The Synthesis and Use of Nano Nickel Catalysts

Adél Anna Ádám1 2, Márton Szabados1 2, Ádám Polyákovics2, Katalin Musza1 2, Zoltán Kónya3 4, Ákos Kukovecz3, Pál Sipos2 5, and István Pálinkó1 2 ∗

1Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, H-6720, Hungary 2Material and Solution Structure Research Group, Institute of Chemistry, University of Szeged, Aradi Vértanúk tere 1, Szeged, H-6720, Hungary 3Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, Szeged, H-6720, Hungary 4MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Rerrich B tér 1, Szeged, H-6720, Hungary 5Department of Inorganic and Analytical Chemistry, University of Szeged, Dóm tér 7, Szeged, H-6720, Hungary

The hydrazine reduction method was applied for the synthesis of nickel nanoparticles without using inert atmosphere and added surface active agents. The effect of the preparation temperature and the chemical quality of the metal sources as well as the solvents were studied. The generation of nanoparticles were studied primarily by X-ray diffractometry, but scanning and transmission electron microscopies as well as dynamic light scattering measurements were also used for the better understanding of the nanoparticles behaviour. The elevation of temperature was the key point in

transforming Ni(OH)2 into metallic nickel. By selecting the metal source, the obtained crystallite sizes could be tailored between 7 nm and 15 nm; however, the SEM and DLS measurements IP: 188.156.237.54 On: Sun, 28 Oct 2018 16:34:02 revealed signiﬁcant agglomeration resulting in aggregates with spherical or Ni(OH)2 resembling morphologies depending onCopyright: the solvent American used. The Scientific catalytic activities Publishers of the nanoparticles prepared Delivered by Ingenta were tested and compared in a Suzuki-Miyaura cross-coupling reaction. Keywords: Ni Nanoparticles, Hydrazine Reduction, XRD-SEM-TEM-DLS, Suzuki-Miyaura Cross-Coupling.

1. INTRODUCTION gas-phase photonucleation12 and other methods, such as In recent decades, nickel nanoparticles (NPs) have received electro-13 and sonochemical depositions14 and pyrolysis15 significant attention in the material science research com- have been published in relation with the synthesis of nickel munity, due to their extraordinary physical, electrical, opti- fine powders and nanoparticles. cal and chemical properties as the consequence of the Nanometals find wide-ranging applications from increased surface to bulk atomic ratio.1 2 These attributes catalysis,16 which can be regarded as the most intensely depend strongly on the size and shape of NPs, there- investigated area, through photochemistry,17 medicine18 to fore numerous synthetic methods were developed for electronics.19 Nobel metal NP catalysts have high density their controlled production. Chemical reduction is the of states (DOS) near their Fermi levels,20 high chemical most frequently used route, where a strong reducing activities and stabilities, therefore, they gained increased agent, for instance polyol,3 supercritical alcohols,4 hydro- attention in practical applications in recent years.21 In the gen gas,5 hydrazine,6 7 metal hydrides8 9 are applied to meantime, the commonly researched and used noble met- obtain nickel NPs from dissolved nickel salts. These meth- als, such as platinum, palladium, silver, or gold, became ods usually offer high mass production and well-tailored scarcer and thus more and more expensive, the more nano- and microstructures under relatively mild condi- accessible transition metals like Ni, Cu, Fe, Co could be a tions. However, chemical reduction is not always feasi- suitable and economical alternative in chemical processes. ble, therefore many physical methods like thermal plasma To the best of our knowledge, this is the first report for synthesis,10 mechanochemical technique,11 laser-assisted the synthesis of nickel NPs directly from nickel hydroxide and a nickel complex based on the hydrazine reduction ∗Author to whom correspondence should be addressed. method in ethanol-water mixture without surface-active

