Lilla Nánai1,2, Anna Szabó2, Tamás Gyulavári2, Zsejke Réka Tóth2,3, Klára Hernádi1
1Institute of Physical Metallurgy, Metal forming and Nanotechnology, University of Miskolc, H-3515 Miskolc, Hungary
2Department of Applied and Environmental Chemistry, University of Szeged, H-6720 Szeged, Rerrich Béla tér 1, Hungary
3Nanostructured Materials and Bio-Nano-Interfaces Center, Interdisciplinary Research Institute on Bio-Nano-Sciences, Babes-Bolyai University, T. Laurian 42, 400271
Cluj-Napoca, Romania
e-mail: nanai.lilla@student.uni-miskolc.hu Abstract
Nowadays, environmental protection and sustainability are getting more and more attention.
Thus, our aim was to develop a cost and energy efficient catalyst layer building method for the synthesis of carbon nanotube forests. A simple spray coating method was used to develop a catalyst layer on the surface of the titanium substrates. Then vertically aligned carbon nanotubes (VACNTs) were synthesized directly on the substrate via catalytic chemical vapor deposition (CCVD) method. During our research, the effect of catalyst layer deposition parameters on the structure of CNTs was investigated and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy.
Introduction
Vertically aligned carbon nanotubes (VACNTs) have been in the focus of intense research over the years due to their remarkable mechanical and chemical properties such as exceptional electrical and thermal conductivities. VACNTs and their composites are also getting more attention in environmental engineering applications based on their features, for example, sorption capacity, component separation and catalytic activity as well [1,2]. Catalytic chemical vapor deposition (CCVD) is a favored method for the mass production of CNTs it is cost-efficient and simple to use, moreover, it is the only suitable process to synthesize VACNTs [3]. Comparing the possibilities for the production of VACNTs, there are several methods for the formation of thin catalyst layers (i.e., atomic layer deposition (ALD), physical vapor deposition (PVD), pulsed laser deposition (PLD), magnetron sputtering (MS), dip-coating etc.
[4,5]) which are able to control layer thickness and morphology, however, they require rather expensive instruments. Among these methods, dip-coating might be an exception, because it requires less complicated instruments, thus, it is widely used for catalyst layer deposition. Spray coating might be an even cheaper method than dip-coating to build thin layers and it also has more variations (thermal, plasma, manual etc. [6,7]). In this research our aim was to study the efficiency of manual spray coating for building bimetallic catalyst layers containing iron and cobalt. Then, their applicability for VACNTs synthesis via CCVD was investigated, which was carried out on the surface of titanium substrates. To gain knowledge about the efficiency of manual spray coating in this specific application, the parameters of the layer formation (substrate pretreatment, coating temperature and number of spraying cycles) were investigated.
43 Experimental
During the synthesis, a titanium substrate was used to build up the catalyst layers with a spray gun, also used for car painting or art drawing. The concentration of catalyst ink was 0.11 M and the ratio of Fe:Co was 2:3. The latter was prepared from Fe(NO3)3×9H2O and Co(NO3)2×6H2O precursors that were dissolved in absolute ethanol. The Ti substrate was heat treated for 1 h at 400 °C before and after the deposition of the catalyst. This was carried out in a static oven to form a native TiO2 layer on the surface and oxidize the catalytic particles. The TiO2 layer might block the diffusion of catalyst particles into the substrate. In some cases, heat treatment process was changed. To prepare thin layers, the pretreated Ti substrate was placed on a heated plate (120-200 °C), then the catalyst ink was sprayed onto the substrate surface. For this purpose, compressed air and a spray gun was used that was operated at constant speed and distance. In one spraying cycle the ink was sputtered 10 times from both directions. The whole process was repeated 5 times (5×10) applying a 30 second-pause to evaporate the solvent from the surface.
For the growth of VACNTs, CCVD was used during which the gas feed contained ethylene (70 cm3/min) as carbon source, nitrogen (50 cm3/min) as carrier gas, hydrogen (50 cm3/min) for reductive environment and water vapor (30 cm3/min) for prolonging the activity of catalyst particles. The synthesis time was 35 min and the temperature was 700 °C. The scheme of the production of VACNTs is presented in Fig. 1.
