CNTs are commonly existed in the shape of powders and are typically required to be chemically modified in order to have a good dispersion in the final electrodes, which always requires a great deal of time and energy, and is also a tedious process [30], [31]. Furthermore, the release of individual CNTs into water or air is likely to cause safety issues during practical applications [32], [33]. If the CNTs can be welded together and utilized as hydrophilic free-standing CNT membrane electrodes, they will be more effective for CDI applications. Compared to CNT-doped composite electrodes, free-standing CNT membrane electrodes with randomly interlaced CNTs in the form of fabrics are a kind of promising carbon-based materials, exhibiting three-dimensionally (3D) interconnected nanoporous networks with higher specific surface areas [34], [35]. Owing to their excellent thermal, chemical, mechanical and electronic properties, this type of CNT membranes has been used in diverse applications [36], [37], [38], [39]. However, the inherent strong hydrophobicity of CNTs dramatically hinders their applications in the field of CDI as water cannot adequately wet the fine pores in the membranes. Creating a hydrophilic interface on the CNT membranes could markedly reduce the contact resistance between the water and CNT membranes [40]. However, the interface engineering of CNT membranes involves complex interactions and dynamics. Hence, there is a strong demand for effective techniques which can realize the hydrophilic modification and functionalization of CNT membrane electrodes in a simple way.
Atomic layer deposition (ALD), based on sequential self-limiting reactions of alternately injected gaseous precursors, is a promising thin-film-coating technique which could obtain excellent conformity, highly controllable thickness and morphology on the surfaces of various substrates. ALD has the capability to deposit a variety of target materials on porous substrates and yields strong chemical bonding between the substrate and the deposited materials, thus precisely regulating surface properties, pore sizes and separation/adsorption applications by simply varying deposition conditions such as temperatures and cycle numbers [41], [42], [43], [44], [45], [46]. Recently, ALD has been successfully employed to deposit metal oxides on carbon-based substrates for increasing charge storage for supercapacitor and a variety of electrochemical applications [47], [48]. Even though CNT membranes have been extensively investigated for CDI applications [28], [49], [50], there are still some shortcomings during the electrode fabrication and application processes. Herein, for the first time, we used ALD of titanium dioxide (TiO2), which is a highly hydrophilic, low-cost and eco-friendly metal oxide [51], onto free-standing CNT membranes to achieve binder-free CNT-TiO2 composite electrodes. Significant enhancement in hydrophilicity, electrochemical behavior and CDI performance were obtained by ALD of TiO2 on CNT membrane electrodes. The TiO2-deposited electrodes also displayed superior reusability during CDI process. This strategy of “ALD on carbon substrates” opens a new avenue to produce advanced electrodes for various electrochemical applications in addition to CDI considering that a large number of materials can be controllably ALD-deposited on various carbon-based substrates.
Section snippets
Materials
Sheets of multi-walled CNT membranes (Suzhou Jiedi Nanotechnology Co., Ltd) with a thickness of ∼8 µm were chosen as the substrates in this work. Titanium tetrachloride (TiCl4, 99.99%, Metalorganic Center, Nanjing University) and deionized (DI) water (8–20 µs/cm, Wahaha) were selected for TiO2 deposition. Ultrahigh purity nitrogen (99.999%) and high purity nitrogen (99.9%) were used as the carrier gas and purge gas in the ALD reactor, respectively. Sodium chloride, hydrochloric acid, anhydrous
Morphology evolution of the CNT membranes during ALD of TiO2
Fig. 2 shows the morphologies of pristine CNT, CNT@20TiO2 and CNT@60TiO2 membrane electrodes, respectively. It can be seen from Fig. 2a that the pristine CNT membrane with nanoporous structure arising from intertwined CNTs and larger pores contributed by the matrix of smooth fibers is consisted by a uniform, highly interconnected CNTs network. As displayed in Fig. 2b and c, the TiO2 coverage on CNTs increases with more ALD cycles. Below 20 ALD cycles, TiO2 nanoparticulates distributed on the
Conclusions
In summary, we have demonstrated the successful modification of CNT membranes via atomic-layer-deposited TiO2 to produce superior CDI electrodes. The surface coverage of CNTs and TiO2 nanoparticulates loading amounts could be precisely controlled by the ALD cycle numbers. The wettability of the TiO2-deposited CNT membrane is progressively transformed from strongly hydrophobic to hydrophilic. Compared to pristine CNT membrane, the functionalized CNT electrode with moderate ALD cycles presents
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
Financial supports from the National Basic Research Program of China (2015CB655301) and the Jiangsu Natural Science Foundation (BK20150063) are gratefully acknowledged. We also thank the support from the Program of Excellent Innovation Teams of Jiangsu Higher Education Institutions, and the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Mr. Yimin Guo, a student in Nanjing Foreign Language School, also contributed to this work by taking part in
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