J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 1 1533-4880/2019/19/453/006 doi:10.1166/jnn.2019.15781 453 The Synthesis and Use of Nano Nickel Catalysts Ádám et al. additive and O2-free atmosphere. Applying this technique, In the second step, the direct reduction of nickel hydrox- the first step is the dissolution of the nickel salt and the ide particles to nickel NPs took place in hydrazine solution precipitation of hydroxide by adding alkali. The second with or without ethanol addition. Beside the lack of alkali, step is the reduction of the hydrazine on the surface of the the reduction conditions were the same as in the synthesis precipitates. Our further aim was to investigate the effect starting from nickel chloride. The molar ratio was 10 for 2+ of the synthesis temperature and the elimination of the N2H4/Ni , and the solution was stirred at 50 Cfor1or first step, i.e., the reduction of nickel hydroxide powder to 4 h. The black precipitate was filtered and stored under metal NPs in ethanolic and aqueous medium directly. For- acetone in inert atmosphere. mation of the nanoparticles were mainly studied by X-ray diffractometry (XRD), but the obtained samples were char- 2.4. The Synthesis of NiNPs from acterized by dynamic light scattering (DLS), scanning Tetraethylammonium Tetrachloronickelate

(SEM) and transmission (TEM) electron microscopy mea- ([(C2H5)4N]2NiCl4) surements as well. The synthesis route followed the procedure of direct The catalytic activity of the nanoparticles prepared were nickel hydroxide reduction. The tetraethylammonium studied in the Suzuki-Miyaura cross-coupling reaction of tetrachloronickelate complex was synthesized following aryl halide and arylboronic acid,22 and compared to com- the a study of Lopez-Salinas et al.23 The complex pre- mercial nickel powder and nickel nanoparticles generated pared (2.9 g) was added to absolute ethanol (12.5 cm3 from nickel chloride by the unmodified method.7 and mixed with the already described ethanolic hydrazine and the base at 50 C and stirred for 4 h. Finally, the black precipitate was collected and protected from O in the way 2. EXPERIMENTAL DETAILS 2 described already. 2.1. Materials All reagents and solutions applied in these experiments 2.5. Procedure for the Suzuki-Miyaura were of reagent grade and used without further purifi- Cross-Coupling Reaction cation. Metallic nickel (99.7% purity, average diameter The catalytic tests were carried out in a 5 cm3 glass <50 m), nickel chloride hexahydrate (NiCl × 6H O), 2 2 reactor immersed into a preheated oil bath using hot >78 wt% hydrazine monohydrate (N H × H O) solution, 2 4 2 plate magnetic stirrer. Aryl halide (4 -bromoacetophenone) potassium hydroxide (KOH), 4IP:-bromoacetophenone 188.156.237.54 On: and Sun, 28 Oct 2018 16:34:02 (1.0 mmol), arylboronic acid (phenylboronic acid) phenylboronic acid were purchased fromCopyright: Sigma-Aldrich. American Scientific Publishers Delivered by(1.2 Ingenta mmol), base (2.5 mmol of anhydrous pyridine) and Absolute ethanol, toluene, pyridine, octanol and acetone metallic nickel catalyst (9.6 mg) were stirred with 3 cm3 of were obtained from VWR International. solvent (anhydrous toluene) at 90 Cinair.Attheendof the reaction, the obtained suspension was filtered, and the 2.2. Synthesis of NiNPs from Nickel Chloride clear liquid was injected into the gas chromatograph (GC) In the first step, nickel chloride (0.7 g) was dis- to determine the conversion using 1-octanol as the internal solved directly in absolute ethanol (12.5 cm3. In paral- standard. The GC was a Hewlett-Packard 5890 Series II lel, in another vessel solid potassium hydroxide (1.6 g, instrument equipped with 50-m-long Agilent HP-1 column + KOH/Ni2 ratio was 10) and hydrazine monohydrate and flame ionization detector. The heating was set in stages 2+ 3 (1.4 g, N2H4/Ni was 10) were mixed in 12.5 cm from 90 C to 350 C, and the chromatographic peaks were of ethanol. The nickel chloride solution and the base- identified applying authentic samples. hydrazine-ethanol mixture were mixed and stirred at 25 C, 50 Cor75 C for 4 h. The obtained precip- 2.6. Methods of Structural Characterization itates were collected on a 0.45 m filter, washed by For registering the powder X-ray diffractograms, a Rigaku deionized water and acetone. Until characterization, the Miniflex II powder X-ray diffractometer was applied in the particles were stored under acetone in oxygen-free (N2 = 5–80 range with 4 /min scan speed using CuK ( = atmosphere. 15418 Å) radiation. To identify the reflections of the NPs, the JCPDS (Joint Committee of Powder Diffraction Stan- 2.3. Preparation of NiNPs from Nickel Hydroxide dards) database was utilized. The average crystal sizes of In this synthesis, the above-mentioned procedure was sep- the particles prepared were calculated from the full width arated into two steps. First, the precipitation of nickel at half maximum of the most intense reflection of metal- hydroxide occurred in the reaction of dissolved nickel lic nickel applying the Scherrer equation using 0.9 as the chloride (0.7 g in 25 cm3 absolute ethanol) and solid potas- shape factor, after fitting Gaussian curve on the reflection sium hydroxide (1.6 g). Then, the mixture was stirred for measured. 1 h under 25 C, and the green precipitate was filtered, The morphology and the size of the nanocrystals were washed by deionized water and dried at room temperature examined by scanning electron microscope (SEM–Hitachi for48hinair. S-4700 instrument) and transmission electron microscopy

