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CuZnCo nanosized mixed oxides prepared from hydroxycarbonate precursors
Journal of Alloys and Compounds 688 (2016) 202e209
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CueZneCo nanosized mixed oxides prepared from hydroxycarbonate precursors Marilena Carbone Dept. Chemical Sciences and Technologies, University of Rome Tor Vergata, Via della Ricerca Scientifica, 1, 00133 Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2016 Received in revised form 12 July 2016 Accepted 17 July 2016 Available online 18 July 2016
Cu/Zn/Co mixed hydroxycarbonates with Cu/Zn atomic ratio ¼ 2.34 and Co atomic percentage variable between 3 and 20% were synthesized by coprecipitation at constant pH. The temperature of the solutions prior to the mixing (RT or 65 C) and the Co percentage determine the phases of the hydroxycarbonate samples. XRD pattern and IR spectra indicate a malachite-like structure for the samples with 3% and 8% Co. The sample with 14% Co is crystalline, only if prepared with solutions at 65 C, whereas the 20% Co samples are amorphous, regardless of the preparation temperature. Thermogravimetric analysis is performed to choose the calcination conditions. High surface area mixed oxides are obtained from the hydroxycarbonates by calcination at 350 C for 3 h. XRD and SEM analyses show that the malachite-like structured precursors yield nanosized mixed oxides with a dominant tenorite-like phase and the amorphous ones give rise to polydispersed or amorphous oxides. EDS maps indicate a homogeneous distribution of the elements in the oxides. © 2016 Elsevier B.V. All rights reserved.
Keywords: Mixed oxides Nanosized materials Preparation methods
1. Introduction The unique chemico-physical properties on nanosized oxides have attracted increasing interest in recent years [1]. Among them, nanoparticles of materials such as zinc, copper and cobalt oxides received much attention because of their potential applications in several fields, including chemical and bio-sensing, gas sensing, photocatalysis and photodetection [2e4]. Zinc oxide is an essential semiconductor with a bandgap energy of 3.37 eV and interesting optical and electrical properties [5,6]. It has great a potential in applications in solar cells, gas sensors, photocatalysis, photodetectors, photodiodes, optical modulator and transparent thin film transistors [7,8]. Copper oxide is an important inorganic semiconductor with the direct band-gap value of 1.85 eV [9,10], used for gas sensors, magnetic storage media, solar energy transformation, electronics, semiconductors, varistors, and catalysis and has been studies with respect to its applications as a photothermally active and photoconductive compound [11]. Cobalt oxides are technologically important materials. The spinel form Co3O4 has applications in lithium ion batteries, heterogeneous catalysts, gas sensing, ceramic pigments, and electrochemical devices [12e14]. CoO is efficient in the water splitting [15]. Mixed oxides often have
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performances better than the corresponding single oxides especially when used in heterogeneous catalysis [16,17] as well as photocatalysis [18,19]. Cu1-xZnxO oxides, for instance, have been the subject of extensive investigations, due to their enhanced performances in methanol synthesis [20] and alcohol steam reforming [21]. Mixed Cu/Co oxides play a role in several reactions, such as oxygen evolution reaction [22], CO and propene oxidation [23] and ammonium perchlorate decomposition [24]. Also mixed Zn/Co oxides are efficient catalysts, for instance, in N2O abatement [25]. More complex oxides are often employed in catalysis. Ternary oxides such as CueZneAl oxides are used in the water gas shift reaction [26] whereas CueZneGa oxides are employed in hydrogenolysis of glycerol. In both cases, Cu and Zn have an active catalytic role, the additional cation improving the catalytic performances by increasing the oxide surface area (Al3þ) [27] or by preventing sintering-related deactivation effects (Ga3þ) [28]. Ternary CueCoeAl oxides are also catalysts for the methanol synthesis [29], whereas quaternary mixed CueCoeZneAl oxides are employed for higher alcohols synthesis [30] and CueCoeZneCr oxides for hydrocarbon synthesis [31]. CueZneCo ternary oxides did not receive much attention so far, though there are premises to use them as catalysts. Furthermore, their synthesis and characterization may help the understanding of the chemico-physical properties of quaternary catalysts and pave the way to new applications. Furthermore, obtaining these materials in nanosized form allows
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the tuning of their physicochemical properties [32,33] and several synthesis techniques have been probed through the years to ensure the achievement of controlled, possibly monodispersed nanomaterials. The synthesis methods include solegel techniques [9,10,34e37], hydrothermal synthesis [38] microvawe assisted techniques [39] using liquid ammonia as solvent [40], as well as precipitation of hydroxycarbonate [41]. Among the methods of synthesis of mixed oxides [42,43] coprecipitation with NaHCO3, followed by calcination is a rather simple one [44,45], and therefore, it is highly desirable to use it for the achieving mixed nanosized materials. According to this synthesis pathway, the calcination temperature is also a way to control the size of the nanoparticles [46,47]. Here, the synthesis of CueZneCo ternary mixed oxides is reported by coprecipitation of hydrocarbonate precursors from the cations nitrate solutions and subsequent calcination. The cationic Cu/Zn atomic ratio was kept fixed to 2.34, i.e. a typical value for catalysts, and the Co content was varied between 3 and 20%. Temperature and pH during the precipitation phase were tuned to allow the homogeneity of the reaction mixture as much as possible. This implied keeping the pH constant at 8 and an ageing of 3 h. Furthermore, the precipitation was made by mixing cation nitrates and hydroxycarbonate solutions which were kept at the same temperature. This resulted in two preparation procedures: in the first one, the room temperature solutions are mixed and then heated up at 65 C, in the second one the solutions at heated up at 65 C and, then, mixed. This appeared to play a role on the crystallinity of the precursors at a critical Co content. The hydrocarbonate precursors were characterized by XRD, attenuated total reflectance FTIR (ATR-FTIR) and by TGA. The mixed oxides are characterized by XRD, SEM, EDS mapping and their surface area was determined by the BET method. 2. Synthesis methods Cu/ZnO compounds are catalysts for methanol synthesis [48], steam reforming [49] and water gas shift [50], their performances depending on their “chemical memory”, i.e. the way they are synthesized. This stimulated numerous studies on the synthesis conditions of hydroxycarbonates precursors and subsequent treatments. In early studies, it became soon evident that the base used (Na2CO3 or NaHCO3) for the synthesis and the order of reactants addition has a crucial role in determining the Cu/Zn precursor composition phase, the main difference being the pH during the reaction. A dropwise addition of a concentrated Na2CO3 solution to well stirred Cu, Zn nitrate solutions (Cu/Zn atomic ratio 2:1) occurs at various pH values, as reported by Porta et al. [51], whereas the addition of the mixed Cu, Zn nitrate solutions to a to a vigorously stirred concentrated NaHCO3 solution ensures a constant pH ¼ 8 [52]. According to the XRD analysis of the precursors, in the former case a mixture of hydroxynitrate and the hydroxycarbonate aurichalcite is obtained, in the latter the only hydroxycarbonates are obtained, i.e. zincian-malachite and aurichalcite. These two methods are generally identified as constant and decreasing pH synthesis. The phases of the precursors appear to be the outcome of a subtle balance between cationic atomic ratio and solution ageing. As evidenced by Bems et al. [53], the co-precipitation is a kineticcontrolled synthesis where cations nitrates of different acidity are mixed in the same aqueous solution. They performed a systematic study of hydroxycarbonates with Cu/Zn ratios varying from 100:0 to 0:100 in steps of 10 mol%, and hypothesized that the initial fractional precipitation of single metal hydroxides is followed by a series of transformations whose final outcome depends on the pH of the solution and on the time issued to the reactions to occur, i.e. the ageing. The different initial cations ratio determines the
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products, as it determines the ratio between “faster” and “slower” initial hydroxides precipitates, each of which has a final preferential product. Therefore, Cu rich solutions afford zincian-malachite [Cu2(1x)Zn2xCO3(OH)2, whereas aurichalcite [(Cu,Zn)5(OH)6(CO3)2], is also formed, when higher zinc concentrations are used. At a lower pH the formation of aurichalcite is favored, therefore, it is found in higher amount in the decreasing pH preparations. The aging time of the solution has been directly related to the catalytic properties of the final Cu/ZnO catalysts [54]. The kinetic control on the final hydroxycarbonates phases is exerted also via the rate of addition of the Cu/Zn nitrates solutions into the stirred NaHCO3 solution [55]. Finally, a way to control the pH, hence the precursors phases, is the mixing the cations nitrates and the NaHCO3 solution in stoichiometric ratio and adjustment of the pH by NaOH addition [56]. The synthesis at constant pH of Cu/Co hydroxycarbonates with varying atomic ratios in the range 100:0 to 0:100 affords compounds with a composition-dependent phase. Cu-rich hydroxycarbonates have a malachite-like strucucture (Cu:Co 0:100 and 67:33). Co-rich compounds crystallize in a sphaeorcobaltite (CoCO3) structure (Cu:Co, 0:100 and 15:85) [57]. The phase obtained for the intermediate atomic ratios Cu:Co 50:50 and 33:67 are, instead, isomorphous to a Co basic carbonate chloride [58]. In the current study, ternary Cu/Zn/Co hydroxycarbonates are synthesized by coprecipitation and, then, calcined. The aim is at obtaining as small and dispersed mixed oxides as possible. The Cu/ Zn atomic ratio was chosen as ¼ 2.34, since it is the one used for Cu/ ZnO catalysts and it also represents the limit for obtaining monophasic zincian-malachite. The Co atomic content is varied between 3 and 20. The preparation conditions are chosen to achieve as much as possible monophasic mixed hydroxycarbonate, i.e. the constant pH method was used and a long ageing of the precipitate in the mother solution was ensured. During a long ageing the pH decreases due to the decomposition of the excess of NaHCO3. Furthermore, Co2þ is more basic than Cu2þ and Zn2þ and may afford a fractional precipitation of CoCO3. Therefore, the pH was adjusted at regular intervals, by dropwise addition of HNO3. The temperature of the solutions during coprecipitation and ageing has been hardly an issue. Regardless of the order and rate of additions, ageing or pH of the solutions, synthesis are performed by mixing a room temperature solution to another one heated up at 65(±5) C and kept under vigorous stirring, However, this can be a crucial point for reactions which are supposed to be kinetically controlled. Therefore, in the current study, it was opted for the strategy of either mixing solutions kept at room temperature and, then, heating the mixture up at 65 C, or heating up both solutions prior to mix them. This may have a dramatic effect on the initial precipitation phase, and, as a consequence, on the final products. The two types of preparations are indicated as cold preparation (CP) or warm preparation (WP). The samples are, then, calcined to obtain the corresponding mixed oxides, the calcination temperature being chosen after performing a thermogravimetric analysis. This should ensure the conditions for achieving as small nanoparticles as possible. 2.1. Materials and hydroxcarbonates preparation Precursors were prepared with nominal Cu/Zn atomic ratio kept at 2.34 and Co atomic percentage at 3%, 8%, 14% or 20% (the complete list of values is reported in Table 1). The synthesis was achieved by coprecipitation from a nitrate solution of Cu, Zn and Co in suitable proportions and a solution of NaHCO3 in excess by 10%, according to the reaction 2M(NO3)2 þ 4NaHCO3 / M2CO3(OH)2 þ 4NaNO3 þ 3CO2. Both