Figure 1. The scheme of manual spray coating layer deposition and production of VACNTs via CCVD
Results and discussion
Since the parameters of the catalyst layer significantly affect the growth VACNTs during CCVD synthesis, they were examined during this work. The effects of heat treatment of Ti substrate, spraying temperature on the properties of VACNTs growth, and number of sputtering cycles were investigated in detail.
In the first sample series, the effect of heat treatment of Ti substrate was investigated during the fabrication of catalyst layer. For this purpose, four different samples were prepared: heat treatment a) before and after – as a reference –, b) only before, c) only after spray coating, and d) without any treatment (Fig.2). VACNTs were grown successfully in the first three cases, while in the fourth case only amorphous carbon was deposited onto the surface of Ti substrate.
Heat treatment before spray coating forms a native titania layer that might allow the better adhesion of catalyst ink to the surface. At the same time, heat treatment after spray coating stabilizes the catalyst layer by converting metal nitrate layer into metal oxid layer. The Raman spectroscopy results indicated that heat treating the substrate was benefictial for the graphitic
44
properties of VACNTs. For the verification of carbon deposit, samples were investigated by SEM (Fig. 2).
The thickness of the catalyst layer can be controlled by the number of spraying cycles, which also influences the formation of separated catalyst particles. Therefore, the number of spraying cycles were varied between 1, 2, 3, 4, 5 and 10 cycles. SEM measurements showed that 1×10 spraying could not provide enough catalyst particles on the surface for the growth of VACNTs.
However, applying 10×10 spraying cycles yielded too many catalyst particles that merged together resulting in thick carbon fibers grown on the surface. Applying 2×10 or 5×10 spraying cycles resulted in VACNTs with 8.1 µm or 12.2 µm of height, respectively.
Figure 2. SEM images of VACNTs grown on different heat treated Ti substrates and their corresponding heights
The TEM images also verified that VACNTs grown on the surface of Fe:Co = 2:3 and 5×10 spraying cycles catalyst layer show good graphitic properties with only few defect sites in their walls, which is in good agreement with Raman spectroscopy results.
Finally, the effect of temperature during spray coating on the formation of VACNTs was also investigated. During spraying, the Ti plate was heated between 120-200 °C to evaporate absolute ethanol rapidly from the surface, on which the catalyst particles were uniformly deposited. VACNTs only formed at 120 °C and 140 °C; the corresponding SEM images and height distributions of these samples are shown in Fig. 3.
45
Figure 3. Height distribution and SEM images of VACNTs synthesized at different spraying temperatures
The evaporation rate above 140 °C was too high, thus no homogenous catalyst layer was formed on the Ti substrate.
Conclusions
In summary, spray coating process was proved to be a suitable method for the formation of thin catalyst layers during the production of VACNTs. Heat treatment of Ti substrate, especially after catalyst layer deposition, is needed to sustain stable catalyst layer and synthesize VACNTs with satisfactory orientation. The optimal number of spraying cycles to obtain VACNTs on the surface of Ti substrate was 5 cycles. Increasing the temperature to 140 °C during spray coating can result in higher VACNTs with better quality.
Acknowledgements
The author appreciates the financial support of NKFIH 2019-2.1.11-TÉT-2020-00134.
References
[1] W. Shi, D.L. Plata, Green Chem., 2018, 20, 5245-5260
[2] M. Bodzek, K. Konieczny, A. Kwiecińska-Mydlak, Archives of Environmental Protection, 2021, 43, 3-27
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46
INFLUENCE OF INORGANIC CONSISTUENTS ON PHOTOCATALYTIC DEGRADATION OF IBUPROFEN
Mladenka Novaković1, Goran Štrbac2, Maja Petrović1, Dragana Štrbac1, Ivana Mihajlović1
1Department of Environmental Engineering and Occupational Safety and Health, Faculty of Technical Sciences, Trg Dositeja Obradovića 6, 21000, Novi Sad, Serbia
2 Department of Physics, Faculty of Sciences, Trg Dositeja Obradovića 3, 21000, Novi Sad Serbia
e-mail: mladenkanovakovic@uns.ac.rs Abstract
Non-steroidal anti-inflammatory pollutants such as ibuprofen are continuously introduced in water media through various environmental routes. Due to their variability in physico-chemical properties, characteristics of sludge used in secondary treatment and other features, pharmaceutical residues are partially removed in conventional wastewater treatment plants. The effect of inorganic constituent (nitrates) present in real aquatic matrices was examined to assess the overall efficacy of the photocatalytic removal of ibuprofen by nanostructured mixture ZnO/SnO2.