454 J. Nanosci. Nanotechnol. 19, 453–458, 2019 Ádám et al. The Synthesis and Use of Nano Nickel Catalysts

(TEM–FEI TECNAI G220 X-TWIN instrument) at various (111) 7 nm magniﬁcations at 10 kV and 200 kV acceleration voltages, (200) from nickel complex (220) respectively. For sharper SEM imaging, the particles were 11 nm coated with few nanometers of gold layer, and the elemen- from nickel hydroxide, no ethanol tal analysis was performed with energy dispersive X-ray 13 nm analysis (EDX) measurements (Röntec QX2 spectrometer from nickel hydroxide equipped with Be window and coupled to the microscope). 15 nm A Malvern NanoZSD dynamic light scattering (DLS) Intensity / a.u. from nickel chloride instrument, installed with a 4 mW helium-neon laser light 34 nm source ( = 633 nm), was applied to map the size distri- the commercial Ni powder bution of the samples at room temperature. For the DLS 10 20 30 40 50 60 70 80 measurements, the detection was performed in back scat- 2θ / º tering mode at 173, and the nanoparticles were dispersed Figure 2. X-ray diffractograms of the powders obtained from various inethyleneglycolwith1hultrasonicirradiation.Thepar- ticle concentration was 2 mg/mL. metal sources at 50 C.

