Novel microwave assisted approach to large scale nickel nanoparticle fabrication

Chemical Engineering Journal 240 (2014) 155–160

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Novel microwave assisted approach to large scale nickel nanoparticle fabrication Julius Motuzas a, Martin Drobek a, João C. Diniz da Costa b, Anne Julbe a,⇑ a b

Institut Européen des Membranes, UMR 5635-CNRS-ENSCM-UM2, Université Montpellier 2, cc 047, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France FIMLab – Films and Inorganic Membrane Laboratory School of Chemical Engineering, The University of Queensland, Brisbane, Qld 4072, Australia

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Nickel nanoparticles were produced

Production of Ni0 nanoparticle from Ni(OH)2 and ethylene glycol under MW.

by reduction of nickel hydroxide under MW-assisted heating.  No special organic additives and pH regulators were applied.  Auto-catalytic behavior of nickel nanoparticles was proven.  Reduction agent (ethylene glycol) multiple usage/recyclability was demonstrated.  Highly concentrated nickel nanoparticle suspensions were produced.

a r t i c l e

i n f o

Article history: Received 25 July 2013 Received in revised form 5 November 2013 Accepted 14 November 2013 Available online 22 November 2013 Keywords: Nickel nanoparticles Microwaves Catalysis Solvent recycling Concentrated suspensions

a b s t r a c t This work shows a novel environmentally benign microwave (MW) assisted method for the fabrication of Ni0 nanoparticles. The objective was the development of a rapid and self-sustainable solvothermal method, starting from concentrated nickel hydroxide suspensions. Suspensions of nickel hydroxide (Ni(OH)2) in ethylene glycol (EG) were converted to Ni0 nanoparticles at 260 °C without the need for any supplementary catalysts. The MW irradiation initiated the dissociation of nickel hydroxide to Ni2+ and mobile OH- ions thus triggering the partial catalytic oxidation of EG. As a consequence, two electrons became available to reduce Ni2+ to metallic Ni0 nanoparticles. This process is self-sustainable as the Ni0 nanoparticles then become the catalytic domains for further oxidation of EG and resulting in a faster kinetics for the complete reduction of nickel hydroxide within 60 min. Interestingly, the MW-assisted process was also effective in multiple recycling of EG, forming Ni0 nanoparticles after each reduction cycle. This process dispenses the need for using a high amount of solvents as required in conventional solvothermal methods, and greatly reduces solvent waste generation. In addition, this novel process led to almost 100% conversion of highly concentrated suspensions (1.2 M Ni(OH)2) to Ni0 nanoparticles, which is very attractive for a large scale production. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Nickel nanoparticles are versatile materials and potentially attractive for a large number of applications ranging from batteries [1], catalysis [2,3] and photocatalysis [4], to electro-oxidation [5], ⇑ Corresponding author. Tel.: +33 467 14 91 42; fax: +33 467 14 91 19. E-mail address: [email protected] (A. Julbe). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.11.032

magnetic devices [6], medicine [7], fuel cells [8,9], and composite materials of carbon [10] and ceramics [11]. The most common method to prepare nickel nanoparticles is based on solvothermal reduction process, a simple and facile method to control the particle size. However, the solvothermal method requires a large amount of solvents which are often wasted or require further processing for reuse. Hence, there is a need to further reduce the solvent requirement to make this method environmentally benign.