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Table 1 Atomic ratios of the cations in the mixed oxides precursors: N¼Nominal value, E ¼ Experimental value. CP indicates the preparation at room temperature (Cold Preparation), WP, the preparation at 65 C (Warm Preparation). Sample
N% Cu
E% Cu
N% Zn
E% Zn
N% Co
E% Co
Co3 e CP Co8 e CP Co14 e CP Co14 e WP Co20 e CP Co20 e WP
68 64 60 60 56 56
67.8 64.1 60.2 60.3 55.9 55.7
29 28 26 26 24 24
29.2 27.8 25.7 25.7 24.0 24.1
3 8 14 14 20 20
3.0 8.1 14.1 14.0 20.1 20.2
solutions were kept at the same temperature before mixing them, i.e. either at room temperature or at 65 C. The initially blue solution was sealed in a vessel, while keeping a vigorous stirring and the temperature was raised to 65 C (or kept at 65 C, in case of preheated reagents). The pH was monitored every 20 min and adjusted by dropwise addition of HNO3 when exceeding 8. The reaction was carried on for three hours after the color of the solution turned from blue to pale green. The samples were, then, filtered and repeatedly washed with distilled water to remove the presence of Naþ and NO 3 . After washing, the sodium content was analyzed by atomic absorption and found to be less than 100 ppm. The atomic ratios of Cu, Zn and Co, obtained by elemental analyses are reported in Table 1. In the first set of experiments, all samples with different Co% were prepared by mixing the solutions at room temperature. The samples which appeared to be amorphous at the X-ray analysis, i.e. the ones with 14% or 20% Co content, were prepared again by mixing the solutions at 65 C. This ensured a higher degree of crystallinity to the 14% Co sample. In Scheme 1, the two routes of preparations are sketched. The blue background indicates a solution at room temperature, the reddish one, a solution at 65 C. The synthesized samples are labelled Co3, Co8, Co14 and Co20 according to the Co nominal atomic percentage, and CP or WP depending on the temperature of the solutions prior to the mixing. 2.2. Samples characterization The samples obtained according to the two synthetic routes were characterized by X-ray diffraction, infrared spectroscopy and thermogravimetry, to assess the phases of the coprecipitation products and the optimal decomposition temperature. Afterwards, they were calcined to obtain the corresponding oxides which were,
then, analyzed by XRD and Scanning Electron Microscopy, to determine the nanoparticles phase, shape and size. Furthermore, the surfaces areas were measured by nitrogen adsorption at liquid nitrogen temperature, after outgassing at 180 C for 1 h at a reduced of pressure of 104 mbar. The powder diffraction patterns were obtained with a Philips automated PW 1729 diffractometer. Scans were taken with a 2q step size of 0.01, using Cu Ka (nickel-filtered) radiation. Attenuated Total Reflectance Fourier Transformed Infrared spectra were recorded on a Shimadzu Prestige 21 spectrophotometer. Thermogravimetric determinations (TGA) were carried out under a flow of N2 (24 cm3 min-1) with a Cahn RG electrobalance, employing a silica sample-holder. The temperature was raised from room temperature up to 800 C at a heating rate of 5 K min1. The surface morphology of the oxides was determined with Zeiss Auriga Field Emission-Scanning Electron Microscope (SEM) instrument operating at 6e8 kV. The EDS maps were taken by coupling the Field Emission Scanning Electron Microscope (SUPRA™ 35, Carl Zeiss SMT, Oberkochen, Germany) with the Energy Dispersive Microanalysis (EDS/EDX, INCAx-sight, Model: 7426, Oxford Instruments, Abingdon, Oxfordshire, UK), operating at 20 KV. 3. Results and discussion 3.1. Cu/Zn/Co hydroxycarbonate precursors The degree of crystallinity and the crystallization phases of the precursors were determined by X-ray diffraction. The diffraction patterns are reported in Fig. 1a) through f) and the crystalline ones are identified by comparison with reference data of malachite [59], aurichalcite [60], hydrozincite [61] sphaerocobaltite [62], azurite [63], gherardtite [64] as well as literature data [65,66]. The samples with 3% and 8% Co are crystalline, the diffraction patterns having a rather high background, though. The peaks have been identified as belonging to the monoclinic malachite with some shifts, and indexed accordingly (the indexes of the main ones are reported in Fig. 1b). When Zn2þ substitutes Cu2þ into the malachite monoclinic lattice, the cell volume shrinks, in spite of the ionic similar radii. In a simplified picture, the d-spacing of (20-1) and (21-1) can be regarded as being correlated to the average JahneTeller-elongated CueO distances in the malachite structure. Zn2þ is not a Jahn-Teller ion and its substitution into the malachite structure causes all the reflexes to shift to higher values, though to a different extent, the most pronounced being the 20-1 and 21-1. It is evaluated that 28%
Scheme 1. Cold and Warm Preparations of the samples. a) CP: Metal nitrates and NaHCO3 solutions are mixed at room temperature and, then, the resulting solution is heated up at 65 C. b) WP: The metal nitrates and the NaHCO3 solutions are pre-heated at 65 C and, then, mixed. The preparation procedure affects the crystallinity of the samples.
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209
Fig. 1. XRD patterns of the Cu/Zn/Co hydroxycarbonates: a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display. Samples a), b) and d) show the typical peaks related to the monoclinic malachite structure.
Zn2þ can enter the malachite structure, by constant pH synthesis. A similar peaks shift is observed for the Co3 and Co8 samples, whose 20-1 and 21-1 reflexes are shifted towards higher values by 0.6 . Co2þ, at variance with Zn2þ, is Jahn-Teller ion and its substitution into the malachite structure causes less strain to the lattice, allowing a total larger amount of guest cations in the malachite structure. Nonetheless, samples with high Co content (14% and 20%), prepared by room temperature mixing are amorphous as shown in Fig. 1, panels c) and e). The cations atomic ratio, however, is not the only parameter determining the crystallinity of the samples: the synthesis conditions also play a role. In general, higher temperatures lower the nucleation rate by increasing the critical size of the nuclei [67,68], which grow into crystalline structures. Therefore, when prepared by mixing solutions at 65 C, the Co14 sample displays a rather crystalline structure, also identified as malachite-like. A higher temperature is not sufficient for the sample at 20% Co, which is amorphous also upon warm preparation. It must be noted that, a preparation of Cu/Zn/Co hydroxycarbonate precursors with similar Cu/Zn ratio and Co content in the 3e20% only yielded amorphous samples [69], the main differences in the synthesis procedure being the temperature of the solutions to be mixed and the pH throughout the reaction course. The IR spectra of the hydroxycarbonate precursors are reported in Fig. 2 in the region of 1800e400 cm1. The six panels correspond to the samples with different Co content and preparation methods. Spectra of the samples with Co content up to 14% are typical of a malachite-like structure, whereas the Co20 samples are significantly different from the others. The malachite-like samples have
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Fig. 2. Infrared spectra of the Cu/Zn/Co hydroxycarbonates a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display. The arrow between the two bars corresponds to 20% transmission.
four bands in the region of 1570e1390 cm1, which are attributed to the n3 mode (2Auþ2Bu, nAs CO2 3 ) of the carbonate ion, under correlation field splitting. The n1 mode (nS CO2 3 ) appears at 1051e1053 cm1 and the corresponding band overlaps with that of the OH librations. The comparatively strong and sharp band at 817 cm1 and a shoulder at 830 cm1 are assigned to n2 (out-of2 plane CO2 3 bending). Two n4 components (in-plane CO3 bending) appear between 744 and 748, and 704 and 709 cm1 depending on the sample. Compared to pure malachite, the inclusion of Zn2þ and Co2þ guest cations into the lattice leads to the shifting of the high frequency components of n3 to higher frequencies and the low frequency components to lower frequencies. The spectra of the two Co14 samples are similar, though the more amorphous one synthesized according to the cold pathway has more smeared features. The spectra of both Co20 samples are far less structured. Only three peaks are present in the 1800400 cm1 range, at 1465 cm1, 1382 cm1, and 832 cm1, and can be associated to the asymmetric stretching and out-of-plane CO2 3 bending. All spectra show the typical rather broad band of the nO-H stretching vibration at 3327 cm1. This allows an estimate of the ratio between the CO2 3 and OH in the various samples as proportional to the ratio between the intensities of the nO-H and n2 As CO3 stretching vibrations. Similar ratios are found in all samples, sug gesting that CO2 3 and OH moieties are nearly 1:2 also in the amorphous samples, as in malachite-like structures. The calcination temperature of the mixed hydroxycarbonates was chosen after a thermogravimetric analysis was performed. The decomposition curves are reported in Fig. 3a) through f). Here, the overall mass loss of all the samples is of ~30%, and corresponds to the release of one H2O and one CO2 molecule from a compound with general formula Cu2-x-yZnxCoyCO3(OH)2.
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In order to obtain the mixed oxides from the hydroxycarbonate precursors, 350 C was chosen as the minimum decomposition temperature, suitable for all the samples. The decomposition was carried on for 3 h. 3.2. Cu/Zn/Co mixed oxides The structural and morphological properties of the Cu/Zn/Co mixed oxides were determined by XRD, SEM and associated EDS mapping and by surface area measurements. The XRD patterns of the calcined samples are reported in Fig. 4a) through f) and are identified by comparison with reference data [70]. The peaks of the Co3 and Co8 oxides have been identified as belonging to a tenorite-like phase, with some shifts and indexed accordingly (Fig. 4a)). In particular, the reflex (110) and is shifted by 0.9 and the reflex (002) by 0.2 , indicating the incorporation of guest cations into the lattice. However, due to the high background of the diffractograms, contributions of small amounts of other phases, such as ZnO [71] cannot be excluded. Also the decomposition of the two Co14 precursors affords similar phases, which are assigned to a dominant tenorite-like phase. The peaks of the Co20 e CP and Co20 e WP mixed oxides XRD patterns are broad and the background noise rather intense. The pattern is compatible with a mixture of tenorite-like and zincitelike phases, though all rather amorphous. In all oxides no CoO nor Co3O4 spinel structures can be identified [72,73]. The SE micrographs of the oxides are reported in Fig. 5 and show a morphology dependency on both the Co content and the synthesis pathway. Co3 and Co8 oxides (panels a) and b)) are narrow sized nanoparticles with a regular shape and a diameter of Fig. 3. Thermogravimetric analysis the Cu/Zn/Co hydroxycarbonates: a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display. The arrow between the two bars corresponds to 20% weight loss.
