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STUDY OF BUCKYPAPER MADE OF CARBON NANOTUBES TAUNIT-4
A.V. Melezhyk1, A.V. Shuklinov2, O.N. Bychkov1, A.G. Tkachev1
Department “Equipment and Technologies of Nanoproduction”, TSTU (1);
Tambov State University named after G.R. Derzhavin (2); [email protected]
Key words and phrases: buckypaper; carbon nanotubes; surfactant; Taunit-4; ultrasonic.
Abstract: Samples of buckypaper were obtained by the method of filtering suspension of carbon nanotubes Taunit-4 through a non-woven polypropylene material. Taunit-4 is few-walled carbon nanotubes with specific surface area of 600...700 m2/g. Individual nanotubes are of 6.8 nm in diameter and are aggregated in bundles up to several tenths of a millimeter. Processing of the initial suspension of nanotubes, which improves the wetting of the surface (oxidation, addition of surfactant Triton X-100, n-butanol), and an increase in the duration of ultrasonic treatment results in splitting bundles of nanotubes into individual nanotubes. Apparent density of the buckypaper from Taunit-4 increases with oxidation of the nanotubes, addition of surfactant or wetting agent, increasing the duration of ultrasonic treatment.
Introduction
Buckypaper made from carbon nanotubes is a promising material [1-32], which can be used in composite materials [12, 15, 20, 22, 25], as electrode material in chemical power sources [17, 24] catalysts carrier [3], sensors [31], for shielding of electromagnetic radiation [8] and conversion of electrical energy into mechanical energy [5], as a flame retardant layer [10, 27], and in other areas. The most common method of producing buckypaper is treating of CNTs with ultrasound and filtering suspension of treated carbon nanotubes (in presence of surfactants or without them) through a microfilter. Also there is known method of buckypaper production by electrophoresis of CNTs [1, 26].
Buckypaper with the best electrical and mechanical characteristics was obtained from singlewalled (SWNTs) and doublewalled carbon nanotubes (DWNTs), due to their higher surface energy, and, as a consequence, the best binding of individual nanotubes by Van der Waals forces. SWNTs and DWNTs produced by CVD method usually are agglomerated in bundles, consisting of a number of individual nanotubes. In the process of buckypaper preparation the bundles of CNTs in the initial dispersion of nanotubes may split to a more or less extent, depending on presence of surfactants, nature of solvent and mode of ultrasonic treatment of the initial suspension. Accordingly, the properties of the resulting buckpaper vary. These issues remain poorly understood.
In the present work we have investigated the properties of buckypaper, obtained by filtration of aqueous suspension of carbon nanotubes Taunit-4, treated in different conditions.
The starting material used for buckypaper preparation, was few-walled CNTs
2
Taunit-4 with specific surface area of 600.700 m /g, average number of carbon layers of 2-4 and an average outer diameter of 6 nm. The length of the nanotubes (estimated from the length of the bundles formed by them) was up to 0.1 mm and more (Fig. 1). The Taunit-4 CNTs were obtained by CVD method on the catalyst (Fe, Co, Mo)/MgO-Al2O3 using acetone or propylene as a carbon source. The yield of nanotubes was 1.5-2 wt. parts from 1 wt. part of the catalyst. Purifying of nanotubes from mineral impurities (catalyst) was carried out by treatment with hydrochloric acid. The resulting aqueous suspension with CNTs mass content of 0.5 % was treated with ultrasound (1.5 l of the suspension for 1 h) using ultrasonic apparatus IL-10. This suspension was used as starting for preparation of buckypaper by filtration through a polypropylene nonwoven fabric on a Buchner funnel. After forming of the precipitate layer on the filter excess water was sucked off under vacuum, the wet layer was set between polypropylene filter material, in its turn placed between the discs of porous ceramics. The package was dried in an oven at 110 oC under pressure.
In some experiments non-ionic surfactant Triton X-100 or n-butanol as a wetting agent were added to the suspension of nanotubes and it was additionally treated with ultrasound for different times. The sample prepared with addition of Triton X-100 was thoroughly washed with water on filter and after drying at 110 oC was additionally kept in a ventilated oven at 200 oC to constant weight (12 h) to remove Triton. It turned out that the sample dried at 110 oC, contained about 22 % of the adsorbed Triton, despite the thorough washing of the starting material on filter.
