{"id":3825,"date":"2015-12-28T11:06:01","date_gmt":"2015-12-28T11:06:01","guid":{"rendered":"http:\/\/biomedpharmajournal.org\/?p=3825"},"modified":"2020-04-25T03:43:25","modified_gmt":"2020-04-25T03:43:25","slug":"ultrathin-metal-oxides-layer-on-a-carbon-nanotube-oriented-arrays-surface-formation-process-development-and-study-for-supercapacitors-electrode-specific-capacity-increasing","status":"publish","type":"post","link":"https:\/\/biomedpharmajournal.org\/staging\/vol8no2\/ultrathin-metal-oxides-layer-on-a-carbon-nanotube-oriented-arrays-surface-formation-process-development-and-study-for-supercapacitors-electrode-specific-capacity-increasing\/","title":{"rendered":"Ultrathin Metal Oxides Layer on A Carbon Nanotube Oriented Arrays Surface Formation Process Development and Study for Supercapacitors Electrode Specific Capacity Increasing"},"content":{"rendered":"

Introduction<\/strong><\/p>\n

One of the most important directions of the development of technologies is the development of efficient and environmentally safe power sources. That kind of devices allows to increase the autonomy of various electric systems from electric cars and electric transport to microsized implanted devices. Electrochemical capacitors with large specific characteristics are promising candidates for a number of applications, because they possess an unique combination of electrophysical characteristics \u2013 considerably larger specific capacity, as compared with regular capacitors, and high specific power as compared with accumulators. However, the main problem for researchers in that direction is the increase of capacity of electrochemical capacitors.<\/p>\n

Conway (1991) divided electrochemical capacitors of large specific capacity into electric double-layer capacitors (EDLC) and supercapacitors, or pseudocapacitors, which are based on Faraday’s pseudocapacity. That classification is based on fundamental differences of mechanisms of accumulation of electric charge: in EDLC due to formation of double electric layer at transition zone electrolyte-electrolyte; in supercapacitors due to occurrence of reversible oxidation-reduction reaction or electric sorption in thin surface layer of electrode material.<\/p>\n

Thus, two main research directions can be specified: firstly, creation of electrodes with high specific surface, and, secondly, development of approaches for the increase of the effect of Faraday’s pseudocapacity. In order to solve the first problem in the last decade all possible modifications of carbon, such as activated carbons, black carbons, graphene and carbon nanotubes, are actively developed and studied (Stoller et al.<\/em>, 2011; Wang et al.<\/em>, 2005; Wang et al.<\/em>, 2012). These materials are cheap and have excellent mechanical, chemical and electrophysical properties, as well as good cyclic stability and long service-life. In order to increase the values of specific capacity and density of energy researches stated to actively developed oxides of transition metals (RuO2<\/sub>, MnO2<\/sub>, NiO, Co3<\/sub>O4<\/sub>, SnO2<\/sub>, ZnO, TiO2<\/sub>, V2<\/sub>O5<\/sub>, CuO, Fe2<\/sub>O, WO3<\/sub>, etc.) as alternative electrode metals (Zhi et al.<\/em>, 2013; Zhao et al.<\/em>, 2009; Amatucci et al.<\/em>, 2001; \u00a0Pell and Conway, 2004; Zhang et al.<\/em>, 2009).<\/p>\n

The presented paper describes the methodology for the development of composite electrode material of supercapacitors based on carbon nanotubes and oxides of transition metals, which, from one point of view, have significant specific surface, and, from another point of view, considerable pseudocapacity. The technology of manufacturing of this kind of materials is based on the application of SILAR and ALD methods from the gas phase on a surface of oriented massifs of carbon nanotubes. Nowadays in MIET ALD method is actively developed and applied; the method is aimed at manufacturing of the antireflection layer of Al2<\/sub>O3<\/sub>, ZnO for solar panels, which allow to increase efficiency of a photoelectric transducer and establish mass-production with implementation of equipment using group loading of slices (Tuzovskii et al.<\/em>, 2015). Further studies in that direction also will allow to use ALD method in the manufacturing technology of planar supercapacitors of increased capacity for very large scale IC and SoC.<\/p>\n