The weight loss of the samples Co3 and Co8 occurs in one broad step in the temperature range 100e370 C (curves a) and b)). For pure malachite the thermogravimetric curve indicates a single narrow decomposition from a monophasic precursor in the temperature range 150 e320 C [65]. Mineral malachite decomposes in a narrower range at higher temperatures, i.e. 320e370 C [66]. The incorporation of zinc into the malachite has the effect of stabilizing the structure, as evidenced by the shift of the DTG peaks from about 250 to 350 C [53]. The decomposition reactions of zincianmalachite occur in the temperature range 100 e450 C and consist of several poorly resolved simultaneous dihydroxylation and decarbonation steps at 180, 260 and 355 C. In comparison, the zincian-cobalt-malachite of the Co3 and Co8 samples is less stable, being the decomposition complete at 370 C. The Co14-WP has a similar weight loss as Co3 and Co8, a broad decomposition in the 100e370 C temperature range (curve c)). The partially amorphous Co 14 e CP sample shows, in comparison, a steeper weight loss, which is complete already at ~320 C (curve d)). The decomposition of the Co20 amorphous samples (curves e) and f)) occurs in two steps of 15% weight loss each, a broader one, complete at 250 C and a steeper one complete at 340 C. The total weight loss, 30%, corresponds to the production of one H2O and one CO2 molecule, but there is no one-to-one correspondence between a single weight loss step and the production of one type of molecules, since this would give rise to a loss of 8% and 21%, respectively. Therefore, the two weight losses must correspond to the breakage of bonds of different strength between the cations and both the hydroxyls and the carbonates within the amorphous structure.
Fig. 4. XRD patterns of the Cu/Zn/Co mixed oxides: a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display.
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209
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Fig. 5. SEM images at magnification 500000 of Cu/Zn/Co mixed oxides a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP.
10e20 nm. The Co14 oxide obtained from the amorphous precursor is more agglomerated and the particles have a diameter distribution in a larger range, 10e35 nm (panel c)). The effects of the synthesis pathway on the morphology are observed on the Co14 oxide obtained from the crystalline precursor (panel d): at variance with the former case, the particles shape is regular and the diameter is 10e20 nm similarly to the low Co content oxides. The samples with 20% Co are markedly amorphous regardless of the preparation route (panels e) and f)). For a more careful evaluation of the nanoparticles size, de-agglomeration was performed by deep sonication of diluted ethanol dispersions of the oxides, prior to deposition on the sample-holder. This procedure worked fine for the nanosized oxides derived from the crystalline precursors. The more agglomerated Co14-CP as well as both Co-20 samples retain a large degree of agglomeration. Micrographs of the oxides at different degree of de-agglomeration and magnifications are reported in Table 1S, along with the size evaluation of the most dispersed nanoparticles. EDS images of the oxides are reported in Fig.1S of the Supplementary Material. They show that in all the oxides, Cu, Zn and Co are evenly distributed in the examined areas. This is compatible with the presence of solid solutions or formations of different phases in close contact with each other. Also in the amorphous samples, there is a homogenous distribution of elements. The surface area of the samples is rather constant, hence it does not depend on the preparation method and, as a consequence, on the morphology. For the narrow dispersed, polydispersed
Table 2 BET surface area of the Cu/Zn/Co mixed oxides. Mixed oxide
BET surface area (m2g1)
Co3 e CP Co8 e CP Co14 e CP Co14 e WP Co20 e CP Co20 e WP
50 52 48 51 52 53
nanoparticles and amorphous oxides, the measured surface area is in the order of 50 ± 3 m2g-1, as reported in Table 2. 4. Conclusions Large surface area Cu/Zn/Co mixed oxides with variable atomic ratios were prepared in nanosized form via hydroxycarbonate precursors. Morphology and size depend on the Co content and on the preparation procedure. The precursors were obtained by coprecipitation from metal nitrate in suitable proportions and NaHCO3 solutions. Co content and temperature of the solutions prior to the mixing, i.e. either both at room temperature or at 65 C, determine the phases of the synthesized hydroxycarbonates. Samples with low Co atomic percentage (8%) or 14% Co, prepared with solutions at 65 C have a malachite-like structure. Samples with 14% Co, prepared with solutions at RT, or with high Co content are quasi-amorphous or amorphous. The precursors display a different behavior with respect to the thermal decomposition: twostepped in the case of the amorphous samples, single-stepped for the crystalline or partially amorphous samples. After carrying on the calcination of the precursors at 350 C for 3 h mixed oxides were obtained. A dominant nanosized tenoritelike phase is obtained from crystalline precursors. Polydispersed or amorphous oxides are obtained from amorphous precursors. The element distribution is homogeneous in all the oxides. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.07.175. References [1] P. Ball, L. Garwin, Science at the atomic scale, Nature 355 (1992) 761e767. [2] X.F. Wang, O. Kitao, E. Hosono, H. Zhoua, S. Sasaki, H. Tamiaki, TiO2 and ZnO based solar cells using a chlorophyll a derivative sensitizer for light-harvesting and energy conversion, J. Photochem. Photobiol. A Chem. 210 (2010) 145e152. [3] R. Seema, S. Poonam, P.K. Shishodia, R.M. Mehra, Synthesis of nanocrystalline
208
[4] [5]
[6]
[7]
[8] [9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30]
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209 ZnO powder via solegel route for dye-sensitized solar cells, Sol. Energy Mat. Sol. Cells 92 (2008) 1639e1645. W.-Y. Li, L.-N. Xu, J. Chen, Co3O4 nanomaterials in lithium-ion batteries and gas sensors, Adv. Funct. Mater. 15 (2005) 851e857. Z. Zhang, J.B. Yi, J. Ding, L.M. Wong, H.L. Seng, S.J. Wang, J.G. Tao, G.P. Li, G.Z. Xing, T.C. Sum, C. Hon, A. Huan, T. Wu, Cu-Doped ZnO nanoneedles and nanonails: morphological evolution and physical properties, J. Phys. Chem. C 112 (2008) 9579e9585. S.S. Kim, J.H. Yum, Y.E. Sung, Flexible dye-sensitized solar cells using ZnO coated TiO2 nanoparticles, J. Photochem. Photobiol. A Chem. 171 (2005) 269e273. A. Al-Hajry, A. Umar, Y.B. Hahn, D.H. Kim, Growth properties ametnd dye sensitized solar cells d applications of ZnO nanorods grown by low temperature solution process, Superlattices Microstruct. 45 (2009) 529e534. Y.Y.I. Bu, Facile synthesis of highly oriented p-type aluminum Co-doped zinc oxide film with aqua ammonia, J. Alloys Compd. 509 (2011) 2874e2878. M.H. Habibi, B. Karimi, Preparation of nanostructure CuO/ZnO mixed oxide by solegel thermal decomposition of a CuCO3 and ZnCO3: TG, DTG, XRD, FESEM and DRS Investigations, J. Ind. Eng. Chem. 20 (2014) 925e929. M.H. Habibi, B. Karimi, Effect of the annealing temperature on crystalline phase of copper oxide nanoparticle by copper acetate precursor and solegel method, J. Therm. Anal. Calorim. 115 (2014) 419e423. C.H. Lo, T.T. Tsung, Fabrication and characterization of CuO nanorods by a submerged arc nanoparticle synthesis system, J. Vac. Sci. Technol. B 23 (2005) 2394e23976. T. Warang, N. Patel, A. Santini, N. Bazzanella, A. Kale, A. Miotello, Pulsed laser deposition of Co3O4 nanoparticles assembled coating: role of substrate temperature to tailor disordered to crystalline phase and related photocatalytic activity in degradation of methylene blue, Appl. Catal. A Gen. 423e424 (2012) 21e27. S.-L. Chou, J.-Z. Wang, H.-K. Liu, S.-X. Dou, Electrochemical deposition of porous Co3O4 nanostructured thin film for lithium-ion battery, J. Power Sources 182 (2008) 359e364. Y.G. Li, B. Tan, Y.Y. Wu, Mesoporous Co3O4 nanowire arrays for lithium ion batteries with high capacity and rate capacity, Nano Lett. 8 (2008) 265e270. L. Liao, Q. Zhang, Z. Su, Z. Zhao, Y. Wang, Y. Li, X. Lu, D. Wei, G. Feng, Q. Yu, X. Cai, J. Zhao, Z. Ren, H. Fang, F. Robles-Hernandez, S. Baldelli, J. Bao, Efficient solar water-splitting using a nanocrystalline CoO photocatalyst, Nat. Nanotechnol. 9 (2014) 69e73. C. Baltes, S. Vukojevi c, F. Schüth, Correlations between synthesis, precursor, and catalyst structure and activityof a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis, J. Catal. 258 (2008) 334e344. A.A. Khassina, H. Jobic, G.A. Filonenko, E.V. Dokuchits, A.V. Khasina, T.P. Minyukova, N.V. Shtertser, L.M. Plyasova, T.M. Yurieva, Interaction of hydrogen with CueZn mixed oxide model methanol synthesis Catalyst, J. Mol. Catal. A Chem. 373 (2013) 151e160. M.H. Habibi, B. Karimi, Nanostructure CueZn mixed-oxide supported photocatalyst fabricated by impregnation method for the photocatalytic degradation of C.I. Reactive Orange 16 (V3R) in water, Spectrochim. Acta A 124 (2014) 629e631. M.B. Gawande, R.K. Pandey, R.V. Jayaram, Role of mixed metal oxides in catalysis sciencedversatile applications in organic synthesis, Catal. Sci. Technol. 