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The template-assisted wet-combustion synthesis of copper oxide nanoparticles on mesoporous network of alumina nanofibers
Materials Chemistry and Physics 192 (2017) 138e146
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The template-assisted wet-combustion synthesis of copper oxide nanoparticles on mesoporous network of alumina nanofibers Marina Aghayan a, Irina Hussainova a, d, e, *, Khachatur Kirakosyan c, Miguel A. Rodríguez b a
Tallinn University of Technology, Ehitajate 5, 19180, Tallinn, Estonia Instituto de Ceramica y Vidrio (ICV-CSIC), C/Kelsen, 5, 28049, Madrid, Spain c Institute of Chemical Physics, P.Sevak, 5/2, Yerevan, 0014, Armenia d ITMO University, Kronverksky 49, St. Petersburg, 197101, Russian Federation e University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering, Urbana, IL, 61801, USA b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Development of novel templateassisted wet-combustion method. Functionalization of mesoporous substrates of aligned fibers with nanoparticles. Evaluation of mechanism of wetcombustion process and influence of fuel on combustion product. Template-assisted synthesis is a flexible technique to tailor morphology.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2016 Received in revised form 11 January 2017 Accepted 23 January 2017 Available online 30 January 2017
A novel wet-combustion method is developed to functionalize mesoporous network of alumina nanofibers. The wet-combustion method includes coating the nanofibers with the solution of the precursors of dopants mixed with fuels, followed by dissociation/oxidation of the precursors in a combustion mode. Due to the high temperature of the flame and short duration of the combustion process, this approach possesses such unique feature as control over the size, morphology and homogeneity of the deposited nanostructures as well as ability to avoid additional procedures of calcination. In this study, urea and glycine as the basic fuels and copper nitrate as the precursor of copper and oxidant as well as mesoporous network of alumina nanofibers (with single fiber diameter of 7 ± 2 nm) as the template are used. As a result, a mesoporous network of highly oriented copper oxide e alumina composite nanofibers are produced. It is shown that the fuel type and the ratio of fuel-to-oxidizer do not evidently influence on the chemical composition of the product, while the morphology of the deposited copper oxide is controlled by the initial composition of reagents. The average crystallite size of the deposited CuO is 16 and 19 nm for the systems containing stoichiometric amount of glycine and urea, respectively. © 2017 Elsevier B.V. All rights reserved.
Keywords: Wet-combustion synthesis Copper oxide nanoparticles Nanofibers Alumina Fuel
* Corresponding author. Tallinn University of Technology, Ehitajate 5, 19180, Tallinn, Estonia. E-mail address: [email protected] (I. Hussainova). http://dx.doi.org/10.1016/j.matchemphys.2017.01.068 0254-0584/© 2017 Elsevier B.V. All rights reserved.
1. Introduction Preparation of well dispersed metal nanoparticles that
M. Aghayan et al. / Materials Chemistry and Physics 192 (2017) 138e146
uniformly cover oxide supports plays a crucial role in many industrially important catalytic applications. Nevertheless, the ability to design and control size, morphology, and dispersion of supported nanoparticles is still a great challenge. Currently, classic synthetic approaches emphasizing on aqueous precursor solutions, such as ion exchange, wet impregnation, chemical vapor deposition and deposition precipitation, are widely adopted for deposition of active species onto the support. One of the main disadvantages of these methods is a low level of control over particle size. Commonly, changes in the spatial distribution occur during drying which is a relevant step of those methods. Therefore, for drying procedure various approaches, such as rotation of the impregnated monolith, freeze-drying, microwave heating, and increasing the precursor viscosity have been recently proposed [1]. However, complete decomposition of the precursors as well as binding of the deposited particles on the oxide supports require elevated temperatures, which may result in sintering of the active phase. In this work, a novel wet-combustion method was developed to prepare homogenously dispersed nanoparticles on mesoporous support. This never-available-before approach is a simple, singlestep wet-combustion method enabling synthesis of various composites nanofibers mesoporous network in a cost-effective way. The wet-combustion method includes coating the nanofibers with precursors of dopants mixed with fuels followed by dissociation/ oxidation of the precursors in a combustion mode. This method is a unique combination of advantages of sol-gel (e.g., excellent compositional control, homogeneous distribution of components, low temperature) [2e4], dip coating (e.g., simplicity, microstructural control, cost-effectiveness) [5,6] and a solution combustion process (e.g., high temperature, fast heating rate and short duration) [1,7e10]. As a result, the nanostructured materials of tailored composition can be produced. Herein, for the first time the described method is applied for producing copper oxide e alumina composite nanofibers due to huge demand of copper oxide and supported copper oxide nanostructures, which are known as materials of primary importance for variety of applications such as heterogeneous catalysis, gas and glucose sensors, etc. [11,12]. Alumina is known as one of the best catalyst support for the Ni and Cu catalyst, and the copper oxide nanocomposites are known to be highly active in CO oxidation [13], catalytic incineration of organic compounds [14], and have been explored as a new class of anode materials for rechargeable lithium ion batteries, electrode materials for supercapacitors, etc. In this work, mechanism of the wet-combustion process and effect of fuel type and its amount on the composition and morphology of the combustion product is thoroughly studied. 2. Experimental The following reagents were used as the precursors: copper nitrate hydrate Cu(NO3)2$6H2O (98, Sigma-Aldrich) as a source of copper and oxidizer; and urea (>99, Merck) or glycine (99, Sigma) as organic fuels. A mesoporous network of self-aligned g-alumina nanofibers (ANF) described in detail elsewhere [15] and demonstrated in Fig. 1aec was used as a template. An average diameter of the well-oriented channels/pores was determined with the help of nitrogen adsorption to be approximately 10 nm, Fig. 1d. To prepare the reactive solution, 7 g of copper nitrate and different amount of fuels as indicated in Fig. 2 were dissolved in 8 g of deionized water. Amount of the precursor was calculated using the stoichiometric Eq. (1) or (2), which demonstrate the combustion reactions between copper nitrate and glycine or urea, respectively.
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Fig. 1. (a) The network of alumina nanofibers; and SEM micrographs of (b) side and (c) top views, (d) differential pore size distribution in the network.