Experimental part<\/strong><\/p>\n

Synthesis of carbon nanotubes<\/strong><\/p>\n

Vertically-oriented multi-wall carbon nanotubes arrays were synthesized directly on a surface of current collectors of supercapacitors by means of the method of plasma-enhanced chemical vapor deposition using Oxford PlasmaLabSystem 100 (Nanofab 800 Agile) device according to the methodology described in the previous studies (Tuzovskii et al.<\/em>, 2015). Firstly, a layer of TiN of 25 nm thickness was deposited on substrates by means of the method of magnetron sputtering; the layer acted as diffusion barrier; also a thin layer of Ni with thickness from 5 to 8 nm was deposited using the same method; that layer was used for the formation of nanoclusters of catalyst as the result of thermal treatment of specimens at 280\u00b0\u0421 temperature in oxygen atmosphere, which was followed by annealing at 450\u00b0\u0421 temperature in hydrogen atmosphere. With the increase of the thickness of Ni layer the density of oriented arrays of CNT is decreasing. Synthesis of CNT was carried out in mixture of gases C2<\/sub>H2<\/sub>, H2<\/sub> and NH3<\/sub> with the working pressure of 2 Torr. Additional treatment with plasm allowed to decrease the process temperature to 500\u00b0\u0421. The temperature of the substrate holder was controlled using the thermal sensor. Speed of growth of CNT massif was, approximately, 600 nm per minute.<\/p>\n

Layer-by-layer Deposition<\/strong><\/p>\n

One of the methodologies of manufacturing of the composite electrode materials of supercapacitor base on CNT arrays was ALD method; according to that method thin layers of Ti and Al oxides were deposited on a surface of carbon nanotubes. For the formation of functional groups on carbon’s surface, which play a role of active centers for further ALD, we carried out preliminary modification CNT in plasma of N2<\/sub>O gas. High-frequency plasma charge due to development of free active radicals allows to provide hydrophilic properties of carbon’s surface and uniformity of the produced coating. Functionalization at Oxford Plasmalab 100 device was carried out at power of 100 W, gas pressure of 400 mTorr, temperature of substrate of 40\u00b0\u0421 and time of processing was 30 s.<\/p>\n

Further ALD processes were carried using Picosun R150 devices with individual loading of substrates. For deposition of Al oxide we used Al(CH3<\/sub>)3<\/sub>(TMA) and water. Deposition parameters: temperature of working chamber is 300\u00b0\u0421, pressure in working chamber is 1.5 mbar, flow of gas-carrier in TMA line is 1 cm3<\/sup>\/min, flow of gas-carrier in H2<\/sub>O line is 100 cm3<\/sup>\/min. Flow of N in reaction chamber was 300 cm3<\/sup>\/min. The deposition consisted from cycles of repeated stages: feeding of TMA into the chamber using gas-carrier, blowing off of the chamber with N, feeding of vapors of deionized water into the chamber, blowing off of the chamber with N. The mentioned stages form one reaction cycle of ALD process for Al oxide, the scheme of which is presented in Figure 1. Speed of deposition of Al oxide film was 0.0097 nm\/cycle. The number of ALD cycles was selected on the basis of the required thickness of the oxide layer.<\/p>\n\n\n\n
\"Figure<\/td>\nFigure 1:\u00a0Scheme of stages of reaction cycle of ALD process<\/strong><\/p>\n

Click here to View figure<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

 <\/p>\n

For deposition of Ti oxide we used Ti(OH5<\/sub>C2<\/sub>)4<\/sub>(EtTi) and water. Deposition parameters: temperature of working chamber is 300\u00b0\u0421, temperature of EtTi source is 145\u00b0\u0421, pressure in working chamber is 1.5 mbar, flow of gas-carrier in EtTi line is 150 cm3<\/sup>\/min, flow of gas-carrier in H2<\/sub>O line is 100 cm3<\/sup>\/min. Flow of N in reaction chamber was 300 cm3<\/sup>\/min. The deposition also consisted from cycles of repeated stages: feeding of EtTi into the chamber using gas-carrier, blowing off of the chamber with N, feeding of vapors of deionized water into the chamber, blowing off of the chamber with N. Speed of deposition of Ti oxide film was 0.033 nm\/cycle.<\/p>\n