2 (2012) 1113e1125. S. Zander, E.L. Kunkes, M.E. Schuster, J. Schumann, G. Weinberg, D. Teschner, €gl, M. Behrens, The role of the oxide component in the N. Jacobsen, R. Schlo development of copper composite catalysts for methanol synthesis, Angew. Chem. Int. Ed. 52 (25) (2013) 6536e6540. B. Lorenzut, T. Montini, L. De Rogatis, P. Canton, A. Benedetti, P. Fornasiero, Hydrogen production through alsohol steam reforming on Cu/ZnO based catalysts, Appl. Catal. B Environ. 101 (2011) 397e408. B. Chi, H. Lin, J. Li, Cations distribution of CuxCo3LxO4 and its electrocatalytic activities for oxygen evolution reaction, Int. J. Hydrog. Energy 33 (2008) 4763e4768. Z.Y. Tian, H. Vieker, P. Mountapmbeme Kouotoub, A. Beyer, In situ characterization of CueCo oxides for catalytic application, Faraday Discuss. 177 (2015) 249e262. A.A. Said, R. A1-Qasmi, The role of copper cobaltite spinel, CuxCo3-xO4 during the thermal decomposition of ammonium perchlorate, Thermochim. Acta 275 (1996) 83e91. L. Yan, T. Ren, X. Wang, Q. Gao, D. Ji, J. Suo, Excellent catalytic performance of ZnxCo1-xCo2O4 spinel catalysts for the decomposition of nitrous oxide, Catal. Commun. 4 (2003) 505e509. s, N. Amadeo, M. Laborde, C.R. Apesteguía, Activity and structureM.J.L. Gine sensitivity of the water-gas shift reaction over Cu-Zn-A1 mixed oxide catalysts, Appl. Catal. A Gen. 131 (1995) 283e296. €gl, B.L. Kniep, T. Ressler, A. Rabis, F. Girgsdies, M. Baenitz, F. Steglich, R. Schlo Rational design of nanostructured copperezinc oxide catalysts for the steam reforming of methanol, Angew. Chem. Int. Ed. 43 (2004) 112e115. A. Bienholz, R. Blume, A. Knop-Gericke, F. Girgsdies, M. Behrens, P. Claus, Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga2O3-modified copper/zinc oxide catalysts, J. Phys. Chem. 115 (2011) 999e1005. J.I. Di Cosimo, A.J. Marchi, C.R. Apesteguia, Preparation of ternary Cu/Co/Al catalyst by amorphous citrate process, J. Catal. 134 (1992) 594e607. J.E. Baker, R. Burch, S.E. Golunski, Synthesis of higher alcohols over copper/
[31] [32]
[33]
[34]
[35]
[36]
[37]
[38]
[39] [40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
cobalt catalysts influence of preparative procedures on the activity and selectivity of Cu/Co/Zn/Al mixed oxide catalysts, Appl. Catal. 53 (1989) 279e297. Y. Souma, H. Ando, M. Fujuwara, R. Kieffer, Catalytic hydrogenation of carbon dioxide to hydrocarbons, Energy Convers. Mat. 36 (1995) 593e596. X.Z. Lin, P. Liu, J.M. Yu, G.W. Yang, Synthesis of CuO nanocrystals and sequential assembly of nanostructures with shape-dependent optical absorption upon laser ablation in liquid, J. Phys. Chem. C 113 (2009) 17543e17547. S. Farhadi, J. Safabakhsh, P. Zaringhadam, Synthesis, characterization, and investigation of optical and magnetic properties of cobalt oxide (Co3O4) nanoparticles, J. Nanostr. Chem. 3 (69) (2013) 1e9. D.G. Lamas, G.E. Lascalea, N.E. Walsoe de Reca, Synthesis and characeterization of nanocrustalline powders of partially stabilized zirconia ceramics, J. Eur. Ceram. Soc. 18 (1998) 1217e1221. R.F. Juarez, D.G. Lamas, G.F. Lascalea, N.F. Walsoe de Reca, Synthesis of nanocrystalline zirconia powders for TZP ceramics by a nitrateecitrate combustion route, J. Eur. Ceram. Soc. 20 (2000) 133e138. J. Chen, J. Tang, F. Yan, H. Ju, A gold nanoparticles/solegel composite architecture for encapsulation of immunoconjugate for reagentless electrochemical immunoassay, Biomaterials 27 (2006) 2313e2321. M. Luo, W. Tang, J. Zhao, C. Pu, Hydrophilic modification of poly(ether sulfone) used TiO2 nanoparticles by a solegel process, J. Mater. Process. Technol. 172 (2006) 431e436. E. Lester, G. Aksomaityte, J. Li, S. Gomez, J. Gonzalez-Gonzalez, M. Poliakoff, Controlled continuous hydrothermal synthesis of cobalt oxide (Co3O4) nanoparticles, Prog. Cryst. Growth Charact. Mater. 58 (2012) 3e13. L.-H. Ai, J. Jiang, Rapid synthesis of nanocrystalline Co3O4 by a microwave assisted combustion method, Powder Tech. 195 (2009) 11e14. L. Sun, Z. Zhang, Z. Wang, Z. Wu, H. Dang, Synthesis and characterization of CuO nanoparticles from liquid ammonia, Mater. Res. Bull. 40 (2005) 1024e1027. M. Farahmandjou, M. Zarinkamar, Synthesis of nano-d ceria (CeO2) particles via a cerium hydroxy carbonate precursor and the effect of reaction temperature on particle morphology, JUFNGSM 48 (2015) 5e10. A. Khaleel, I. Shehadi, M. Al-Shamisi, Nanostructured chromiumeiron mixed oxides: physicochemical properties and catalytic activity, Colloids Surf. A Physicochem. Eng. Asp. 355 (2010) 75e82. A. Cetin, R. Kibar, M. Ayvacikli, N. Can, Ch. Buchal, P.D. Townsend, A.L. Stepanov, T. Karali, S. Selvi, Optical properties of Cu implanted ZnO, Nucl. Instrum. Methods Phys. Res. B 249 (2006) 474. G. Avgouropoulos, T. Ioannides, H.K. Matralis, J. Batista, S. Hocevar, CuOeCeO2 mixed oxide catalysts for the selective oxidation of carbon monoxide in excess hydrogen, Catal. Lett. 73 (2001) 33e40. C. Ngamcharussrivichai, P. Totarat, K. Bunyakiat, Ca and Zn mixed oxide as a heterogeneous base catalyst for transesterification of palm kernel oil, Appl. Cat. A Gen. 341 (2008) 77e85. A. Gaber, M.A. Abdel-Rahim, A.Y. Abdel-Latief, M.N. Abdel-Salam, Influence of calcination temperature on the structure and porosity of nanocrystalline SnO2 synthesized by a conventional precipitation method, Int. J. Elecrochem. Sci. 9 (2014) 81e95. S. Satapathy, A. Ahlawat, A. Paliwal, R. Singh, M.K. Singh, P.K. Gupta, Effect of calcination temperature on nanoparticle morphology and its consequence on optical properties of Nd:Y2O3 transparent ceramics, Cryst. Eng. Comm. 16 (2014) 723e2731. M. Saito, T. Fujitani, M. Takeuchi, T. Watanabe, Development of copper/zinc oxide-based multicomponent catalysts for methanol synthesis from carbon dioxide and hydrogen, Appl. Cat. A Gen. 138 (1996) 311e318. M.M. Günter, T. Ressler, R.E. Jentoft, B. Bems, Redox behavior of copper oxide/ zinc oxide catalysts in the steam reforming of methanol studied by in situ xray diffraction and absorption spectroscopy, J. Catal. 203 (2001) 133e149. N. Thouchprasitchai, A. Luengnaruemitchai, S. Pongstabodee, Water-gas shift reaction over CueZn, CueFe, and CueZneFe composite-oxide catalysts prepared by urea-nitrate combustion, J. Ind. Eng. Chem. 19 (2013) 1483e1492. P. Porta, S. De Rossi, G. Ferraris, F. Pompa, Characterization of copper-zinc mixed oxide system in relation to different precursor structure and morphology, Solid State Ion. 45 (1991) 35e41. , A. Vaccari, Catalysts for lowC. Busetto, G. Del Piero, G. Manara, F. Trifiro temperature methanol synthesis. Preparation of Cu-Zn-Al mixed oxides via hydrotalcite-like precursors, J. Catal. 85 (1984) 260e266. €gl, Relations B. Bems, M. Schur, A. Dassenoy, H. Junkes, D. Herein, R. Schlo between synthesis and microstructural properties of copper/zinc hydroxycarbonates, Chem. Eur. J. 9 (2003) 2039e2052. S.H. Taylor, Graham J. Hutchings, A.A. Mirzaei, The preparation and activity of copper zinc oxide catalysts for ambient temperature carbon monoxide oxidation, Catal. Today 84 (2003) 113e119. G.C. Shen, S.-I. Fujita, N. Takezawa, Preparation of precursors for the Cu/ZnO methanol synthesis catalysts by coprecipitation methods: effects of the preparation conditions upon the structures of the precursors, J. Catal. 138 (1992) 754e758. S. Morpurgo, M. Lo Jacono, P. Porta, CoppereZinceCobalteAluminiumeChromium hydroxycarbonates and mixed oxides, J. Solid State Chem. 122 (1996) 324e332. P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lo Jacono, G. Moretti, Copperecobalt hydroxysalts and oxysalts: bulk and surface characterization,
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209 J. Mater. Chem. 1 (1991) 531e537. [58] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lo Jacono, G. Moretti, Preparation and Characterization of Cobalt-Copper Hydroxysalts and their oxide products of decomposition, J. Chem. Soc. Faraday Trans. 88 (1992) 311e319. [59] JCPDS-ICDD 41-1390. [60] JCPDS-ICDD 38-154. [61] JCPDS-ICDD 72-1100. [62] JCPDS-ICDD 10-240. [63] JCPDS-ICDD 72-539. [64] JCPDS-ICDD 75e1779. [65] P. Porta, S. De Rossi, G. Ferraris, M. Lo Jacono, G. Minelli, G. Moretti, Structural characterization of malachite-like coprecipitated precursors of binary CuOZnO catalysts, J. Catal. 109 (1988) 367e377. €gl, Minerals as model com[66] M. Behrens, F. Girgsdies, A. Trunschke, R. Schlo pounds for Cu/ZnO catalyst precursors: structural and thermal properties and
[67]
[68] [69]
[70] [71] [72] [73]
209
IR spectra of mineral and synthetic (zincian) malachite, rosasite and aurichalcite and a catalyst precursor mixture, Eur. J. Inorg. Chem. 10 (2009) 1347e1357. €zinger, P.G. Menon, B. Delmon, Solid-state reactions, in: G. Ertl, H. Kno J. Weitkamp (Eds.), Preparation of Solid Catalysts, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999. F. Schüth, M. Hesse, K.K. Unger, Precipitation and Coprecipitation, in: Handbook of Heterogeneous Catalysis, vols. 2:2.3:2.3.3, 2008, pp. 100e119. M. Carbone, C. Sadun, R. Caminiti, Structural study by energy dispersive X-ray diffraction of amorphous mixed hydroxycarbonates containing Co, Cu, Zn, A1, J. Mater Chem. 6 (10) (1996) 1709e1716. JCPDS-ICDD 41-254. JCPDS-ICDD 36-1451. JCPDS-ICDD 42-1467. JCPDS-ICDD 78-0431.