10 5 4CH2 NH2 COOH þ ð4 1ÞO2 9 2 20 25 54 þ 9 4CO2 þ 4H2 O þ N2 ¼ CuO þ 9 9 9
CuðNO3 Þ2 þ
3 ð4 1ÞO2 2 ¼ CuO þ 24 CO2 þ 44 H2 O þ ð1 þ 24ÞN2
(1)
CuðNO3 Þ2 þ 24 COðNH2 Þ2 þ
(2)
where f is fuel-to-oxidizer ratio (f ¼ 1 implies that all oxygen required for complete combustion of a fuel derives from the oxidizer; while f > 1 (<1) implies fuel-rich (or lean) conditions, respectively). The process parameters such as amount of a fuel added into reactive solution and a corresponding fuel-to-oxidizer ratio are indicated in Fig. 2. The obtained homogeneous aqueous reactive solutions were dropped onto 2 g of fibers, which subsequently were placed in a muffle furnace preheated to 400 C in order to heat the sample as fast as possible, thus, to have fast elimination of the gases which could prohibit agglomeration of the particles. After 30 min the baked samples were taken out from the furnace and characterized by different methods. The schematic representation of the method, which is a combination of sol-gel, dip coating and combustion synthesis, is illustrated in Fig. 3.
Fig. 2. The amount of a fuel added in reactive solution and a corresponding fuel-tooxidizer ratio.
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Fig. 3. Schematic representation of the wet-combustion method.
Thermodynamic calculations were performed using the software package ‘‘ISMAN-Thermo’’ to calculate the adiabatic combustion temperature and the equilibrium products under the adiabatic conditions. This program is based on optimizing the Gibbs free energy of multiphase and multicomponent systems. It assumes gases to be ideal and condensed phases to be completely immiscible. 3. Results and discussion
The temperature-time history of the synthesis process was recorded by MPAC IGAR 12-LO Digital 2-color pyrometers with a fiber optic and a response time of 2 ms. Simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) (Setsys Evolution Setaram, France) of the supports wetted by solutions containing stoichiometric amount of urea (fur ¼ 1) and glycine (fgly ¼ 1) were performed to reveal decomposition behavior of a nitrate-fuel mixture and a combustion behavior pattern. The TGA/DTA was performed in air at a temperature interval from 25 up to 450 C with a heating rate of 10 C/min. Phase identification of the combustion synthesized powder was carried out by X-ray diffraction analysis using a Bruker diffractometer (D8) with CuKa radiation at 40 kV in a scanning range from 20 to 70 with a step of 0.04 . The size of the crystallites was calculated using the Scherrer’s equation detailed in Ref. [16]. Scanning electron microscopy (SEM Zeiss EVO MA 15, Germany) was used to characterize the microstructure of the final products.
3.1. Progress of combustion reaction by DTA/TGA Fig. 4 (a) demonstrates the DTA-TGA of a cupper nitrateeglycine stoichiometric (f ¼ 1) precursor soaked by the substrate of nanofibers. The glycine fuel batch shows low weight loss of 11 wt% up to 180 C that is accompanied by a slight endothermic effect. This weight loss is likely due to dehydration of copper nitrate hydrate. According to published data, copper nitrate begins to decompose at 165 C yielding CuO and releasing HNO3 along with H2O, NO2, O2, while the decomposition of glycine begins at temperatures over 210 C [9,17]. It is proposed that the released nitric acid from the decomposition of copper nitrate reacts with glycine triggering the combustion reaction, [18]. Subsequently, a slight exothermal peak at 187 C is due to the ignition of the combustion process. This stage is followed by a dramatic mass loss (77 wt% of initial value) and a pronounced change in the base line at 196 C due to release of the
Fig. 4. DTA (curve 1) - TGA (curve 2) analysis: (a) copper nitrate e glycine e alumina nanofibers (fgly ¼ 1), and (b) copper nitrate e urea e alumina nanofibers (fur ¼ 1) system.
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depending on the concentration [18], which corresponds the temperature range of the first stage. It should be mentioned that the evaporation of nitric acid before the ignition of the combustion causes less available nitric acid at the ignition temperature, thus changing the fuel-to-oxidizer ratio, which may lead to the formation of not fully oxidized contaminations after the combustion process. The second stage of mass loss is recorded at ~200 C accompanied by a weak exothermic effect that can be caused by an ignition of combustion accompanied with release of a huge amount of gases and, consequently, loss of energy. This ignition persists in a sharp exothermic process and a weight loss of 44 wt% in the temperature range of 250e280 C corresponds to the self-ignition of combustion process. 3.2. Effect of reaction parameters on combustion characteristics Fig. 5. Temperature-time history of the combustion in stoichiometric (f ¼ 1) copper nitrate - glycine e alumina nanofibers system (curve 1); and copper nitrate - urea alumina nanofibers system (curve 2).
gases. These temperature intervals are well corresponding to the decompositions of the precursors and the self-ignition temperature of the mixtures (Tig) as discussed by Kumar et al. [9]. The DTA-TGA curves for the stoichiometric reactive mixture of copper nitrate-urea-alumina nanofibers obtained under the same condition as for the previous system are presented in Fig. 4 (b). The first stage of mass loss takes place at a temperature range from 80 C up to 170 C with a slight endothermic effect. This stage correspond neither to the decomposition temperature of copper nitrate, nor the decomposition temperature of urea (over 190 C) [19]. Dissolving in water urea forms a basic solution, due to presence of two amino-groups, while the copper nitrate forms highly acidic solution. Upon mixing and heating, a neutralization reaction occurs leading to the formation of copper hydroxide and copper hydroxyl-nitrate accompanied by the release of a nitric acid [20]. The nitric acid evaporates from the reaction mixture along with water as the boiling point of the acid ranges from 86 to 121 C
Typical timeetemperature profiles of combustion reactions in both stoichiometric (f ¼ 1) glycine and urea systems are shown in Fig. 5. At an ignition point (Tig), the combustion reaction is dynamically activated without an additional supply of external heat and, as a result, the viscous and frothed mass burst into incandescent flame with associated sharp rise in temperature. The maximum temperatures were observed to be ~800 C for the copper nitrate - glycine e ANFs system (fgly ¼ 1), and ~1070 C for the copper nitrate - urea - ANFs system (fur ¼ 1). In the second stage of the process, contaminations are not fully oxidized by air oxygen in the copper nitrate - urea - ANFs system. However, only one stage is observed in the copper nitrate glycine e ANFs system. It is worth noting that the duration of the combustion process in copper nitrate - urea - ANFs system is longer as compared to copper nitrate - glycine e ANFs system. 3.3. Thermodynamic calculations Under an adiabatic condition, the flame temperature (Tf) and possible combustion product of both copper nitrate-glycine and copper nitrate-urea combustion systems for different copper/fuel ratio were calculated by ISMAN-THERMO software [21]. This
Fig. 6. Thermodynamic characteristics of the combustion process in fuel-to-oxidizer ratio: (a) adiabatic combustion temperature, (b) equilibrium products in copper nitrate-glycine system, (c) equilibrium products in copper nitrate-urea system.