interaction with the hydrazine reducing agent, several 3. RESULTS AND DISCUSSION metal sources were tested (Fig. 2). In all instances, the 3.1. Nanoparticles Studied by X-ray Diffractometry preparation of nickel nanoparticles was successful without The formation of metallic nickel was followed and veri- any contamination, but with excursive average crystallite fied via the X-ray diffractograms of the samples prepared. sizes (calculated from the most intense (111) reflection fit- Firstly, the effect of synthesis temperature was investigated ting Gaussian curve). using the nickel chloride source (Fig. 1), and it was proved As a reference, commercial metallic nickel powder to be a crucial parameter. At room temperature there was was applied and compared to the as-prepared samples. no sign of nickel nanoparticles, the reflections (JCPDS#74- Although its average grain size is under 50 m written on 2075) of hydroxide solid (i.e., the hydrolysis of the nickel the label, XRD measurement was used to determine the chloride took place) was only observable. The Ni(OH)2 coherently scattering domain, which correlates well with was very stable at 25 C, the hydrazineIP: 188.156.237.54 molecules were On: Sun,the 28 crystallite Oct 2018 or 16:34:02 subgrain sizes obtained by other charac- unable to reduce it in 4 h, but 24 h wereCopyright: sufficient toAmerican gener- Scientific Publishers24 Delivered byterizing Ingenta techniques (except when the powder was pre- ate metallic nickel. On elevating the temperature, the gen- pared by plastic deformation). X-ray line profile analysis eration of nickel (JCPDS#04-0850) nanoparticles became provided with 34 nm value, due to the dislocation bound- possible, and phase-pure solid material was obtained in aries and the dipolar walls; however, SEM images indicate 4 h. Only three reflections were noticed, (111), (200), that the particles are in the micron range. By varying the (220) they were, indicating pure face-centered cubic (fcc) metal source, the size of the prepared crystallites could be structure. Since the crystallite sizes remained practically decreased from 15 nm to 7 nm. constant up to 75 C, further synthetic work was carried On the X-ray patterns of nanoparticles derived from out at 50 C. Ni(OH)2, a small, but observable baseline rise was To examine the effect of the quality of the nickel- detected indicating the presence of amorphous nickel containing reactants, which may have direct or indirect phase. It cannot be residual hydroxide, since oxygen was not observed by SEM-EDX measurements. Curiously, 4 h (111) stirring was necessary for all metal source to arrive at

17 nm metallic Ni; however, in water, Ni(OH)2 could be trans- (200) (220) formed to metallic nickel in shorter than 1 h. 75ºC 15 nm 3.2. Morphology Characteristics The SEM micrographs of the nanoparticles prepared in 50ºC ethanol show close to spherical Ni aggregates in the (011) (110)

Intensity / a.u. (100) range of 100–300 nm with the narrowest size distribu- (012) (111) (001) tion in the images of nickel hydroxide source (Fig. 3). 25ºC In the absence of ethanol, aggregation occurred producing morphology resembling that of nickel hydroxide.25 The 10 20 30 40 50 60 70 80 EDX spectrum conﬁrmed the synthesis of metallic nickel, θ 2 / º peaks corresponding to oxygen or chlorine did not turn up Figure 1. XRD patterns of the samples prepared at various tempera- (their appearance would have been the sign of incomplete tures from nickel chloride as the metal source. transformation). Nevertheless, the transmission electron

J. Nanosci. Nanotechnol. 19, 453–458, 2019 455 The Synthesis and Use of Nano Nickel Catalysts Ádám et al.

(A)

(B)

IP: 188.156.237.54 On: Sun, 28 Oct 2018 16:34:02 Copyright: American Scientific Publishers Delivered by Ingenta (C)

(D)

Figure 3. SEM images of the as-prepared Ni nanoparticles obtained from A—from nickel chloride, B—from nickel complex, C—nickel hydroxide, D—nickel hydroxide in the absence of ethanol. microscope, operating at advanced angular resolution, obtained from the nickel complex verifying that the val- could provide more detailed picture from the nanoparticles ues of coherently scattering domains determined from the (Fig. 4). The measurements revealed close ﬁtting square diffractograms were of similar sizes to the nanoparticles groups with <10 nm diagonals in the images of the powder grown.

456 J. Nanosci. Nanotechnol. 19, 453–458, 2019 Ádám et al. The Synthesis and Use of Nano Nickel Catalysts

(260–340 nm) and a wider (1110–1280 nm). The metal source was able to dissolve in the reaction medium par-

tially. As a result, hydrazine could react with Ni(OH)2 via the dissolved Ni2+.Thein situ generated nickel hydroxide particles were originated from the precipitation reaction between the nickel cations and the potassium hydroxide when the nickel complex was the metal source; however, for the synthesis starting from nickel hydroxide, KOH pel- lets were not added, and the basic condition was provided by the dissolution of hydrazine. This interpretation was conﬁrmed by the unimodal distribution when the starting metal source was nickel chloride. During the nanoparticle 2+ synthesis, various nickel forms, such as [Ni(C2H5OH)6] , 2+ [Ni(N2H4 m] , and ﬁnally Ni(OH)2, could be formed in the order of their kinetic and thermodynamic stabilities.7 Thus, in the larger part of the stirring time, only one form was present and transformed to nanoparticles.