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CueZneCo nanosized mixed oxides prepared from hydroxycarbonate precursors Marilena Carbone Dept. Chemical Sciences and Technologies, University of Rome Tor Vergata, Via della Ricerca Scientifica, 1, 00133 Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2016 Received in revised form 12 July 2016 Accepted 17 July 2016 Available online 18 July 2016
Cu/Zn/Co mixed hydroxycarbonates with Cu/Zn atomic ratio ¼ 2.34 and Co atomic percentage variable between 3 and 20% were synthesized by coprecipitation at constant pH. The temperature of the solutions prior to the mixing (RT or 65 C) and the Co percentage determine the phases of the hydroxycarbonate samples. XRD pattern and IR spectra indicate a malachite-like structure for the samples with 3% and 8% Co. The sample with 14% Co is crystalline, only if prepared with solutions at 65 C, whereas the 20% Co samples are amorphous, regardless of the preparation temperature. Thermogravimetric analysis is performed to choose the calcination conditions. High surface area mixed oxides are obtained from the hydroxycarbonates by calcination at 350 C for 3 h. XRD and SEM analyses show that the malachite-like structured precursors yield nanosized mixed oxides with a dominant tenorite-like phase and the amorphous ones give rise to polydispersed or amorphous oxides. EDS maps indicate a homogeneous distribution of the elements in the oxides. © 2016 Elsevier B.V. All rights reserved.
Keywords: Mixed oxides Nanosized materials Preparation methods
1. Introduction The unique chemico-physical properties on nanosized oxides have attracted increasing interest in recent years [1]. Among them, nanoparticles of materials such as zinc, copper and cobalt oxides received much attention because of their potential applications in several fields, including chemical and bio-sensing, gas sensing, photocatalysis and photodetection [2e4]. Zinc oxide is an essential semiconductor with a bandgap energy of 3.37 eV and interesting optical and electrical properties [5,6]. It has great a potential in applications in solar cells, gas sensors, photocatalysis, photodetectors, photodiodes, optical modulator and transparent thin film transistors [7,8]. Copper oxide is an important inorganic semiconductor with the direct band-gap value of 1.85 eV [9,10], used for gas sensors, magnetic storage media, solar energy transformation, electronics, semiconductors, varistors, and catalysis and has been studies with respect to its applications as a photothermally active and photoconductive compound [11]. Cobalt oxides are technologically important materials. The spinel form Co3O4 has applications in lithium ion batteries, heterogeneous catalysts, gas sensing, ceramic pigments, and electrochemical devices [12e14]. CoO is efficient in the water splitting [15]. Mixed oxides often have
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performances better than the corresponding single oxides especially when used in heterogeneous catalysis [16,17] as well as photocatalysis [18,19]. Cu1-xZnxO oxides, for instance, have been the subject of extensive investigations, due to their enhanced performances in methanol synthesis [20] and alcohol steam reforming [21]. Mixed Cu/Co oxides play a role in several reactions, such as oxygen evolution reaction [22], CO and propene oxidation [23] and ammonium perchlorate decomposition [24]. Also mixed Zn/Co oxides are efficient catalysts, for instance, in N2O abatement [25]. More complex oxides are often employed in catalysis. Ternary oxides such as CueZneAl oxides are used in the water gas shift reaction [26] whereas CueZneGa oxides are employed in hydrogenolysis of glycerol. In both cases, Cu and Zn have an active catalytic role, the additional cation improving the catalytic performances by increasing the oxide surface area (Al3þ) [27] or by preventing sintering-related deactivation effects (Ga3þ) [28]. Ternary CueCoeAl oxides are also catalysts for the methanol synthesis [29], whereas quaternary mixed CueCoeZneAl oxides are employed for higher alcohols synthesis [30] and CueCoeZneCr oxides for hydrocarbon synthesis [31]. CueZneCo ternary oxides did not receive much attention so far, though there are premises to use them as catalysts. Furthermore, their synthesis and characterization may help the understanding of the chemico-physical properties of quaternary catalysts and pave the way to new applications. Furthermore, obtaining these materials in nanosized form allows
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209
the tuning of their physicochemical properties [32,33] and several synthesis techniques have been probed through the years to ensure the achievement of controlled, possibly monodispersed nanomaterials. The synthesis methods include solegel techniques [9,10,34e37], hydrothermal synthesis [38] microvawe assisted techniques [39] using liquid ammonia as solvent [40], as well as precipitation of hydroxycarbonate [41]. Among the methods of synthesis of mixed oxides [42,43] coprecipitation with NaHCO3, followed by calcination is a rather simple one [44,45], and therefore, it is highly desirable to use it for the achieving mixed nanosized materials. According to this synthesis pathway, the calcination temperature is also a way to control the size of the nanoparticles [46,47]. Here, the synthesis of CueZneCo ternary mixed oxides is reported by coprecipitation of hydrocarbonate precursors from the cations nitrate solutions and subsequent calcination. The cationic Cu/Zn atomic ratio was kept fixed to 2.34, i.e. a typical value for catalysts, and the Co content was varied between 3 and 20%. Temperature and pH during the precipitation phase were tuned to allow the homogeneity of the reaction mixture as much as possible. This implied keeping the pH constant at 8 and an ageing of 3 h. Furthermore, the precipitation was made by mixing cation nitrates and hydroxycarbonate solutions which were kept at the same temperature. This resulted in two preparation procedures: in the first one, the room temperature solutions are mixed and then heated up at 65 C, in the second one the solutions at heated up at 65 C and, then, mixed. This appeared to play a role on the crystallinity of the precursors at a critical Co content. The hydrocarbonate precursors were characterized by XRD, attenuated total reflectance FTIR (ATR-FTIR) and by TGA. The mixed oxides are characterized by XRD, SEM, EDS mapping and their surface area was determined by the BET method. 2. Synthesis methods Cu/ZnO compounds are catalysts for methanol synthesis [48], steam reforming [49] and water gas shift [50], their performances depending on their “chemical memory”, i.e. the way they are synthesized. This stimulated numerous studies on the synthesis conditions of hydroxycarbonates precursors and subsequent treatments. In early studies, it became soon evident that the base used (Na2CO3 or NaHCO3) for the synthesis and the order of reactants addition has a crucial role in determining the Cu/Zn precursor composition phase, the main difference being the pH during the reaction. A dropwise addition of a concentrated Na2CO3 solution to well stirred Cu, Zn nitrate solutions (Cu/Zn atomic ratio 2:1) occurs at various pH values, as reported by Porta et al. [51], whereas the addition of the mixed Cu, Zn nitrate solutions to a to a vigorously stirred concentrated NaHCO3 solution ensures a constant pH ¼ 8 [52]. According to the XRD analysis of the precursors, in the former case a mixture of hydroxynitrate and the hydroxycarbonate aurichalcite is obtained, in the latter the only hydroxycarbonates are obtained, i.e. zincian-malachite and aurichalcite. These two methods are generally identified as constant and decreasing pH synthesis. The phases of the precursors appear to be the outcome of a subtle balance between cationic atomic ratio and solution ageing. As evidenced by Bems et al. [53], the co-precipitation is a kineticcontrolled synthesis where cations nitrates of different acidity are mixed in the same aqueous solution. They performed a systematic study of hydroxycarbonates with Cu/Zn ratios varying from 100:0 to 0:100 in steps of 10 mol%, and hypothesized that the initial fractional precipitation of single metal hydroxides is followed by a series of transformations whose final outcome depends on the pH of the solution and on the time issued to the reactions to occur, i.e. the ageing. The different initial cations ratio determines the
203
products, as it determines the ratio between “faster” and “slower” initial hydroxides precipitates, each of which has a final preferential product. Therefore, Cu rich solutions afford zincian-malachite [Cu2(1x)Zn2xCO3(OH)2, whereas aurichalcite [(Cu,Zn)5(OH)6(CO3)2], is also formed, when higher zinc concentrations are used. At a lower pH the formation of aurichalcite is favored, therefore, it is found in higher amount in the decreasing pH preparations. The aging time of the solution has been directly related to the catalytic properties of the final Cu/ZnO catalysts [54]. The kinetic control on the final hydroxycarbonates phases is exerted also via the rate of addition of the Cu/Zn nitrates solutions into the stirred NaHCO3 solution [55]. Finally, a way to control the pH, hence the precursors phases, is the mixing the cations nitrates and the NaHCO3 solution in stoichiometric ratio and adjustment of the pH by NaOH addition [56]. The synthesis at constant pH of Cu/Co hydroxycarbonates with varying atomic ratios in the range 100:0 to 0:100 affords compounds with a composition-dependent phase. Cu-rich hydroxycarbonates have a malachite-like strucucture (Cu:Co 0:100 and 67:33). Co-rich compounds crystallize in a sphaeorcobaltite (CoCO3) structure (Cu:Co, 0:100 and 15:85) [57]. The phase obtained for the intermediate atomic ratios Cu:Co 50:50 and 33:67 are, instead, isomorphous to a Co basic carbonate chloride [58]. In the current study, ternary Cu/Zn/Co hydroxycarbonates are synthesized by coprecipitation and, then, calcined. The aim is at obtaining as small and dispersed mixed oxides as possible. The Cu/ Zn atomic ratio was chosen as ¼ 2.34, since it is the one used for Cu/ ZnO catalysts and it also represents the limit for obtaining monophasic zincian-malachite. The Co atomic content is varied between 3 and 20. The preparation conditions are chosen to achieve as much as possible monophasic mixed hydroxycarbonate, i.e. the constant pH method was used and a long ageing of the precipitate in the mother solution was ensured. During a long ageing the pH decreases due to the decomposition of the excess of NaHCO3. Furthermore, Co2þ is more basic than Cu2þ and Zn2þ and may afford a fractional precipitation of CoCO3. Therefore, the pH was adjusted at regular intervals, by dropwise addition of HNO3. The temperature of the solutions during coprecipitation and ageing has been hardly an issue. Regardless of the order and rate of additions, ageing or pH of the solutions, synthesis are performed by mixing a room temperature solution to another one heated up at 65(±5) C and kept under vigorous stirring, However, this can be a crucial point for reactions which are supposed to be kinetically controlled. Therefore, in the current study, it was opted for the strategy of either mixing solutions kept at room temperature and, then, heating the mixture up at 65 C, or heating up both solutions prior to mix them. This may have a dramatic effect on the initial precipitation phase, and, as a consequence, on the final products. The two types of preparations are indicated as cold preparation (CP) or warm preparation (WP). The samples are, then, calcined to obtain the corresponding mixed oxides, the calcination temperature being chosen after performing a thermogravimetric analysis. This should ensure the conditions for achieving as small nanoparticles as possible. 2.1. Materials and hydroxcarbonates preparation Precursors were prepared with nominal Cu/Zn atomic ratio kept at 2.34 and Co atomic percentage at 3%, 8%, 14% or 20% (the complete list of values is reported in Table 1). The synthesis was achieved by coprecipitation from a nitrate solution of Cu, Zn and Co in suitable proportions and a solution of NaHCO3 in excess by 10%, according to the reaction 2M(NO3)2 þ 4NaHCO3 / M2CO3(OH)2 þ 4NaNO3 þ 3CO2. Both