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Fig. 7. XRD patterns of the combustion products obtained for the copper nitrate - glycine eANF system at different fuel-to-oxidizer ratios: fgly¼ (a) 0.5, (b) 0.85, (c) 1, (d) 3.
software is specially designed to calculate adiabatic temperature (Tad) and product equilibrium compositions in heterogeneous chemical processes. The calculated flame temperature (Tad) values for copper nitrate-glycine and copper nitrate-urea systems and different fuelto-oxidizer (f) ratios are presented in Fig. 6. The adiabatic temperature is higher when glycine is used (about 2160 K for the copper nitrate-glycine system, and 1800 K for the copper nitrateurea system) (Fig. 6 (a)). This discrepancy with the data obtained from the pyrometric analysis can be caused by the process pathway and amount of released gases during the process, which were not taken into account during thermodynamic calculation. However, the adiabatic combustion temperature shows the maximum at f ¼ 1 for both systems. The addition of excess fuel causes incomplete combustion reaction in both systems due to oxidant starvation. Therefore, the flame temperature reduces and carbon-residue may remain in the obtained product. The maximum combustion temperature can shift to the fuel-rich region if the reaction is carried out in oxygen containing environment. It is worthy outlining that five different regions are distinguished (Fig. 6 (b) e glycine; and Fig. 6 (c) e urea) based on the predicted equilibrium products in both systems. For the copper
nitrate-glycine system at a relatively low n < 0.53 (region I), only pure CuO phase is presented in an oxygen rich atmosphere. For the region II (0.53 < n < 0.67), the presence of Cu2O along with CuO is predicted in a decreasing oxygen concentration, whereas Cu2O (region III) is obtained for 0.67 < n < 1.06. Reduced Cu with Cu2O present when 1.06 < n < 1.33 (region IV). For n > 1.33 (region V), only pure Cu phase is predicted in a reducing hydrogen atmosphere. It is obvious that for urea system the regions are shifted to the higher amount of fuel. 3.4. Phase analysis of the combustion product X-ray diffraction analyses indicate that in both systems copper (II) oxide is synthesized-deposited at the fuel-lean and stoichiometric (f 1) reactive mixtures (Fig. 7aec and Fig. 8 a, b). Increasing the amount of fuel (fgly ¼ 3) in the glycine-copper nitrate-ANFs system results in appearance of traces of reduced copper along with copper (II) oxide (Fig. 7d). Moreover, the thermodynamic analysis (Fig. 6b) and the experimental data obtained by Kumar el al [9] show that only pure copper phase are obtained when a high amount of glycine is used (f ¼ 3). Such inconsistency of the data can be explained by the ultra-fine particle
Fig. 8. XRD patterns of the combustion products obtained for the copper nitrate-urea-ANF system at different fuel-to-oxidizer ratios fur¼ (a) 0.5, (b) 0.75, (c) 1, (d) 1.5.
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increasing the glycine-to-oxidizer ratio from 0.5 to 1.0. Although with increase in fgly the amount of eliminated gases increases, the flame temperature increases as well, which prevents growth of the crystallite size. 3.5. Microstructural characterization
Fig. 9. The size of the crystallites of deposited copper oxide obtained by using different amount of glycine (orange columns) and urea (blue columns) in the corresponding systems; copper nitrate - glycine e alumina nanofibers and copper nitrate - urea alumina nanofibers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. (a) The schematic representation of the network of as fabricated combustion product, SEM micrographs of (b) a top, and (c) a side views of CuO-ANFs mesoporous network.
size of the deposited product, which immediately oxidize in air at 400 C [22]. Traces of copper (I) oxide along with copper (II) oxide form when fur 1 in the urea-copper nitrate system (Fig. 8c and d). The alumina shows very low intensity peaks and can hardly be detected by XRD in this particular case as the signal is masked by presence of copper oxide homogeneously distributed over the surface of fibers. The size of the crystallites is measured by X-ray diffraction and shown in Fig. 9. The crystallite size of CuO depends on the fuel type. It is obvious that the crystallite size of as-synthesized CuO is relatively small when glycine is used as a fuel. This can be caused by a lower flame temperature (~800 C) generated during a combustion process as compared to the copper nitrate e urea - ANFs system (fur ¼ 1) (~1070 C). Moreover, the duration of the combustion process is longer when urea is used as a fuel, which results in increase of the crystallite size during the process. In the copper nitrate e glycine - ANFs system, the crystallite size increases with
The micro-mesoporous network of the highly aligned fibrous composite is developed as a result of wet-combustion method, Fig. 10. Figs. 11 and 12 represent the SEM micrographs of assynthesized-deposited product on the network of nanofibers obtained from various fuel containing batches of glycine and urea, respectively. At the fuel-lean glycine system, copper oxide with a fine particle size (<50 nm) is deposited quite homogeneously on the surface of nanofibers (Fig. 11a and b). However, with increase in the amount of glycine, the particle size of the deposited copper oxide increases, which can be explained by an increase in the maximum combustion temperature with increase in 4. High-resolution TEM analysis (Fig. 12) of CuO-ANF obtained using a small amount of glycine (fgly ¼ 0.75) provides an evidence for the core-shell structure of the fibers. The alumina nanofibers with average sizes of ~7 nm are covered with a smooth layer of CuO with thickness of 3e8 nm. The corresponding selected area electron diffraction (SAED) pattern of the composite core-shell structured fibers reveals amorphous structure. The deposited particles differ in size and morphology when urea is used as the fuel (Fig. 13). Particles with inhomogeneous size and shape is deposited on nanofibers network at fur ¼ 0.5 (Fig. 13a). Increase in the amount of fuel results in more pronounced inhomogeneous distribution of the particles size of nonuniform shape. However, the particles of as-synthesized-deposited copper oxide are bigger in size when urea is used, which is supposed to be caused by a higher flame temperature, prolonged combustion process and formation of copper hydroxide and hydroxyl-nitrate before the combustion ignition as it was described above. An increase in the amount of urea leads to bi-modal particles size distribution: up to 30 nm particles tightly squeezed the fibers, and 100e200 nm particles dispersed all over the bundle of fibers (Fig. 13c and d). In the fuel-rich systems, the size of particles increases. Multiple factors influence the formation and evolution of size and morphology of the particles synthesized in a combustion regime. The flame temperature and the volume of evolved gas are not the only criteria determining the particle size. Possible interaction of a fuel and metal ions also affects the obtained particle size. This interaction and the time of oxidation of the fuel influence on both nucleation and growth rates. Complex of factors leads to formation of the finer particles when glycine is used as a fuel. 4. Conclusions The network of the self-aligned alumina nanofibers of a huge aspect ratio 107 was deposited by copper oxide with the help of a novel single-step wet-combustion synthesis without further calcination. It was demonstrated that the morphology of the product changes altering the type of the fuel. Therefore, the template-assisted wet-combustion synthesis approach is a flexible cost-effective technique to produce composite nanofibers of tailored morphology. The reaction pathway drastically depends on the functional groups of the fuels involved, the decomposition temperatures, and the reactions occurring before combustion ignition. It was shown that the particle size of the deposited CuO is highly influenced by the fuel type and its concentration. As a fuel, urea
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Fig. 11. SEM micrographs of as fabricated combustion impregnation product using different amount of glycine: fgly¼ (a) 0.75; (b) 0.85; (c) 1.0; (d) 2.0.