It was studied also influence of oxidation of nanotubes with potassium
permanganate on the properties of buckypaper obtained. As is known oxidation of
CNTs results in appearance of phenolic, quinoid and carboxyl groups on surface of nanotubes. The oxidized nanotubes much better wetted by water. Oxidation of the surface of CNTs can be accomplished by different reagents (nitric acid, ammonium persulfate, hydrogen peroxide, sodium hypochlorite, and others). In laboratory
experiments it is convenient using potassium permanganate as an oxidizing agent for CNTs in acidic medium, as the reaction proceeds rapidly and the completion of the reaction is well distinguished visually by disappearance of the permanganate color. In a typical experiment, to 150 ml of 0.5 % suspension of CNTs solution of 1 g of potassium permanganate in 150 ml of 1M sulfuric acid was added, after which the mixture was heated under stirring until disappearance of permanganate color. Then 5 g of citric acid was added and the mixture was heated to boiling for reduction of manganese dioxide,
which could be produced in the system. The suspension was filtered, washed with water until neutral pH, adjusted to total volume of 300 ml, treated with ultrasound for 10 min, and then samples of buckypaper were molded on filter.
In the Table, the duration of additional treatment with ultrasound
(where it took place) is assigned to the volume of suspension. That is, it was roughly assumed that at constant power of the ultrasonic emitter 100-min treatment of 1 liter of a suspension was equivalent to 5-min treatment of 50 ml
Fig. 1. SEM image of CNTs Taunit-4 as is from CVD reactor without any additional treatment
The molding conditions and properties of the buckypaper samples obtained from CNTs Taunit-4
The molding conditions Diameter of
Sample addition of surfactant, wetting agent, additional treatment additional treatment with ultrasound, min/l the disk after drying in % from diameter of the initial wet sample h, mm d, g/cm3
1 No No 100 0.68 0.18
2 Triton X-100, 1 % in water 40 94 0.66 0.34
3 «-butanol, 48 97 0.27 0.25
4 5 vol. % in water 140 95 0.23 0.29
5 280 94 0.22 0.30
6 CNTs oxidized with potassium permanganate 33 Nearly 80 %, the sample cracked 0.14 0.41
7 33 78 0.31 0.56
8 The initial 0.5 % No 100 0.39 0.12
9 suspension of CNTs was diluted with water 1:1 20 100 0.29 0.18
10 60 100 0.24 0.20
Note: h - thickness of a sample (average from measurements in
10 points); d - apparent density of a sample.
of the suspension and was referred as 100 min/l. Of course, this assumption was approximate, but it could be bigger mistake to not take into account the volume of the treated suspensions.
Images of the samples were recorded using two beam scanning electron microscopy complex Neon 40, Carl Zeiss.
The thickness of samples (disks) was measured by mechanical micrometer M-0-25-0.01 in 10 points and the average was calculated.
Results and discussion
The Table shows the molding conditions and properties of the buckypaper samples obtained from CNTs Taunit-4.
From the data presented in the Table it is seen that addition of the surfactant Triton X-100 to the initial suspension of CNTs (sample 2) leads to significant increase in the apparent density of the molded paper compared to the sample 1 without surfactant. Addition to the initial suspension of CNTs «-butanol as a wetting agent (samples 3-5) also increases the density of the buckypaper, although to less extent than with the Triton. The greatest increase in density was achieved by oxidation of the initial CNTs with potassium permanganate (sample 6). Linear shrinkage along the plane of the samples after drying was the greater the greater the greater the density of a sample. All these facts can be explained if we take into account that the presence of surfactants, wetting agents or grafting of polar groups to the surface of carbon nanotubes by oxidation facilitates sliding of the nanotubes and their bundles while molding an
aqueous slurry on the filter, resulting in more dense packing of nanotubes. The same effect was caused by increase of treatment time of nanotubes suspension by ultrasound. High density and very large shrinkage during drying of buckypaper samples obtained from oxidized nanotubes most likely was caused by contraction of nanotubes by capillary forces.
To determine the extent to which bundles of nanotubes present in the original nanotubes Taunit-4, remain while molding of buckypaper, we have studied the samples of buckypaper using scanning electron microscope.
As can be seen from Fig. 2, buckypaper, obtained from the initial suspension of the CNTs Taunit-4 (sample 1), consists of long intertwined bundles of nanotubes. Each bundle consists of many individual nanotubes. Thus, treatment of the initial suspension of CNTs with ultrasound did not lead to a significant splitting of the bundles into individual nanotubes.