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M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209
Table 1 Atomic ratios of the cations in the mixed oxides precursors: N¼Nominal value, E ¼ Experimental value. CP indicates the preparation at room temperature (Cold Preparation), WP, the preparation at 65 C (Warm Preparation). Sample
N% Cu
E% Cu
N% Zn
E% Zn
N% Co
E% Co
Co3 e CP Co8 e CP Co14 e CP Co14 e WP Co20 e CP Co20 e WP
68 64 60 60 56 56
67.8 64.1 60.2 60.3 55.9 55.7
29 28 26 26 24 24
29.2 27.8 25.7 25.7 24.0 24.1
3 8 14 14 20 20
3.0 8.1 14.1 14.0 20.1 20.2
solutions were kept at the same temperature before mixing them, i.e. either at room temperature or at 65 C. The initially blue solution was sealed in a vessel, while keeping a vigorous stirring and the temperature was raised to 65 C (or kept at 65 C, in case of preheated reagents). The pH was monitored every 20 min and adjusted by dropwise addition of HNO3 when exceeding 8. The reaction was carried on for three hours after the color of the solution turned from blue to pale green. The samples were, then, filtered and repeatedly washed with distilled water to remove the presence of Naþ and NO 3 . After washing, the sodium content was analyzed by atomic absorption and found to be less than 100 ppm. The atomic ratios of Cu, Zn and Co, obtained by elemental analyses are reported in Table 1. In the first set of experiments, all samples with different Co% were prepared by mixing the solutions at room temperature. The samples which appeared to be amorphous at the X-ray analysis, i.e. the ones with 14% or 20% Co content, were prepared again by mixing the solutions at 65 C. This ensured a higher degree of crystallinity to the 14% Co sample. In Scheme 1, the two routes of preparations are sketched. The blue background indicates a solution at room temperature, the reddish one, a solution at 65 C. The synthesized samples are labelled Co3, Co8, Co14 and Co20 according to the Co nominal atomic percentage, and CP or WP depending on the temperature of the solutions prior to the mixing. 2.2. Samples characterization The samples obtained according to the two synthetic routes were characterized by X-ray diffraction, infrared spectroscopy and thermogravimetry, to assess the phases of the coprecipitation products and the optimal decomposition temperature. Afterwards, they were calcined to obtain the corresponding oxides which were,
then, analyzed by XRD and Scanning Electron Microscopy, to determine the nanoparticles phase, shape and size. Furthermore, the surfaces areas were measured by nitrogen adsorption at liquid nitrogen temperature, after outgassing at 180 C for 1 h at a reduced of pressure of 104 mbar. The powder diffraction patterns were obtained with a Philips automated PW 1729 diffractometer. Scans were taken with a 2q step size of 0.01, using Cu Ka (nickel-filtered) radiation. Attenuated Total Reflectance Fourier Transformed Infrared spectra were recorded on a Shimadzu Prestige 21 spectrophotometer. Thermogravimetric determinations (TGA) were carried out under a flow of N2 (24 cm3 min-1) with a Cahn RG electrobalance, employing a silica sample-holder. The temperature was raised from room temperature up to 800 C at a heating rate of 5 K min1. The surface morphology of the oxides was determined with Zeiss Auriga Field Emission-Scanning Electron Microscope (SEM) instrument operating at 6e8 kV. The EDS maps were taken by coupling the Field Emission Scanning Electron Microscope (SUPRA™ 35, Carl Zeiss SMT, Oberkochen, Germany) with the Energy Dispersive Microanalysis (EDS/EDX, INCAx-sight, Model: 7426, Oxford Instruments, Abingdon, Oxfordshire, UK), operating at 20 KV. 3. Results and discussion 3.1. Cu/Zn/Co hydroxycarbonate precursors The degree of crystallinity and the crystallization phases of the precursors were determined by X-ray diffraction. The diffraction patterns are reported in Fig. 1a) through f) and the crystalline ones are identified by comparison with reference data of malachite [59], aurichalcite [60], hydrozincite [61] sphaerocobaltite [62], azurite [63], gherardtite [64] as well as literature data [65,66]. The samples with 3% and 8% Co are crystalline, the diffraction patterns having a rather high background, though. The peaks have been identified as belonging to the monoclinic malachite with some shifts, and indexed accordingly (the indexes of the main ones are reported in Fig. 1b). When Zn2þ substitutes Cu2þ into the malachite monoclinic lattice, the cell volume shrinks, in spite of the ionic similar radii. In a simplified picture, the d-spacing of (20-1) and (21-1) can be regarded as being correlated to the average JahneTeller-elongated CueO distances in the malachite structure. Zn2þ is not a Jahn-Teller ion and its substitution into the malachite structure causes all the reflexes to shift to higher values, though to a different extent, the most pronounced being the 20-1 and 21-1. It is evaluated that 28%
Scheme 1. Cold and Warm Preparations of the samples. a) CP: Metal nitrates and NaHCO3 solutions are mixed at room temperature and, then, the resulting solution is heated up at 65 C. b) WP: The metal nitrates and the NaHCO3 solutions are pre-heated at 65 C and, then, mixed. The preparation procedure affects the crystallinity of the samples.
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209
Fig. 1. XRD patterns of the Cu/Zn/Co hydroxycarbonates: a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display. Samples a), b) and d) show the typical peaks related to the monoclinic malachite structure.
Zn2þ can enter the malachite structure, by constant pH synthesis. A similar peaks shift is observed for the Co3 and Co8 samples, whose 20-1 and 21-1 reflexes are shifted towards higher values by 0.6 . Co2þ, at variance with Zn2þ, is Jahn-Teller ion and its substitution into the malachite structure causes less strain to the lattice, allowing a total larger amount of guest cations in the malachite structure. Nonetheless, samples with high Co content (14% and 20%), prepared by room temperature mixing are amorphous as shown in Fig. 1, panels c) and e). The cations atomic ratio, however, is not the only parameter determining the crystallinity of the samples: the synthesis conditions also play a role. In general, higher temperatures lower the nucleation rate by increasing the critical size of the nuclei [67,68], which grow into crystalline structures. Therefore, when prepared by mixing solutions at 65 C, the Co14 sample displays a rather crystalline structure, also identified as malachite-like. A higher temperature is not sufficient for the sample at 20% Co, which is amorphous also upon warm preparation. It must be noted that, a preparation of Cu/Zn/Co hydroxycarbonate precursors with similar Cu/Zn ratio and Co content in the 3e20% only yielded amorphous samples [69], the main differences in the synthesis procedure being the temperature of the solutions to be mixed and the pH throughout the reaction course. The IR spectra of the hydroxycarbonate precursors are reported in Fig. 2 in the region of 1800e400 cm1. The six panels correspond to the samples with different Co content and preparation methods. Spectra of the samples with Co content up to 14% are typical of a malachite-like structure, whereas the Co20 samples are significantly different from the others. The malachite-like samples have
205
Fig. 2. Infrared spectra of the Cu/Zn/Co hydroxycarbonates a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display. The arrow between the two bars corresponds to 20% transmission.
four bands in the region of 1570e1390 cm1, which are attributed to the n3 mode (2Auþ2Bu, nAs CO2 3 ) of the carbonate ion, under correlation field splitting. The n1 mode (nS CO2 3 ) appears at 1051e1053 cm1 and the corresponding band overlaps with that of the OH librations. The comparatively strong and sharp band at 817 cm1 and a shoulder at 830 cm1 are assigned to n2 (out-of2 plane CO2 3 bending). Two n4 components (in-plane CO3 bending) appear between 744 and 748, and 704 and 709 cm1 depending on the sample. Compared to pure malachite, the inclusion of Zn2þ and Co2þ guest cations into the lattice leads to the shifting of the high frequency components of n3 to higher frequencies and the low frequency components to lower frequencies. The spectra of the two Co14 samples are similar, though the more amorphous one synthesized according to the cold pathway has more smeared features. The spectra of both Co20 samples are far less structured. Only three peaks are present in the 1800400 cm1 range, at 1465 cm1, 1382 cm1, and 832 cm1, and can be associated to the asymmetric stretching and out-of-plane CO2 3 bending. All spectra show the typical rather broad band of the nO-H stretching vibration at 3327 cm1. This allows an estimate of the ratio between the CO2 3 and OH in the various samples as proportional to the ratio between the intensities of the nO-H and n2 As CO3 stretching vibrations. Similar ratios are found in all samples, sug gesting that CO2 3 and OH moieties are nearly 1:2 also in the amorphous samples, as in malachite-like structures. The calcination temperature of the mixed hydroxycarbonates was chosen after a thermogravimetric analysis was performed. The decomposition curves are reported in Fig. 3a) through f). Here, the overall mass loss of all the samples is of ~30%, and corresponds to the release of one H2O and one CO2 molecule from a compound with general formula Cu2-x-yZnxCoyCO3(OH)2.
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In order to obtain the mixed oxides from the hydroxycarbonate precursors, 350 C was chosen as the minimum decomposition temperature, suitable for all the samples. The decomposition was carried on for 3 h. 3.2. Cu/Zn/Co mixed oxides The structural and morphological properties of the Cu/Zn/Co mixed oxides were determined by XRD, SEM and associated EDS mapping and by surface area measurements. The XRD patterns of the calcined samples are reported in Fig. 4a) through f) and are identified by comparison with reference data [70]. The peaks of the Co3 and Co8 oxides have been identified as belonging to a tenorite-like phase, with some shifts and indexed accordingly (Fig. 4a)). In particular, the reflex (110) and is shifted by 0.9 and the reflex (002) by 0.2 , indicating the incorporation of guest cations into the lattice. However, due to the high background of the diffractograms, contributions of small amounts of other phases, such as ZnO [71] cannot be excluded. Also the decomposition of the two Co14 precursors affords similar phases, which are assigned to a dominant tenorite-like phase. The peaks of the Co20 e CP and Co20 e WP mixed oxides XRD patterns are broad and the background noise rather intense. The pattern is compatible with a mixture of tenorite-like and zincitelike phases, though all rather amorphous. In all oxides no CoO nor Co3O4 spinel structures can be identified [72,73]. The SE micrographs of the oxides are reported in Fig. 5 and show a morphology dependency on both the Co content and the synthesis pathway. Co3 and Co8 oxides (panels a) and b)) are narrow sized nanoparticles with a regular shape and a diameter of Fig. 3. Thermogravimetric analysis the Cu/Zn/Co hydroxycarbonates: a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display. The arrow between the two bars corresponds to 20% weight loss.