Fig. 12. High-resolution TEM (HRTEM) image and corresponding SAED patterns of CuO-ANF (fgly ¼ 0.75) nanofibers: Magnification increases from panel (a) to (b).
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Fig. 13. SEM micrographs of as fabricated combustion impregnation product using different amount of urea: fur¼ (a) 0.5; (b) 0.75; (c) 1.0; (d) 1.5.
facilitates the formation of the larger particles of copper oxide as compared to the particles grown when glycine is used. The deposition of pure CuO and CuO with traces of mixed valence copper oxide phases as well as metallic copper was demonstrated by variations in the fuel and fuel-to-oxidizer ratio. Acknowledgements This work was supported by the Estonia Research Council under PUT1063 (I. Hussainova), Estonian Ministry of Higher Education and Research under Projects IUT19-29, as well as Spanish Ministry of Economy and Competitiveness under the project MAT201348426-C2-1R. The authors would like to thank Prof. V. Sammelselg and PhD Mihkel Rahn from University of Tartu, Estonia, for their valuable help with TEM analysis. References [1] V. Dhanasekaran, T. Mahalingam, V. Ganesan, SEM and AFM studies of dipcoated CuO nanofilms, Microsc. Res. Tech. 76 (2013) 58e65. [2] S.T. Aruna, A.S. Mukasyan, Combustion synthesis and nanomaterials, Curr. Opin. Solid State Mater. Sci. 12 (2008) 44e50. [3] G.R. Rao, B.G. Mishra, H.R. Sahu, Synthesis of CuO, Cu and CuNi alloy particles by solution combustion using carbohydrazide and N-tertiarybutoxy-carbonylpiperazine fuels, Mater. Lett. 58 (2004) 3523e3527. [4] A. Kumar, E.E. Wolf, A.S. Mukasyan, Solution combustion synthesis of metal nanopowders: copper and copper/nickel alloys, AIChE J. 57 (2011)
3473e3479. [5] A.J. Christy, L.C. Nehru, M. Umadevi, A novel combustion method to prepare CuO nanorods and its antimicrobial and photocatalytic activities, Powder Technol. 235 (2013) 783e786. [6] Z. Kebin, W. Ruipu, X. Boqing, L. Yadong, Synthesis, characterization and catalytic properties of CuO nanocrystals with various shapes, Nanotechnology 17 (2006) 3939. [7] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, CuO nanostructures: synthesis, characterization, growth mechanisms, fundamental properties, and applications, Prog. Mater. Sci. 60 (2014) 208e337. [8] Y. Feng, X. Zheng, Plasma-enhanced catalytic CuO nanowires for CO oxidation, Nano Lett. 10 (2010) 4762e4766. [9] C.-H. Wang, S.-S. Lin, C.-L. Chen, H.-S. Weng, Performance of the supported copper oxide catalysts for the catalytic incineration of aromatic hydrocarbons, Chemosphere 64 (2006) 503e509. [10] M. Aghayan, I. Hussainova, M. Gasik, M. Kutuzov, M. Friman, Coupled thermal analysis of novel alumina nanofibers with ultrahigh aspect ratio, Thermochim. Acta 574 (2013) 140e144. € gl, Microwave-assisted [11] U. Zavyalova, F. Girgsdies, O. Korup, R. Horn, R. Schlo self-propagating combustion synthesis for uniform deposition of metal nanoparticles on ceramic monoliths, J. Phys. Chem. C 113 (2009) 17493e17501. [12] R.M. Allaf, L.J. Hope-Weeks, Synthesis of ZnO-CuO nanocomposite aerogels by the sol-gel route, J. Nanomater. 2014 (2014) 9. [13] P. Wang, P. Li, T.-F. Yi, X. Lin, Y.-R. Zhu, L. Shao, M. Shui, N. Long, J. Shu, Fabrication and electrochemical properties of CuCrO2 anode obtained by a solegel method, Ceram. Int. 41 (2015) 6668e6675. [14] A. Mirzaei, K. Janghorban, B. Hashemi, M. Bonyani, S.G. Leonardi, G. Neri, Highly stable and selective ethanol sensor based on a-Fe2O3 nanoparticles prepared by Pechini solegel method, Ceram. Int. 42 (2016) 6136e6144. [15] R. Ahmad, J.-H. Ha, I.-H. Song, Enhancement of the compressive strength of highly porous Al2O3 foam through crack healing and improvement of the
146
M. Aghayan et al. / Materials Chemistry and Physics 192 (2017) 138e146
surface condition by dip-coating, Ceram. Int. 40 (2014) 3679e3685. [16] U. Holzwarth, N. Gibson, The Scherrer equation versus the ‘Debye-Scherrer equation’, Nat. Nanotechnol. 6 (2011) 534. [17] V.Y. Yablokov, I.L. Smel’tsova, I.A. Zelyaev, S.V. Mitrofanova, Studies of the rates of thermal decomposition of glycine, alanine, and serine, Russ. J. General Chem. 79 (2009) 1704e1706. [18] P. Erri, P. Pranda, A. Varma, OxidizerFuel interactions in aqueous combustion synthesis. 1. Iron(III) NitrateModel fuels, Ind. Eng. Chem. Res. 43 (2004) 3092e3096. [19] P.M. Schaber, J. Colson, S. Higgins, D. Thielen, B. Anspach, J. Brauer, Thermal
decomposition (pyrolysis) of urea in an open reaction vessel, Thermochim. Acta 424 (2004) 131e142. [20] L.H. Dalman, Ternary systems of urea and acids. I. Urea, nitric acid and water. II. Urea, sulfuric acid and water. III. Urea, oxalic acid and water, J. Am. Chem. Soc. 56 (1934) 549e553. [21] A.A. Shiryaev, Thermodynamics of SHS processes: advanced approach, Int. J. Self Propagating High Temp. Synth. 4 (4) (1995) 351e362. [22] A. Yabuki, S. Tanaka, Oxidation behavior of copper nanoparticles at low temperature, Mater. Res. Bull. 46 (12) (2011) 2323e2327.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
The template-assisted wet-combustion synthesis of copper oxide nanoparticles on mesoporous network of alumina nanofibers Marina Aghayan a, Irina Hussainova a, d, e, *, Khachatur Kirakosyan c, Miguel A. Rodríguez b a
Tallinn University of Technology, Ehitajate 5, 19180, Tallinn, Estonia Instituto de Ceramica y Vidrio (ICV-CSIC), C/Kelsen, 5, 28049, Madrid, Spain c Institute of Chemical Physics, P.Sevak, 5/2, Yerevan, 0014, Armenia d ITMO University, Kronverksky 49, St. Petersburg, 197101, Russian Federation e University of Illinois at Urbana-Champaign, Department of Mechanical Science and Engineering, Urbana, IL, 61801, USA b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Development of novel templateassisted wet-combustion method. Functionalization of mesoporous substrates of aligned fibers with nanoparticles. Evaluation of mechanism of wetcombustion process and influence of fuel on combustion product. Template-assisted synthesis is a flexible technique to tailor morphology.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2016 Received in revised form 11 January 2017 Accepted 23 January 2017 Available online 30 January 2017