As can be seen from Fig. 2, the length of fragments of some bundles emerging to the surface and entering into the field of view is greater than 100 microns. If we assume that nanotubes have the same length as the bundles, their length can be several tenths of millimeter, i.e., macroscopic. One can also see from Fig. 2 that individual nanotubes in bundles have spiral shape to different extent. It can be assumed that such shape of nanotubes arises because of inhomogeneity of catalyst surface and different rate of growth of individual nanotubes. Those nanotubes, which grow more slowly, are stretched and define the length of the bundle. However, some of the nanotubes grow faster and thus form a spiral shape. Probably the entire bundle is an elastic formation, as is evidenced by characteristic folds on the bend fragments (Fig. 1, top left corner).
Macroscopic length of the nanotubes Taunit-4 is proved also by behavior of their agglomerates in aqueous dispersion. If you attempt to loosen them by glass rod the agglomerates behave like wet cotton wads.
Additional treatment of the suspension by ultrasound (sample 10) led to partial dissagregation of the nanotube bundles (Fig. 3).
Fig. 4 shows SEM images of buckypaper, molded from nanotubes, oxidized with potassium permanganate (sample 6). It is evident that oxidation in conjunction with action of ultrasound in aqueous suspension leads to disaggregation of bundles into individual nanotubes, due to better wettability of oxidized nanotubes with water. After drying of the wet buckypaper layer molded on filter structure with much greater apparent density formed compared to untreated nanotubes, because of contraction of nanotubes by capillary forces. The nanotubes in the resulting buckypaper are arranged predominantly chaotically.
For the sample 2 of buckypaper obtained with treating the suspension of Taunit-4 by ultrasound in presence of surfactant (Triton X-100), most of bundles are randomized (Fig. 5), possibly in a little less extent than for the sample oxidized with permanganate.
Fig. 2. Images (SEM) of buckypaper molded from aqueous suspension of Taunit-4 without any additional processing
Fig. 3. Images (SEM) of buckypaper from CNTs Taunit-4 molded with additional processing of the initial suspension by ultrasound
Fig. 5. Images (SEM) of buckypaper, molded from aqueous suspension of nanotubes Taunit-4 treated with ultrasound in presence of surfactant Triton X-100
Almost the same result was achieved by treatment of the suspension of Taunit-4 with ultrasound in presence of «-butanol, which in this case improves the wetting of nanotube surface with water (Fig. 6).
Thus, treatment of the initial suspension of nanotubes, which improves the wetting of the surface of nanotubes (oxidation, addition of surfactant, «-butanol), and increasing the duration of ultrasound treatment allows breaking bundles of nanotubes into individual nanotubes. This provides more dense packing of nanotubes in the final buckypaper. However, one should note that in this paper the initial suspensions of nanotubes, which were used for molding buckypaper, were sufficiently concentrated (0.25-0.5 %). At such concentration in any case nanotubes are agglomerated. In order to
Fig. 6. Images (SEM) of buckypaper, molded from aqueous suspension of nanotubes Taunit-4, with addition of «-butanol and treatment with ultrasound
obtain the “true” solutions of nanotubes (not agglomerated) in water, even in presence of surfactants, the mass concentration of nanotubes should not exceed a few hundredths and even thousandths of %. It is obvious that buckypaper with regular structure with best mechanical properties can be obtained from the “true” solution of nanotubes by molding a layer of buckypaper on micro-filter or by method of electrophoresis. However, the buckypaper obtained by methods described above, the structure of which is formed by bundles of nanotubes or their agglomerates, can be useful for applications where rapid diffusion of the reagents or ions in a layer of buckypaper is desirable. In fact, the samples of buckypaper from Taunit-4 are materials with bimodal porosity, where there are nanometer-sized pores (between individual nanotubes in bundles or agglomerates), and macropores between the agglomerates or bundles. Such materials can be effective for creation chemical power sources, catalysts, sensors, and for other similar applications.
Conclusion
1. Samples of buckypaper from carbon nanotubes Taunit-4 were obtained by method of filtering aqueous suspension of nanotubes through a non-woven polypropylene material.
2. Apparent density of the buckypaper made from CNTs Taunit-4 increases with oxidation of nanotubes, addition of surfactant or wetting agent, increasing the processing time of the suspension by ultrasound.
3. Processing of the initial suspension of nanotubes, which improves wetting of the surface of nanotubes (oxidation, addition of surfactant, n-butanol), and increase of duration of treatment with ultrasound facilitates breaking the bundles of nanotubes into individual nanotubes.