3.4. Catalytic Test of the Synthesized Nanoparticles The catalytic activities of the nanoparticles and the com- Figure 4. TEM image of nickel nanoparticles from nickel complex as mercial nickel powder were compared in a Suzuki- the metal source. Miyaura cross-coupling reaction producing biaryl, which has received increased attention by the pharmaceutical 3.3. Dynamic Light Scattering Investigations industry as scaffold in the structure of anti-inflammatory, 26 In order to study the degree of the aggregation processes antihypertensive and antitumor drugs. In the reactions, 22 during nanoparticles synthesis in detail, size distribution the work of Park et al. was followed with several modi- curves were recorded by the dynamic light scattering fications (the solvent was toluene and the applied temper- IP: 188.156.237.54 On: Sun,ature 28 Oct was 2018 decreased 16:34:02 to 90 C). Beside pyridine, sodium method (Fig. 5). Two types of aggregationCopyright: tendencies American were Scientific Publishers recognizable: unimodal starting from nickel chlorideDelivered or bymethoxide Ingenta and trimethylamine as stronger bases were also hydroxide in ethanol and bimodal for the rest. The latter tested as well as the duration of the synthesis. The obtained indicate two competing routes of nanoparticle formation. experimental results were similar; formation of polymer- Nickel hydroxide is slightly soluble in water, but insolu- ized product was detected after 4 h with pyridine, but it ble in ethanol, while the solubility of the nickel complex occurred instantly with the stronger bases. in ethanol was sufficient to generate nickel-ethanol com- The nickel powder and the nanoparticles from hydrox- plex in minute amounts. In this latter instance, aggregation ide obtained from aqueous medium showed similar activi- process followed bimodal distribution, and the aggre- ties (Table I), while the yield was the lowest applying the gates were grown in different size ranges, in a narrower nanoparticles from ethanolic nickel hydroxide. Curiously, the nickel particles originating from the complex proved to be the most efficient catalyst, presumably because of their 530 25 smallest crystallite size, thus the increased fraction of edge

1480 sites with enhanced surface energies. Let us mention that 20 the other nanoparticles did not follow this trend indicating that other, still unknown parameters are also operational. 15 1110

10 1280 Table I. The Suzuki-Miyaura cross-coupling reaction of 4-bromoacetophenone and phenylboronic acid yielding a biaryl derivate. Frequency / % 260 5 340 0 100 1000 Catalyst Yield (%) Diameter / nm Without catalyst 0 from nickel complex from nickel hydroxide, no ethanol Commercial Ni powder 27 from nickel hydroxide (no water) From nickel hydroxide, no water 12 from nickel chloride From nickel hydroxide, no ethanol 23 From nickel chloride 43 Figure 5. Uni-and bimodal aggregate intensity-weighted size distribu- From nickel complex 52 tion curves for the nickel nanoparticles.