A novel wet-combustion method is developed to functionalize mesoporous network of alumina nanofibers. The wet-combustion method includes coating the nanofibers with the solution of the precursors of dopants mixed with fuels, followed by dissociation/oxidation of the precursors in a combustion mode. Due to the high temperature of the flame and short duration of the combustion process, this approach possesses such unique feature as control over the size, morphology and homogeneity of the deposited nanostructures as well as ability to avoid additional procedures of calcination. In this study, urea and glycine as the basic fuels and copper nitrate as the precursor of copper and oxidant as well as mesoporous network of alumina nanofibers (with single fiber diameter of 7 ± 2 nm) as the template are used. As a result, a mesoporous network of highly oriented copper oxide e alumina composite nanofibers are produced. It is shown that the fuel type and the ratio of fuel-to-oxidizer do not evidently influence on the chemical composition of the product, while the morphology of the deposited copper oxide is controlled by the initial composition of reagents. The average crystallite size of the deposited CuO is 16 and 19 nm for the systems containing stoichiometric amount of glycine and urea, respectively. © 2017 Elsevier B.V. All rights reserved.
Keywords: Wet-combustion synthesis Copper oxide nanoparticles Nanofibers Alumina Fuel
* Corresponding author. Tallinn University of Technology, Ehitajate 5, 19180, Tallinn, Estonia. E-mail address: [email protected] (I. Hussainova). http://dx.doi.org/10.1016/j.matchemphys.2017.01.068 0254-0584/© 2017 Elsevier B.V. All rights reserved.
1. Introduction Preparation of well dispersed metal nanoparticles that
M. Aghayan et al. / Materials Chemistry and Physics 192 (2017) 138e146
uniformly cover oxide supports plays a crucial role in many industrially important catalytic applications. Nevertheless, the ability to design and control size, morphology, and dispersion of supported nanoparticles is still a great challenge. Currently, classic synthetic approaches emphasizing on aqueous precursor solutions, such as ion exchange, wet impregnation, chemical vapor deposition and deposition precipitation, are widely adopted for deposition of active species onto the support. One of the main disadvantages of these methods is a low level of control over particle size. Commonly, changes in the spatial distribution occur during drying which is a relevant step of those methods. Therefore, for drying procedure various approaches, such as rotation of the impregnated monolith, freeze-drying, microwave heating, and increasing the precursor viscosity have been recently proposed [1]. However, complete decomposition of the precursors as well as binding of the deposited particles on the oxide supports require elevated temperatures, which may result in sintering of the active phase. In this work, a novel wet-combustion method was developed to prepare homogenously dispersed nanoparticles on mesoporous support. This never-available-before approach is a simple, singlestep wet-combustion method enabling synthesis of various composites nanofibers mesoporous network in a cost-effective way. The wet-combustion method includes coating the nanofibers with precursors of dopants mixed with fuels followed by dissociation/ oxidation of the precursors in a combustion mode. This method is a unique combination of advantages of sol-gel (e.g., excellent compositional control, homogeneous distribution of components, low temperature) [2e4], dip coating (e.g., simplicity, microstructural control, cost-effectiveness) [5,6] and a solution combustion process (e.g., high temperature, fast heating rate and short duration) [1,7e10]. As a result, the nanostructured materials of tailored composition can be produced. Herein, for the first time the described method is applied for producing copper oxide e alumina composite nanofibers due to huge demand of copper oxide and supported copper oxide nanostructures, which are known as materials of primary importance for variety of applications such as heterogeneous catalysis, gas and glucose sensors, etc. [11,12]. Alumina is known as one of the best catalyst support for the Ni and Cu catalyst, and the copper oxide nanocomposites are known to be highly active in CO oxidation [13], catalytic incineration of organic compounds [14], and have been explored as a new class of anode materials for rechargeable lithium ion batteries, electrode materials for supercapacitors, etc. In this work, mechanism of the wet-combustion process and effect of fuel type and its amount on the composition and morphology of the combustion product is thoroughly studied. 2. Experimental The following reagents were used as the precursors: copper nitrate hydrate Cu(NO3)2$6H2O (98, Sigma-Aldrich) as a source of copper and oxidizer; and urea (>99, Merck) or glycine (99, Sigma) as organic fuels. A mesoporous network of self-aligned g-alumina nanofibers (ANF) described in detail elsewhere [15] and demonstrated in Fig. 1aec was used as a template. An average diameter of the well-oriented channels/pores was determined with the help of nitrogen adsorption to be approximately 10 nm, Fig. 1d. To prepare the reactive solution, 7 g of copper nitrate and different amount of fuels as indicated in Fig. 2 were dissolved in 8 g of deionized water. Amount of the precursor was calculated using the stoichiometric Eq. (1) or (2), which demonstrate the combustion reactions between copper nitrate and glycine or urea, respectively.
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Fig. 1. (a) The network of alumina nanofibers; and SEM micrographs of (b) side and (c) top views, (d) differential pore size distribution in the network.