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Исследование углеродной бумаги из нанотрубок Таунит-4 А.В. Мележик1, А.В. Шуклинов2, О.Н. Бычков1, А.Г. Ткачев1
Кафедра «Техника и технологии производства нанопродуктов»,
ФГБОУ ВПО «ТГТУ» (1); ФГБОУ ВПО «Тамбовский государственный университет имени Г.Р. Державина» (2); [email protected]
Ключевые слова и фразы: поверхностно-активное вещество; Таунит-4; углеродная бумага; углеродные нанотрубки; ультразвук.
Аннотация: Методом фильтрования суспензии нанотрубок через нетканый полипропиленовый материал получены образцы бумаги из углеродных нанотрубок Таунит-4, представляющих собой малослойные нанотрубки с удельной поверхностью 600...700 м2/г. Индивидуальные нанотрубки диаметром 6...8 нм собраны в пучки длиной до нескольких десятых долей миллиметра. Обработка исходной суспензии нанотрубок, улучшающая смачивание их поверхности (окисление, добавка ПАВ Тритон Х-100, н-бутанола), и увеличение продолжительности обработки ультразвуком позволяют расщепить пучки нанотрубок на индивидуальные нанотрубки. Кажущаяся плотность бумаги из Таунита-4 увеличивается при окислении нанотрубок, добавке ПАВ или смачивателя, увеличении времени обработки суспензии ультразвуком.
Forschung des Kohlenstoffpapiers aus den Nanorohren Taunit-4
Zusammenfassung: Von der Methode der Filterung der Suspension der Nanorohren durch das nichtgewebte Polypropylenmaterial sind die Muster des Papiers aus den Kohlenstoffnanorohren Taunit-4, die die schichtarmen Nanorohre mit der
spezifischen Oberflache 600... 700 m2/g darstellen, erhalten. Die individuellen Nanorohre mit dem Durchmesser 6... 8 nm sind in die Bundel von der Lange bis zu einigen zehnten Anteil des Millimeters gesammelt. Die Bearbeitung der
Ausgangssuspension der Nanorohre, die das Anfeuchten ihrer Oberflache verbessert (die Oxydierung, der Zusatz von Tenside Triton X-100, «-Butanol), und die
Vergrofierung der Dauer der Bearbeitung vom Ultralaut lassen die Bundel der Nanorohre auf die individuellen Nanorohre zu spalten. Die scheinende Dichte des Papiers aus Taunit-4 vergrossert sich bei der Oxydierung der Nanorohre, bei dem
Zusatz von Tenside oder vom Befeuchter, bei der Vergrofierung der Zeit der
Bearbeitung der Suspension durch den Ultraschall.
Etude du papier carbonique a partir des nanotubes Taunit-4
Resume: Par la methode du filtrage de la matiere en suspension des nanotubes a partir un materiel polypropylene non-tisse sont regus les exemples du papier carbonique a partir des nanotubes Taunit-4 presentant les nanotubes de peu de couches avec une surface specifique de 600...700 m2/g. Les nanotubes individuels du diametre de 6.. .8 nm sont ramasses en faisceau de la longeur de quelques dixiemes du millimetre. Le traitement de la suspension initiale des nanotubes ameliorant l’arrosage de la surface (oxydation, addition du surfactant Triton X-100, «-butanol) et l’augmentation de longevite du traitement par l’ultrason permettent de desagreger les faisceaux des nanotubes sur ceux individuels. La densite apparente du papier augmente lors de l’oxydation des nanotubes, l’addition du surfactant ou du mouillante, l’augmentation du temps du traitement de la suspension par l’ultrason.
Авторы: Мележик Александр Васильевич - кандидат химических наук, доцент кафедры «Техника и технологии производства нанопродуктов», ФГБОУ ВПО «ТГТУ»; Шуклинов Алексей Васильевич - кандидат физико-математических наук, ведущий научный сотрудник Тамбовского государственного университета имени Г.Р. Державина; Бычков Олег Николаевич - магистрант кафедры «Техника и технологии производства нанопродуктов»; Ткачев Алексей Григорьевич - доктор технических наук, профессор, заведующий кафедрой «Техника и технологии производства нанопродуктов», ФГБОУ ВПО «ТГТУ».
Рецензент: Леонтьева Альбина Ивановна - доктор технических наук, профессор, заведующая кафедрой «Химические технологии органических веществ», ФГБОУ ВПО «ТГТУ».