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Finally, the catalysts were recovered after the reaction 4. J. Kim, D. Kim, B. Veriansyah, J. W. Kang, and J.-D. Kim, Matter. and their X-ray diffractograms were taken. The calculated Lett. 63, 1880 (2009). average crystallite sizes and the reflections did not show 5. J. Forsman, U. Tapper, A. Auvinen, and J. Jokiniemi, J. Nanopart. Res. 10, 745 (2008). significant alterations, and there were no signs of oxide or 6. S.-H. Wu and D.-H. Chen, J. Colloid. Interf. Sci. 259, 282 (2003). other nickel-containing secondary products. 7. Z. G. Wu, M. Munoz, and O. Montero, Adv. Powder Technol. 21, 165 (2010). 8. P. K. Khanna, P. V. More, J. P. Jawalkar, and B. G. Bharate, Mater. 4. CONCLUSIONS Lett. 63, 1384 (2009). During the synthesis of nickel nanoparticles by the 9. Y. Y. Titova, L. B. Belykh, A. V. Rokhin, V. A. Umanets, and F. K. hydrazine reduction method, it was experienced that the Schmidt, Kinet. Katal. 53, 577 (2012). 10. Y.-M.Kim,K-H.Kim,B.Kim,andH.Choi,J. Alloy. Compd. temperature of the reaction and the quality of the metal 658, 824 (2016). source were crucial in size control, while the application of 11. J. Ding, T. Tsuzuki, P. G. McCormick, and R. Street, J. Phys. D: ethanol in the synthesis mixture was decisive in the shape Appl. Phys. 29, 2365 (1996). control of the aggregates. 12. H. He, R. H. Heist, B. L. McIntyre, and T. N. Blanton, Nanostruct. Ni nanoparticles obtained from either metal source Mater. 8, 879 (1997). 13. T. Hang, M. Li, Q. Fei, and D. Mao, Nanotechnology 19, 035201 applied were active and recyclable catalysts in the Suzuki- (2007). Miyaura cross-coupling reaction; however, those generated 14. S. Ramesh, Y. Koltypin, R. Prozorov, and A. Gedanken, Chem. from the nickel complex exhibited the highest activity. In Mater. 9, 546 (1997). addition, the X-ray diffractograms taken after use verified 15. W.-N. Wang, Y. Itoh, I. W. Lenggoro, and K. Okuyama, Mater. Sci. that the catalysts were not degraded during reaction and it Eng. B 111, 69 (2004). 16. F.-S. Han, Chem. Soc. Rev. 42, 5270 (2013). remained in the metallic state. 17. A. F. Koenderik, A. Alú, and A. Polman, Science 348, 516 (2015). 18. J. Kreuter, Int. J. Pharm. 331, 1 (2007). Acknowledgment: This work was supported by the 19. D. V. Talapin, J.-S. Lee, M. V. Kovalenko, and E. V. Shevchenko, grant GINOP-2.3.2-15-2016-00013. The financial help is Chem. Rev. 9, 389 (2010). 20. E. Gross and G. A. Somorjai, J. Catal. 328, 91 (2015). highly appreciated. 21. B. F. G. Johnson, Coord. Chem. Rev. 190–192, 1269 (1999). 22. J. Park, E. Kang, S. U. Son, H. M. Park, M. K. Lee, J. Kim, K. W. Kim, H.-J. Noh, J.-H. Park, C. J. Bae, J.-G. Park, and T. Hyeon, References and Notes IP: 188.156.237.54 On: Sun, 28Adv. Oct Mater. 201817, 16:34:02 429 (2005). 1. C. M. Welch and R. G. Compton, Anal. Bioanal.Copyright: Chem. American384, 601 Scientific23. E. Lopez-Salinas, Publishers N. Tomita, T. Matsui, E. Suzuki, and Y. Ono, (2006). Delivered by IngentaJ. Mol. Catal. 81, 397 (1993). 2. M. I. Din and A. Rani, Int. J. Anal. Chem. Article ID: 3512145 24. T. Ungár, G. Tichy, J. Gubicza, and R. J. Hellmig, Powder Diffr. (2016). 20, 366 (2005). 3. F. Fiévet and R. Brayner, The polyol process, Nanomateri- 25. Y.Tanga,Y.Liu,S.Yu,Y.Zhao,S.Mu,andF.Gao,Electrochim. als: A Danger or a Promise? Springer-Verlag, London (2013), Acta 123, 158 (2014). pp. 1–25. 26. O. Baudoin, Angew. Chem. Int. Ed. 46, 1373 (2007).

Received: 30 September 2017. Accepted: 30 October 2017.

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