10 5 4CH2 NH2 COOH þ ð4 1ÞO2 9 2 20 25 54 þ 9 4CO2 þ 4H2 O þ N2 ¼ CuO þ 9 9 9
CuðNO3 Þ2 þ
3 ð4 1ÞO2 2 ¼ CuO þ 24 CO2 þ 44 H2 O þ ð1 þ 24ÞN2
(1)
CuðNO3 Þ2 þ 24 COðNH2 Þ2 þ
(2)
where f is fuel-to-oxidizer ratio (f ¼ 1 implies that all oxygen required for complete combustion of a fuel derives from the oxidizer; while f > 1 (<1) implies fuel-rich (or lean) conditions, respectively). The process parameters such as amount of a fuel added into reactive solution and a corresponding fuel-to-oxidizer ratio are indicated in Fig. 2. The obtained homogeneous aqueous reactive solutions were dropped onto 2 g of fibers, which subsequently were placed in a muffle furnace preheated to 400 C in order to heat the sample as fast as possible, thus, to have fast elimination of the gases which could prohibit agglomeration of the particles. After 30 min the baked samples were taken out from the furnace and characterized by different methods. The schematic representation of the method, which is a combination of sol-gel, dip coating and combustion synthesis, is illustrated in Fig. 3.
Fig. 2. The amount of a fuel added in reactive solution and a corresponding fuel-tooxidizer ratio.
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Fig. 3. Schematic representation of the wet-combustion method.
Thermodynamic calculations were performed using the software package ‘‘ISMAN-Thermo’’ to calculate the adiabatic combustion temperature and the equilibrium products under the adiabatic conditions. This program is based on optimizing the Gibbs free energy of multiphase and multicomponent systems. It assumes gases to be ideal and condensed phases to be completely immiscible. 3. Results and discussion
The temperature-time history of the synthesis process was recorded by MPAC IGAR 12-LO Digital 2-color pyrometers with a fiber optic and a response time of 2 ms. Simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA) (Setsys Evolution Setaram, France) of the supports wetted by solutions containing stoichiometric amount of urea (fur ¼ 1) and glycine (fgly ¼ 1) were performed to reveal decomposition behavior of a nitrate-fuel mixture and a combustion behavior pattern. The TGA/DTA was performed in air at a temperature interval from 25 up to 450 C with a heating rate of 10 C/min. Phase identification of the combustion synthesized powder was carried out by X-ray diffraction analysis using a Bruker diffractometer (D8) with CuKa radiation at 40 kV in a scanning range from 20 to 70 with a step of 0.04 . The size of the crystallites was calculated using the Scherrer’s equation detailed in Ref. [16]. Scanning electron microscopy (SEM Zeiss EVO MA 15, Germany) was used to characterize the microstructure of the final products.
3.1. Progress of combustion reaction by DTA/TGA Fig. 4 (a) demonstrates the DTA-TGA of a cupper nitrateeglycine stoichiometric (f ¼ 1) precursor soaked by the substrate of nanofibers. The glycine fuel batch shows low weight loss of 11 wt% up to 180 C that is accompanied by a slight endothermic effect. This weight loss is likely due to dehydration of copper nitrate hydrate. According to published data, copper nitrate begins to decompose at 165 C yielding CuO and releasing HNO3 along with H2O, NO2, O2, while the decomposition of glycine begins at temperatures over 210 C [9,17]. It is proposed that the released nitric acid from the decomposition of copper nitrate reacts with glycine triggering the combustion reaction, [18]. Subsequently, a slight exothermal peak at 187 C is due to the ignition of the combustion process. This stage is followed by a dramatic mass loss (77 wt% of initial value) and a pronounced change in the base line at 196 C due to release of the
Fig. 4. DTA (curve 1) - TGA (curve 2) analysis: (a) copper nitrate e glycine e alumina nanofibers (fgly ¼ 1), and (b) copper nitrate e urea e alumina nanofibers (fur ¼ 1) system.
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depending on the concentration [18], which corresponds the temperature range of the first stage. It should be mentioned that the evaporation of nitric acid before the ignition of the combustion causes less available nitric acid at the ignition temperature, thus changing the fuel-to-oxidizer ratio, which may lead to the formation of not fully oxidized contaminations after the combustion process. The second stage of mass loss is recorded at ~200 C accompanied by a weak exothermic effect that can be caused by an ignition of combustion accompanied with release of a huge amount of gases and, consequently, loss of energy. This ignition persists in a sharp exothermic process and a weight loss of 44 wt% in the temperature range of 250e280 C corresponds to the self-ignition of combustion process. 3.2. Effect of reaction parameters on combustion characteristics Fig. 5. Temperature-time history of the combustion in stoichiometric (f ¼ 1) copper nitrate - glycine e alumina nanofibers system (curve 1); and copper nitrate - urea alumina nanofibers system (curve 2).
gases. These temperature intervals are well corresponding to the decompositions of the precursors and the self-ignition temperature of the mixtures (Tig) as discussed by Kumar et al. [9]. The DTA-TGA curves for the stoichiometric reactive mixture of copper nitrate-urea-alumina nanofibers obtained under the same condition as for the previous system are presented in Fig. 4 (b). The first stage of mass loss takes place at a temperature range from 80 C up to 170 C with a slight endothermic effect. This stage correspond neither to the decomposition temperature of copper nitrate, nor the decomposition temperature of urea (over 190 C) [19]. Dissolving in water urea forms a basic solution, due to presence of two amino-groups, while the copper nitrate forms highly acidic solution. Upon mixing and heating, a neutralization reaction occurs leading to the formation of copper hydroxide and copper hydroxyl-nitrate accompanied by the release of a nitric acid [20]. The nitric acid evaporates from the reaction mixture along with water as the boiling point of the acid ranges from 86 to 121 C
Typical timeetemperature profiles of combustion reactions in both stoichiometric (f ¼ 1) glycine and urea systems are shown in Fig. 5. At an ignition point (Tig), the combustion reaction is dynamically activated without an additional supply of external heat and, as a result, the viscous and frothed mass burst into incandescent flame with associated sharp rise in temperature. The maximum temperatures were observed to be ~800 C for the copper nitrate - glycine e ANFs system (fgly ¼ 1), and ~1070 C for the copper nitrate - urea - ANFs system (fur ¼ 1). In the second stage of the process, contaminations are not fully oxidized by air oxygen in the copper nitrate - urea - ANFs system. However, only one stage is observed in the copper nitrate glycine e ANFs system. It is worth noting that the duration of the combustion process in copper nitrate - urea - ANFs system is longer as compared to copper nitrate - glycine e ANFs system. 3.3. Thermodynamic calculations Under an adiabatic condition, the flame temperature (Tf) and possible combustion product of both copper nitrate-glycine and copper nitrate-urea combustion systems for different copper/fuel ratio were calculated by ISMAN-THERMO software [21]. This
Fig. 6. Thermodynamic characteristics of the combustion process in fuel-to-oxidizer ratio: (a) adiabatic combustion temperature, (b) equilibrium products in copper nitrate-glycine system, (c) equilibrium products in copper nitrate-urea system.