The weight loss of the samples Co3 and Co8 occurs in one broad step in the temperature range 100e370 C (curves a) and b)). For pure malachite the thermogravimetric curve indicates a single narrow decomposition from a monophasic precursor in the temperature range 150 e320 C [65]. Mineral malachite decomposes in a narrower range at higher temperatures, i.e. 320e370 C [66]. The incorporation of zinc into the malachite has the effect of stabilizing the structure, as evidenced by the shift of the DTG peaks from about 250 to 350 C [53]. The decomposition reactions of zincianmalachite occur in the temperature range 100 e450 C and consist of several poorly resolved simultaneous dihydroxylation and decarbonation steps at 180, 260 and 355 C. In comparison, the zincian-cobalt-malachite of the Co3 and Co8 samples is less stable, being the decomposition complete at 370 C. The Co14-WP has a similar weight loss as Co3 and Co8, a broad decomposition in the 100e370 C temperature range (curve c)). The partially amorphous Co 14 e CP sample shows, in comparison, a steeper weight loss, which is complete already at ~320 C (curve d)). The decomposition of the Co20 amorphous samples (curves e) and f)) occurs in two steps of 15% weight loss each, a broader one, complete at 250 C and a steeper one complete at 340 C. The total weight loss, 30%, corresponds to the production of one H2O and one CO2 molecule, but there is no one-to-one correspondence between a single weight loss step and the production of one type of molecules, since this would give rise to a loss of 8% and 21%, respectively. Therefore, the two weight losses must correspond to the breakage of bonds of different strength between the cations and both the hydroxyls and the carbonates within the amorphous structure.
Fig. 4. XRD patterns of the Cu/Zn/Co mixed oxides: a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP. The spectra are rigidly shifted for a better display.
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209
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Fig. 5. SEM images at magnification 500000 of Cu/Zn/Co mixed oxides a) Co3 e CP b) Co8 e CP c) Co14 e CP d) Co14 e WP, e) Co20 e CP and f) Co20 e WP.
10e20 nm. The Co14 oxide obtained from the amorphous precursor is more agglomerated and the particles have a diameter distribution in a larger range, 10e35 nm (panel c)). The effects of the synthesis pathway on the morphology are observed on the Co14 oxide obtained from the crystalline precursor (panel d): at variance with the former case, the particles shape is regular and the diameter is 10e20 nm similarly to the low Co content oxides. The samples with 20% Co are markedly amorphous regardless of the preparation route (panels e) and f)). For a more careful evaluation of the nanoparticles size, de-agglomeration was performed by deep sonication of diluted ethanol dispersions of the oxides, prior to deposition on the sample-holder. This procedure worked fine for the nanosized oxides derived from the crystalline precursors. The more agglomerated Co14-CP as well as both Co-20 samples retain a large degree of agglomeration. Micrographs of the oxides at different degree of de-agglomeration and magnifications are reported in Table 1S, along with the size evaluation of the most dispersed nanoparticles. EDS images of the oxides are reported in Fig.1S of the Supplementary Material. They show that in all the oxides, Cu, Zn and Co are evenly distributed in the examined areas. This is compatible with the presence of solid solutions or formations of different phases in close contact with each other. Also in the amorphous samples, there is a homogenous distribution of elements. The surface area of the samples is rather constant, hence it does not depend on the preparation method and, as a consequence, on the morphology. For the narrow dispersed, polydispersed
Table 2 BET surface area of the Cu/Zn/Co mixed oxides. Mixed oxide
BET surface area (m2g1)
Co3 e CP Co8 e CP Co14 e CP Co14 e WP Co20 e CP Co20 e WP
50 52 48 51 52 53
nanoparticles and amorphous oxides, the measured surface area is in the order of 50 ± 3 m2g-1, as reported in Table 2. 4. Conclusions Large surface area Cu/Zn/Co mixed oxides with variable atomic ratios were prepared in nanosized form via hydroxycarbonate precursors. Morphology and size depend on the Co content and on the preparation procedure. The precursors were obtained by coprecipitation from metal nitrate in suitable proportions and NaHCO3 solutions. Co content and temperature of the solutions prior to the mixing, i.e. either both at room temperature or at 65 C, determine the phases of the synthesized hydroxycarbonates. Samples with low Co atomic percentage (8%) or 14% Co, prepared with solutions at 65 C have a malachite-like structure. Samples with 14% Co, prepared with solutions at RT, or with high Co content are quasi-amorphous or amorphous. The precursors display a different behavior with respect to the thermal decomposition: twostepped in the case of the amorphous samples, single-stepped for the crystalline or partially amorphous samples. After carrying on the calcination of the precursors at 350 C for 3 h mixed oxides were obtained. A dominant nanosized tenoritelike phase is obtained from crystalline precursors. Polydispersed or amorphous oxides are obtained from amorphous precursors. The element distribution is homogeneous in all the oxides. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.07.175. References [1] P. Ball, L. Garwin, Science at the atomic scale, Nature 355 (1992) 761e767. [2] X.F. Wang, O. Kitao, E. Hosono, H. Zhoua, S. Sasaki, H. Tamiaki, TiO2 and ZnO based solar cells using a chlorophyll a derivative sensitizer for light-harvesting and energy conversion, J. Photochem. Photobiol. A Chem. 210 (2010) 145e152. [3] R. Seema, S. Poonam, P.K. Shishodia, R.M. Mehra, Synthesis of nanocrystalline
208
[4] [5]
[6]
[7]
[8] [9]
[10]
[11]
[12]
[13]
[14] [15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29] [30]
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209 ZnO powder via solegel route for dye-sensitized solar cells, Sol. Energy Mat. Sol. Cells 92 (2008) 1639e1645. W.-Y. Li, L.-N. Xu, J. Chen, Co3O4 nanomaterials in lithium-ion batteries and gas sensors, Adv. Funct. Mater. 15 (2005) 851e857. Z. Zhang, J.B. Yi, J. Ding, L.M. Wong, H.L. Seng, S.J. Wang, J.G. Tao, G.P. Li, G.Z. Xing, T.C. Sum, C. Hon, A. Huan, T. Wu, Cu-Doped ZnO nanoneedles and nanonails: morphological evolution and physical properties, J. Phys. Chem. C 112 (2008) 9579e9585. S.S. Kim, J.H. Yum, Y.E. Sung, Flexible dye-sensitized solar cells using ZnO coated TiO2 nanoparticles, J. Photochem. Photobiol. A Chem. 171 (2005) 269e273. A. Al-Hajry, A. Umar, Y.B. Hahn, D.H. Kim, Growth properties ametnd dye sensitized solar cells d applications of ZnO nanorods grown by low temperature solution process, Superlattices Microstruct. 45 (2009) 529e534. Y.Y.I. Bu, Facile synthesis of highly oriented p-type aluminum Co-doped zinc oxide film with aqua ammonia, J. Alloys Compd. 509 (2011) 2874e2878. M.H. Habibi, B. Karimi, Preparation of nanostructure CuO/ZnO mixed oxide by solegel thermal decomposition of a CuCO3 and ZnCO3: TG, DTG, XRD, FESEM and DRS Investigations, J. Ind. Eng. Chem. 20 (2014) 925e929. M.H. Habibi, B. Karimi, Effect of the annealing temperature on crystalline phase of copper oxide nanoparticle by copper acetate precursor and solegel method, J. Therm. Anal. Calorim. 115 (2014) 419e423. C.H. Lo, T.T. Tsung, Fabrication and characterization of CuO nanorods by a submerged arc nanoparticle synthesis system, J. Vac. Sci. Technol. B 23 (2005) 2394e23976. T. Warang, N. Patel, A. Santini, N. Bazzanella, A. Kale, A. Miotello, Pulsed laser deposition of Co3O4 nanoparticles assembled coating: role of substrate temperature to tailor disordered to crystalline phase and related photocatalytic activity in degradation of methylene blue, Appl. Catal. A Gen. 423e424 (2012) 21e27. S.-L. Chou, J.-Z. Wang, H.-K. Liu, S.-X. Dou, Electrochemical deposition of porous Co3O4 nanostructured thin film for lithium-ion battery, J. Power Sources 182 (2008) 359e364. Y.G. Li, B. Tan, Y.Y. Wu, Mesoporous Co3O4 nanowire arrays for lithium ion batteries with high capacity and rate capacity, Nano Lett. 8 (2008) 265e270. L. Liao, Q. Zhang, Z. Su, Z. Zhao, Y. Wang, Y. Li, X. Lu, D. Wei, G. Feng, Q. Yu, X. Cai, J. Zhao, Z. Ren, H. Fang, F. Robles-Hernandez, S. Baldelli, J. Bao, Efficient solar water-splitting using a nanocrystalline CoO photocatalyst, Nat. Nanotechnol. 9 (2014) 69e73. C. Baltes, S. Vukojevi c, F. Schüth, Correlations between synthesis, precursor, and catalyst structure and activityof a large set of CuO/ZnO/Al2O3 catalysts for methanol synthesis, J. Catal. 258 (2008) 334e344. A.A. Khassina, H. Jobic, G.A. Filonenko, E.V. Dokuchits, A.V. Khasina, T.P. Minyukova, N.V. Shtertser, L.M. Plyasova, T.M. Yurieva, Interaction of hydrogen with CueZn mixed oxide model methanol synthesis Catalyst, J. Mol. Catal. A Chem. 373 (2013) 151e160. M.H. Habibi, B. Karimi, Nanostructure CueZn mixed-oxide supported photocatalyst fabricated by impregnation method for the photocatalytic degradation of C.I. Reactive Orange 16 (V3R) in water, Spectrochim. Acta A 124 (2014) 629e631. M.B. Gawande, R.K. Pandey, R.V. Jayaram, Role of mixed metal oxides in catalysis sciencedversatile applications in organic synthesis, Catal. Sci. Technol. 