The composite electrodes, which were manufactured using layer-by-layer deposition method, were formed on current collectors made from stainless steel film of round shape with 25 mm diameter.<\/p>\n

The second method for manufacturing of a composite electrode material of supercapacitors is SILAR (Lindroos et al.<\/em>, 1995). That method was used for deposition of Ni oxide on a surface of the carbon material. Prior to that massifs of CNT were processed in Ar-O2<\/sub> plasma with incident power of 50 W during 120 s using NIITM MVU Plasma RIT-T device.<\/p>\n

As the cation precursor we used 0.01 \u041c Ni(NO3<\/sub>)2<\/sub>\u00b76H2<\/sub>O and liquid ammonia, which was added to the solution in order to create alkaline media (pH=12). As the anion precursor we used water at the temperature of 90 \u00b0C. Ni ions were absorbed on the surface of the substrate after it was submerged into the cation solution at room temperature for 30 seconds. After that the substrate was transported to water bath at the temperature of 90 \u00b0C for additional 30 s. At that Ni oxide was formed. At the final stage of the cycle the substrate was submerged in deionized water for 30 s for removal of not binded ions of Ni hydroxide. The mentioned stages form one reaction cycle of SILAR process, the scheme of which is presented in Figure 2. The necessary number of deposition cycles was carried out to obtain the coating of a required thickness.<\/p>\n

After deposition the specimen were annealed in air at the temperature of 250\u00b0C during 2 hours in order to decompose Ni hydroxide, which is formed together with the oxide during the deposition.<\/p>\n

The composite electrodes, which were manufactured using layer-by-layer deposition method, were formed on current collectors made from stainless steel film of rectangular shape with dimensions of 20×25 mm2<\/sup>.<\/p>\n\n\n\n
\"Figure<\/td>\nFigure 2:\u00a0Scheme of stages of the reaction cycle of deposition from liquid phase using SILAR method<\/strong><\/p>\n

Click here to View figure<\/a><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

 <\/p>\n

Assembly of specimens and measurement of electrophysical characteristics<\/strong><\/p>\n

The study of the influence of the parameters of the composite material comprising CNT and metal oxide on the electrophysical properties of supercapacitors, which are based on them, was carried out on the basis of their charge-discharge characteristics. In order to do that the authors prepared experimental specimens, which consisted of two current collectors, made from stainless steel; on the specimen of those collectors composite electrodes, which comprised a separator, made from porous polypropylene, and electrolyte, made from water solution of sulphuric acid, were synthesized.<\/p>\n

Measurement of charge-discharge characteristics was carried out using Elins P-30J potentiometer. Current of charge and discharge varied from 0.5 to 2 mA, and range of voltages was from 0 to 2 V.<\/p>\n

Morphology and composition of CNT arrays and composite materials, which are based on them, was controlled by means of SEM method using Agilent 8500 FE-SEM. Control of composition of composite material was carried out by means of the method of EDX analysis.<\/p>\n

Results and Discussion<\/strong><\/p>\n

It is known that there are virtually no active groups (except surface defects) on a surface of carbon materials, including carbon nanotubes. Those materials are hydrophobic and are inert to the majoring known reagent used in ALD process (Knez et al.<\/em>, 2007; Lee et al.<\/em>, 2008; Liu et al.<\/em>, 2009), which results in nonunifromity of the obtained coating. The same problem appears in the case of deposition of oxide layers using SILAR method (Lokhande et al.<\/em>, 2011; Lee et al.<\/em>, 2009; Lin et al.<\/em>, 2010), which is based on the application of liquid-phase precursors.<\/p>\n

For the formation of functional groups on carbon’s surface, which play a role of active centers for further ALD, we carried out preliminary modification CNT in plasma of N2<\/sub>O gas (Markeev et al.<\/em>, 2013).<\/p>\n

In order to form functional groups on a surface of CNT for further deposition by means of SILAR method the authors used the method of surface treatment in plasma of gas mix of Ar and O, which is responsible for physical and chemical mechanisms of activation (Cho et al.<\/em>, 2007; Yang et al.<\/em>, 2014; Hong et al.<\/em>, 2013). For the optimization of modes of treatment we carried out a series of experiments, which included the following stages:<\/p>\n