Introduction
Pharmaceutical active compounds (PhACs) represent one of most dominant group of micropollutants which are continuously introduced into aquatic media, primarily through untreated or inadequately treated wastewater. After usage, the pharmaceuticals undergo to different reactions in which they are transformed into metabolites. These metabolites are often more polar and persistent than the parent compounds. Numerous studies have confirmed that ibuprofen exhibits negative ecotoxicological effects on various aquatic species [1,2]. The aim of this study was to investigate the impact of nitrate ions on photocatalytic degradation of ibuprofen.
Experimental
The synthesis of nanostructured materials was carried out using mechanochemical solid-state method. Initial precursors (ZnO, and SnO2, purity 99.9%) were grounded in an agate mortar for 10 minutes in a molar ratio of 2:1, then annealed in furnace for two hours at 700 °C and once more grounded for 10 minutes [3,4].
Photocatalytic experiment was performed on laboratory scale. A mercury high pressure lamp of 125 W was used as the radiation source, (manufacturer Philips, HPL-N) emission spectrum in the UV range at 304, 314, 335 and 366 nm with maximum emission at 366 nm. The initial concentration of nanostructured mixture ZnO/SnO2 and analyzed pharmaceutical (ibuprofen) was 0,40 mg mL-1 and 5 mg L-1, respectively. In order to demonstrate the effect of inorganic ions on overall efficiency of ibuprofen decomposition, the nitrate ions were selected. At certain time intervals, 1 mL of treated sample was filtrated through 0,45 µm syringe filters in order to remove nanoparticles. Detection in changes in the ibuprofen concentration is followed by the application of high-performance liquid chromatography (HPLC, Agilent 1260). The concentration of nitrate used for experiment was in the range of 5 to 20 mg L-1. 10 mL of samples were filtered and quantitatively transferred to UV-VIS cuvettes. HACH NitraVer 5 reagent was added to the cuvette containing the sample. The concentration of nitrate ions was determined on UV-VIS spectrophotometer (DR5000, HACH, Germany).
47 Results and discussion
The obtained results are shown in the Table 1. In order to compare the effect of nitrate ions on the decomposition of ibuprofen, a pseudo first-order constant was used.
Table 1. Influence of nitrate ions on photocatalytic degradation of ibuprofen Initial concentration of nitrate degradation constant was recorded. The decrease in the value of the degradation constant was 85 % when analyzing concentrations of 10 and 20 mg L-1. The inhibitory effect during the photocatalytic process is caused by an increase of NO3- charge, which emphasizes the electrostatic repulsion between ions and leads to reducing the rate of active adsorption sites on the surface of photocatalysts [5].
Conclusion
Pharmaceutical compounds are micropollutants which are continuously introduced in water streams considering their inefficient removal by conventional wastewater treatment. The heterogenous photocatalytic treatment by newly synthesized nanomaterial ZnO/SnO2 for removal of nonsteroidal anti-inflammatory pollutants is demonstrated to be efficient. According to obtained results, the inhibitory effect on ibuprofen removal was proven by increasing the concentration of nitrate ions.
Acknowledgements
The presented research is partly financed within a project of the Government of Vojvodina
“Synthesis and application of new nanostructured materials for the degradation of organic pollutants from municipal landfill leachate in Vojvodina“ 142-451- 2387/2018-01/01, the Ministry of Education, Science and Technological Development through the project no. 451-03-68/2020-14/200156: “Innovative scientific and artistic research from the FTS domain”, the Bilateral project funded by the Ministry of Education, Science and Technological Development (contract no. 337-00-107/2019-09/16) and Slovak Research and Development Agency under contract SK-SRB-18-0020.
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[3] T.B. Ivetić, N.L. Finčur, B.F. Abramović, M. Dimitrievska, G.R. Štrbac, K.O. Čajko, B.B.
Miljević, L.R. Đačanin, S.R. Lukić-Petrović, Ceram. Int. 42 (2016) 3575–3583.
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Yannopoulos, Process Saf. Environ. Prot. 113 (2018) 174-183.
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386 (2020) 112108.
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AMMONIUM REMOVAL FROM AQUEOUS SOLUTIONS USING BANANA