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Fig. 7. XRD patterns of the combustion products obtained for the copper nitrate - glycine eANF system at different fuel-to-oxidizer ratios: fgly¼ (a) 0.5, (b) 0.85, (c) 1, (d) 3.
software is specially designed to calculate adiabatic temperature (Tad) and product equilibrium compositions in heterogeneous chemical processes. The calculated flame temperature (Tad) values for copper nitrate-glycine and copper nitrate-urea systems and different fuelto-oxidizer (f) ratios are presented in Fig. 6. The adiabatic temperature is higher when glycine is used (about 2160 K for the copper nitrate-glycine system, and 1800 K for the copper nitrateurea system) (Fig. 6 (a)). This discrepancy with the data obtained from the pyrometric analysis can be caused by the process pathway and amount of released gases during the process, which were not taken into account during thermodynamic calculation. However, the adiabatic combustion temperature shows the maximum at f ¼ 1 for both systems. The addition of excess fuel causes incomplete combustion reaction in both systems due to oxidant starvation. Therefore, the flame temperature reduces and carbon-residue may remain in the obtained product. The maximum combustion temperature can shift to the fuel-rich region if the reaction is carried out in oxygen containing environment. It is worthy outlining that five different regions are distinguished (Fig. 6 (b) e glycine; and Fig. 6 (c) e urea) based on the predicted equilibrium products in both systems. For the copper
nitrate-glycine system at a relatively low n < 0.53 (region I), only pure CuO phase is presented in an oxygen rich atmosphere. For the region II (0.53 < n < 0.67), the presence of Cu2O along with CuO is predicted in a decreasing oxygen concentration, whereas Cu2O (region III) is obtained for 0.67 < n < 1.06. Reduced Cu with Cu2O present when 1.06 < n < 1.33 (region IV). For n > 1.33 (region V), only pure Cu phase is predicted in a reducing hydrogen atmosphere. It is obvious that for urea system the regions are shifted to the higher amount of fuel. 3.4. Phase analysis of the combustion product X-ray diffraction analyses indicate that in both systems copper (II) oxide is synthesized-deposited at the fuel-lean and stoichiometric (f 1) reactive mixtures (Fig. 7aec and Fig. 8 a, b). Increasing the amount of fuel (fgly ¼ 3) in the glycine-copper nitrate-ANFs system results in appearance of traces of reduced copper along with copper (II) oxide (Fig. 7d). Moreover, the thermodynamic analysis (Fig. 6b) and the experimental data obtained by Kumar el al [9] show that only pure copper phase are obtained when a high amount of glycine is used (f ¼ 3). Such inconsistency of the data can be explained by the ultra-fine particle
Fig. 8. XRD patterns of the combustion products obtained for the copper nitrate-urea-ANF system at different fuel-to-oxidizer ratios fur¼ (a) 0.5, (b) 0.75, (c) 1, (d) 1.5.
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increasing the glycine-to-oxidizer ratio from 0.5 to 1.0. Although with increase in fgly the amount of eliminated gases increases, the flame temperature increases as well, which prevents growth of the crystallite size. 3.5. Microstructural characterization
Fig. 9. The size of the crystallites of deposited copper oxide obtained by using different amount of glycine (orange columns) and urea (blue columns) in the corresponding systems; copper nitrate - glycine e alumina nanofibers and copper nitrate - urea alumina nanofibers. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. (a) The schematic representation of the network of as fabricated combustion product, SEM micrographs of (b) a top, and (c) a side views of CuO-ANFs mesoporous network.
size of the deposited product, which immediately oxidize in air at 400 C [22]. Traces of copper (I) oxide along with copper (II) oxide form when fur 1 in the urea-copper nitrate system (Fig. 8c and d). The alumina shows very low intensity peaks and can hardly be detected by XRD in this particular case as the signal is masked by presence of copper oxide homogeneously distributed over the surface of fibers. The size of the crystallites is measured by X-ray diffraction and shown in Fig. 9. The crystallite size of CuO depends on the fuel type. It is obvious that the crystallite size of as-synthesized CuO is relatively small when glycine is used as a fuel. This can be caused by a lower flame temperature (~800 C) generated during a combustion process as compared to the copper nitrate e urea - ANFs system (fur ¼ 1) (~1070 C). Moreover, the duration of the combustion process is longer when urea is used as a fuel, which results in increase of the crystallite size during the process. In the copper nitrate e glycine - ANFs system, the crystallite size increases with
The micro-mesoporous network of the highly aligned fibrous composite is developed as a result of wet-combustion method, Fig. 10. Figs. 11 and 12 represent the SEM micrographs of assynthesized-deposited product on the network of nanofibers obtained from various fuel containing batches of glycine and urea, respectively. At the fuel-lean glycine system, copper oxide with a fine particle size (<50 nm) is deposited quite homogeneously on the surface of nanofibers (Fig. 11a and b). However, with increase in the amount of glycine, the particle size of the deposited copper oxide increases, which can be explained by an increase in the maximum combustion temperature with increase in 4. High-resolution TEM analysis (Fig. 12) of CuO-ANF obtained using a small amount of glycine (fgly ¼ 0.75) provides an evidence for the core-shell structure of the fibers. The alumina nanofibers with average sizes of ~7 nm are covered with a smooth layer of CuO with thickness of 3e8 nm. The corresponding selected area electron diffraction (SAED) pattern of the composite core-shell structured fibers reveals amorphous structure. The deposited particles differ in size and morphology when urea is used as the fuel (Fig. 13). Particles with inhomogeneous size and shape is deposited on nanofibers network at fur ¼ 0.5 (Fig. 13a). Increase in the amount of fuel results in more pronounced inhomogeneous distribution of the particles size of nonuniform shape. However, the particles of as-synthesized-deposited copper oxide are bigger in size when urea is used, which is supposed to be caused by a higher flame temperature, prolonged combustion process and formation of copper hydroxide and hydroxyl-nitrate before the combustion ignition as it was described above. An increase in the amount of urea leads to bi-modal particles size distribution: up to 30 nm particles tightly squeezed the fibers, and 100e200 nm particles dispersed all over the bundle of fibers (Fig. 13c and d). In the fuel-rich systems, the size of particles increases. Multiple factors influence the formation and evolution of size and morphology of the particles synthesized in a combustion regime. The flame temperature and the volume of evolved gas are not the only criteria determining the particle size. Possible interaction of a fuel and metal ions also affects the obtained particle size. This interaction and the time of oxidation of the fuel influence on both nucleation and growth rates. Complex of factors leads to formation of the finer particles when glycine is used as a fuel. 4. Conclusions The network of the self-aligned alumina nanofibers of a huge aspect ratio 107 was deposited by copper oxide with the help of a novel single-step wet-combustion synthesis without further calcination. It was demonstrated that the morphology of the product changes altering the type of the fuel. Therefore, the template-assisted wet-combustion synthesis approach is a flexible cost-effective technique to produce composite nanofibers of tailored morphology. The reaction pathway drastically depends on the functional groups of the fuels involved, the decomposition temperatures, and the reactions occurring before combustion ignition. It was shown that the particle size of the deposited CuO is highly influenced by the fuel type and its concentration. As a fuel, urea
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Fig. 11. SEM micrographs of as fabricated combustion impregnation product using different amount of glycine: fgly¼ (a) 0.75; (b) 0.85; (c) 1.0; (d) 2.0.