2 (2012) 1113e1125. S. Zander, E.L. Kunkes, M.E. Schuster, J. Schumann, G. Weinberg, D. Teschner, €gl, M. Behrens, The role of the oxide component in the N. Jacobsen, R. Schlo development of copper composite catalysts for methanol synthesis, Angew. Chem. Int. Ed. 52 (25) (2013) 6536e6540. B. Lorenzut, T. Montini, L. De Rogatis, P. Canton, A. Benedetti, P. Fornasiero, Hydrogen production through alsohol steam reforming on Cu/ZnO based catalysts, Appl. Catal. B Environ. 101 (2011) 397e408. B. Chi, H. Lin, J. Li, Cations distribution of CuxCo3LxO4 and its electrocatalytic activities for oxygen evolution reaction, Int. J. Hydrog. Energy 33 (2008) 4763e4768. Z.Y. Tian, H. Vieker, P. Mountapmbeme Kouotoub, A. Beyer, In situ characterization of CueCo oxides for catalytic application, Faraday Discuss. 177 (2015) 249e262. A.A. Said, R. A1-Qasmi, The role of copper cobaltite spinel, CuxCo3-xO4 during the thermal decomposition of ammonium perchlorate, Thermochim. Acta 275 (1996) 83e91. L. Yan, T. Ren, X. Wang, Q. Gao, D. Ji, J. Suo, Excellent catalytic performance of ZnxCo1-xCo2O4 spinel catalysts for the decomposition of nitrous oxide, Catal. Commun. 4 (2003) 505e509. s, N. Amadeo, M. Laborde, C.R. Apesteguía, Activity and structureM.J.L. Gine sensitivity of the water-gas shift reaction over Cu-Zn-A1 mixed oxide catalysts, Appl. Catal. A Gen. 131 (1995) 283e296. €gl, B.L. Kniep, T. Ressler, A. Rabis, F. Girgsdies, M. Baenitz, F. Steglich, R. Schlo Rational design of nanostructured copperezinc oxide catalysts for the steam reforming of methanol, Angew. Chem. Int. Ed. 43 (2004) 112e115. A. Bienholz, R. Blume, A. Knop-Gericke, F. Girgsdies, M. Behrens, P. Claus, Prevention of catalyst deactivation in the hydrogenolysis of glycerol by Ga2O3-modified copper/zinc oxide catalysts, J. Phys. Chem. 115 (2011) 999e1005. J.I. Di Cosimo, A.J. Marchi, C.R. Apesteguia, Preparation of ternary Cu/Co/Al catalyst by amorphous citrate process, J. Catal. 134 (1992) 594e607. J.E. Baker, R. Burch, S.E. Golunski, Synthesis of higher alcohols over copper/
[31] [32]
[33]
[34]
[35]
[36]
[37]
[38]
[39] [40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
[51]
[52]
[53]
[54]
[55]
[56]
[57]
cobalt catalysts influence of preparative procedures on the activity and selectivity of Cu/Co/Zn/Al mixed oxide catalysts, Appl. Catal. 53 (1989) 279e297. Y. Souma, H. Ando, M. Fujuwara, R. Kieffer, Catalytic hydrogenation of carbon dioxide to hydrocarbons, Energy Convers. Mat. 36 (1995) 593e596. X.Z. Lin, P. Liu, J.M. Yu, G.W. Yang, Synthesis of CuO nanocrystals and sequential assembly of nanostructures with shape-dependent optical absorption upon laser ablation in liquid, J. Phys. Chem. C 113 (2009) 17543e17547. S. Farhadi, J. Safabakhsh, P. Zaringhadam, Synthesis, characterization, and investigation of optical and magnetic properties of cobalt oxide (Co3O4) nanoparticles, J. Nanostr. Chem. 3 (69) (2013) 1e9. D.G. Lamas, G.E. Lascalea, N.E. Walsoe de Reca, Synthesis and characeterization of nanocrustalline powders of partially stabilized zirconia ceramics, J. Eur. Ceram. Soc. 18 (1998) 1217e1221. R.F. Juarez, D.G. Lamas, G.F. Lascalea, N.F. Walsoe de Reca, Synthesis of nanocrystalline zirconia powders for TZP ceramics by a nitrateecitrate combustion route, J. Eur. Ceram. Soc. 20 (2000) 133e138. J. Chen, J. Tang, F. Yan, H. Ju, A gold nanoparticles/solegel composite architecture for encapsulation of immunoconjugate for reagentless electrochemical immunoassay, Biomaterials 27 (2006) 2313e2321. M. Luo, W. Tang, J. Zhao, C. Pu, Hydrophilic modification of poly(ether sulfone) used TiO2 nanoparticles by a solegel process, J. Mater. Process. Technol. 172 (2006) 431e436. E. Lester, G. Aksomaityte, J. Li, S. Gomez, J. Gonzalez-Gonzalez, M. Poliakoff, Controlled continuous hydrothermal synthesis of cobalt oxide (Co3O4) nanoparticles, Prog. Cryst. Growth Charact. Mater. 58 (2012) 3e13. L.-H. Ai, J. Jiang, Rapid synthesis of nanocrystalline Co3O4 by a microwave assisted combustion method, Powder Tech. 195 (2009) 11e14. L. Sun, Z. Zhang, Z. Wang, Z. Wu, H. Dang, Synthesis and characterization of CuO nanoparticles from liquid ammonia, Mater. Res. Bull. 40 (2005) 1024e1027. M. Farahmandjou, M. Zarinkamar, Synthesis of nano-d ceria (CeO2) particles via a cerium hydroxy carbonate precursor and the effect of reaction temperature on particle morphology, JUFNGSM 48 (2015) 5e10. A. Khaleel, I. Shehadi, M. Al-Shamisi, Nanostructured chromiumeiron mixed oxides: physicochemical properties and catalytic activity, Colloids Surf. A Physicochem. Eng. Asp. 355 (2010) 75e82. A. Cetin, R. Kibar, M. Ayvacikli, N. Can, Ch. Buchal, P.D. Townsend, A.L. Stepanov, T. Karali, S. Selvi, Optical properties of Cu implanted ZnO, Nucl. Instrum. Methods Phys. Res. B 249 (2006) 474. G. Avgouropoulos, T. Ioannides, H.K. Matralis, J. Batista, S. Hocevar, CuOeCeO2 mixed oxide catalysts for the selective oxidation of carbon monoxide in excess hydrogen, Catal. Lett. 73 (2001) 33e40. C. Ngamcharussrivichai, P. Totarat, K. Bunyakiat, Ca and Zn mixed oxide as a heterogeneous base catalyst for transesterification of palm kernel oil, Appl. Cat. A Gen. 341 (2008) 77e85. A. Gaber, M.A. Abdel-Rahim, A.Y. Abdel-Latief, M.N. Abdel-Salam, Influence of calcination temperature on the structure and porosity of nanocrystalline SnO2 synthesized by a conventional precipitation method, Int. J. Elecrochem. Sci. 9 (2014) 81e95. S. Satapathy, A. Ahlawat, A. Paliwal, R. Singh, M.K. Singh, P.K. Gupta, Effect of calcination temperature on nanoparticle morphology and its consequence on optical properties of Nd:Y2O3 transparent ceramics, Cryst. Eng. Comm. 16 (2014) 723e2731. M. Saito, T. Fujitani, M. Takeuchi, T. Watanabe, Development of copper/zinc oxide-based multicomponent catalysts for methanol synthesis from carbon dioxide and hydrogen, Appl. Cat. A Gen. 138 (1996) 311e318. M.M. Günter, T. Ressler, R.E. Jentoft, B. Bems, Redox behavior of copper oxide/ zinc oxide catalysts in the steam reforming of methanol studied by in situ xray diffraction and absorption spectroscopy, J. Catal. 203 (2001) 133e149. N. Thouchprasitchai, A. Luengnaruemitchai, S. Pongstabodee, Water-gas shift reaction over CueZn, CueFe, and CueZneFe composite-oxide catalysts prepared by urea-nitrate combustion, J. Ind. Eng. Chem. 19 (2013) 1483e1492. P. Porta, S. De Rossi, G. Ferraris, F. Pompa, Characterization of copper-zinc mixed oxide system in relation to different precursor structure and morphology, Solid State Ion. 45 (1991) 35e41. , A. Vaccari, Catalysts for lowC. Busetto, G. Del Piero, G. Manara, F. Trifiro temperature methanol synthesis. Preparation of Cu-Zn-Al mixed oxides via hydrotalcite-like precursors, J. Catal. 85 (1984) 260e266. €gl, Relations B. Bems, M. Schur, A. Dassenoy, H. Junkes, D. Herein, R. Schlo between synthesis and microstructural properties of copper/zinc hydroxycarbonates, Chem. Eur. J. 9 (2003) 2039e2052. S.H. Taylor, Graham J. Hutchings, A.A. Mirzaei, The preparation and activity of copper zinc oxide catalysts for ambient temperature carbon monoxide oxidation, Catal. Today 84 (2003) 113e119. G.C. Shen, S.-I. Fujita, N. Takezawa, Preparation of precursors for the Cu/ZnO methanol synthesis catalysts by coprecipitation methods: effects of the preparation conditions upon the structures of the precursors, J. Catal. 138 (1992) 754e758. S. Morpurgo, M. Lo Jacono, P. Porta, CoppereZinceCobalteAluminiumeChromium hydroxycarbonates and mixed oxides, J. Solid State Chem. 122 (1996) 324e332. P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lo Jacono, G. Moretti, Copperecobalt hydroxysalts and oxysalts: bulk and surface characterization,
M. Carbone / Journal of Alloys and Compounds 688 (2016) 202e209 J. Mater. Chem. 1 (1991) 531e537. [58] P. Porta, R. Dragone, G. Fierro, M. Inversi, M. Lo Jacono, G. Moretti, Preparation and Characterization of Cobalt-Copper Hydroxysalts and their oxide products of decomposition, J. Chem. Soc. Faraday Trans. 88 (1992) 311e319. [59] JCPDS-ICDD 41-1390. [60] JCPDS-ICDD 38-154. [61] JCPDS-ICDD 72-1100. [62] JCPDS-ICDD 10-240. [63] JCPDS-ICDD 72-539. [64] JCPDS-ICDD 75e1779. [65] P. Porta, S. De Rossi, G. Ferraris, M. Lo Jacono, G. Minelli, G. Moretti, Structural characterization of malachite-like coprecipitated precursors of binary CuOZnO catalysts, J. Catal. 109 (1988) 367e377. €gl, Minerals as model com[66] M. Behrens, F. Girgsdies, A. Trunschke, R. Schlo pounds for Cu/ZnO catalyst precursors: structural and thermal properties and
[67]
[68] [69]
[70] [71] [72] [73]
209
IR spectra of mineral and synthetic (zincian) malachite, rosasite and aurichalcite and a catalyst precursor mixture, Eur. J. Inorg. Chem. 10 (2009) 1347e1357. €zinger, P.G. Menon, B. Delmon, Solid-state reactions, in: G. Ertl, H. Kno J. Weitkamp (Eds.), Preparation of Solid Catalysts, Wiley-VCH Verlag GmbH, Weinheim, Germany, 1999. F. Schüth, M. Hesse, K.K. Unger, Precipitation and Coprecipitation, in: Handbook of Heterogeneous Catalysis, vols. 2:2.3:2.3.3, 2008, pp. 100e119. M. Carbone, C. Sadun, R. Caminiti, Structural study by energy dispersive X-ray diffraction of amorphous mixed hydroxycarbonates containing Co, Cu, Zn, A1, J. Mater Chem. 6 (10) (1996) 1709e1716. JCPDS-ICDD 41-254. JCPDS-ICDD 36-1451. JCPDS-ICDD 42-1467. JCPDS-ICDD 78-0431.