Fig. 12. High-resolution TEM (HRTEM) image and corresponding SAED patterns of CuO-ANF (fgly ¼ 0.75) nanofibers: Magnification increases from panel (a) to (b).
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Fig. 13. SEM micrographs of as fabricated combustion impregnation product using different amount of urea: fur¼ (a) 0.5; (b) 0.75; (c) 1.0; (d) 1.5.
facilitates the formation of the larger particles of copper oxide as compared to the particles grown when glycine is used. The deposition of pure CuO and CuO with traces of mixed valence copper oxide phases as well as metallic copper was demonstrated by variations in the fuel and fuel-to-oxidizer ratio. Acknowledgements This work was supported by the Estonia Research Council under PUT1063 (I. Hussainova), Estonian Ministry of Higher Education and Research under Projects IUT19-29, as well as Spanish Ministry of Economy and Competitiveness under the project MAT201348426-C2-1R. The authors would like to thank Prof. V. Sammelselg and PhD Mihkel Rahn from University of Tartu, Estonia, for their valuable help with TEM analysis. References [1] V. Dhanasekaran, T. Mahalingam, V. Ganesan, SEM and AFM studies of dipcoated CuO nanofilms, Microsc. Res. Tech. 76 (2013) 58e65. [2] S.T. Aruna, A.S. Mukasyan, Combustion synthesis and nanomaterials, Curr. Opin. Solid State Mater. Sci. 12 (2008) 44e50. [3] G.R. Rao, B.G. Mishra, H.R. Sahu, Synthesis of CuO, Cu and CuNi alloy particles by solution combustion using carbohydrazide and N-tertiarybutoxy-carbonylpiperazine fuels, Mater. Lett. 58 (2004) 3523e3527. [4] A. Kumar, E.E. Wolf, A.S. Mukasyan, Solution combustion synthesis of metal nanopowders: copper and copper/nickel alloys, AIChE J. 57 (2011)
3473e3479. [5] A.J. Christy, L.C. Nehru, M. Umadevi, A novel combustion method to prepare CuO nanorods and its antimicrobial and photocatalytic activities, Powder Technol. 235 (2013) 783e786. [6] Z. Kebin, W. Ruipu, X. Boqing, L. Yadong, Synthesis, characterization and catalytic properties of CuO nanocrystals with various shapes, Nanotechnology 17 (2006) 3939. [7] Q. Zhang, K. Zhang, D. Xu, G. Yang, H. Huang, F. Nie, C. Liu, S. Yang, CuO nanostructures: synthesis, characterization, growth mechanisms, fundamental properties, and applications, Prog. Mater. Sci. 60 (2014) 208e337. [8] Y. Feng, X. Zheng, Plasma-enhanced catalytic CuO nanowires for CO oxidation, Nano Lett. 10 (2010) 4762e4766. [9] C.-H. Wang, S.-S. Lin, C.-L. Chen, H.-S. Weng, Performance of the supported copper oxide catalysts for the catalytic incineration of aromatic hydrocarbons, Chemosphere 64 (2006) 503e509. [10] M. Aghayan, I. Hussainova, M. Gasik, M. Kutuzov, M. Friman, Coupled thermal analysis of novel alumina nanofibers with ultrahigh aspect ratio, Thermochim. Acta 574 (2013) 140e144. € gl, Microwave-assisted [11] U. Zavyalova, F. Girgsdies, O. Korup, R. Horn, R. Schlo self-propagating combustion synthesis for uniform deposition of metal nanoparticles on ceramic monoliths, J. Phys. Chem. C 113 (2009) 17493e17501. [12] R.M. Allaf, L.J. Hope-Weeks, Synthesis of ZnO-CuO nanocomposite aerogels by the sol-gel route, J. Nanomater. 2014 (2014) 9. [13] P. Wang, P. Li, T.-F. Yi, X. Lin, Y.-R. Zhu, L. Shao, M. Shui, N. Long, J. Shu, Fabrication and electrochemical properties of CuCrO2 anode obtained by a solegel method, Ceram. Int. 41 (2015) 6668e6675. [14] A. Mirzaei, K. Janghorban, B. Hashemi, M. Bonyani, S.G. Leonardi, G. Neri, Highly stable and selective ethanol sensor based on a-Fe2O3 nanoparticles prepared by Pechini solegel method, Ceram. Int. 42 (2016) 6136e6144. [15] R. Ahmad, J.-H. Ha, I.-H. Song, Enhancement of the compressive strength of highly porous Al2O3 foam through crack healing and improvement of the
146
M. Aghayan et al. / Materials Chemistry and Physics 192 (2017) 138e146
surface condition by dip-coating, Ceram. Int. 40 (2014) 3679e3685. [16] U. Holzwarth, N. Gibson, The Scherrer equation versus the ‘Debye-Scherrer equation’, Nat. Nanotechnol. 6 (2011) 534. [17] V.Y. Yablokov, I.L. Smel’tsova, I.A. Zelyaev, S.V. Mitrofanova, Studies of the rates of thermal decomposition of glycine, alanine, and serine, Russ. J. General Chem. 79 (2009) 1704e1706. [18] P. Erri, P. Pranda, A. Varma, OxidizerFuel interactions in aqueous combustion synthesis. 1. Iron(III) NitrateModel fuels, Ind. Eng. Chem. Res. 43 (2004) 3092e3096. [19] P.M. Schaber, J. Colson, S. Higgins, D. Thielen, B. Anspach, J. Brauer, Thermal
decomposition (pyrolysis) of urea in an open reaction vessel, Thermochim. Acta 424 (2004) 131e142. [20] L.H. Dalman, Ternary systems of urea and acids. I. Urea, nitric acid and water. II. Urea, sulfuric acid and water. III. Urea, oxalic acid and water, J. Am. Chem. Soc. 56 (1934) 549e553. [21] A.A. Shiryaev, Thermodynamics of SHS processes: advanced approach, Int. J. Self Propagating High Temp. Synth. 4 (4) (1995) 351e362. [22] A. Yabuki, S. Tanaka, Oxidation behavior of copper nanoparticles at low temperature, Mater. Res. Bull. 46 (12) (2011) 2323e2327.