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In the solvothermal method, Ni0 nanoparticles are produced in liquid media by the reduction of Ni2+ ions. Ethylene glycol (EG) is a classical reducing agent which also serves as a reaction media for Ni nanoparticle formation [12–14]. Hydrazine has also been used as an efficient reducing agent instead of EG [15,16]. The use of a reducing agent requires a pH adjusting media such as NaOH, Na2CO3 [16] or H2SO4 [13]. However, pH adjusting media containing sodium and sulfur, both of which are undesirable in many applications. Hence, the synthesis of Ni0 nanoparticles derived from Ni(OH)2 without any pH additive remains an important challenge. Indeed, only the partial Ni(OH)2 conversion to Ni0 is generally achieved in such case [17], as the Ni2+ reduction to Ni0 strongly depends upon the reducing conditions. EG based reductions by conventional heating are generally carried out at 195–198 °C. In the case of hydrazine, the reduction temperature is typically in the range 60–180 °C [15]. Microwave (MW) assisted reduction using either EG or hydrazine has been considered, though hydrazine results in a fast reaction which is difficult to control. On the other hand, EG has a high dielectric constant (e0 = 41.0 at 25 °C) [18] which is preferred as it favors rapid heating under MW-assisted conditions. Nevertheless, the influence of the reduction temperature is an important fundamental parameter related to Ni0 particle formation, though it has yet to be reported for MW-assisted heating using EG media. Controlling the particle size formation is another parameter of importance. There are two strategies to achieve particle size control. The first strategy is based on the addition of an easily reducible metal Pt, Pd, Au, Ag or Ni precursors or particles [13,14,19], which act as a seeding agent. The second strategy takes into consideration the steric stabilization of the early formed Ni nanoparticle in the polymeric or organic matrices such as polyvinylpyrolidone (PVP) or n-dodecylamine [20,21], or cetyltrimethyl ammonium bromide (CTAB) [22]. The problem of the second strategy is that most of these polymers and organic additives require the supplement of other pH control ionic species (Na+, Cl, or SO2 4 ). Therefore, the pH controlled mixture solutions need further downstream recovery processing especially when Ni0 nanoparticles are produced in a large scale. Possibly, further processing may become economically prohibitive, and the solutions can no longer be re-used or recovered and merely become waste products after the synthesis. This work focuses on the development of an environmentally benign method adapted to large scale production of Ni0 nanoparticles preferably in concentrated suspensions, thus preventing large generation of solvent wastes. Therefore, we have developed a novel method based on a key strategy which (i) encompasses an initial synthesis process from concentrated suspensions of commercial Ni hydroxide (Ni(OH)2) in EG and (ii) carries out the nickel reduction rapidly by coupling metal basic catalysis and MW heating, without any further additives or pH adjusting media. To further understand the outcomes of the proposed novel method for large scale production, this work studies the effect of temperature by MW assisted reduction in the formation of Ni0 nanoparticles. The solutions and resultant Ni0 nanoparticles are analyzed by microscopy and spectroscopy techniques involving XRD, SEM, TEM, FT-IR to elucidate the important parameters leading to particle size and reaction completion.

2. Experimental 2.1. Nanoparticle synthesis Nickel hydroxide (Ni(OH)2, 99% – AlfaAesar), and ethylene glycol (C2H4(OH)2, 98% – Sigma), were used for the synthesis of metallic nickel nanoparticles.

Homogeneous suspensions of Ni(OH)2 (0.3, 1.2, 2.4, 3.6 and 4.8 M) in ethylene glycol (EG) were prepared by mixing 20 g of EG with the corresponding amount of Ni(OH)2. In order to investigate the catalytic effect of the seeds on the formation of Ni0 nanoparticles, the suspension was seeded with either (i) Ni0 nanoparticles, using a molar ratio [Ni0]/[Ni(OH)2] = 102) or (ii) hexachloroplatinic acid (IV) hexahydrate PremionÒ (H2PtCl66H2O, 99.9% metal basis – AlfaAesar) using a molar ratio [H2PtCl66H2O]/ [Ni(OH)2] = 103. The suspensions were poured into high pressure TeflonÒ autoclaves fitting the MW reactor. A scientific MW oven (Milestone Ethos 1600) was used for the synthesis of Ni0 nanoparticles. Sols were irradiated for specific times (10–120 min) at temperatures in the range 160–260 °C, with a maximum MW power fixed at 300 W only in order to better control the temperature regulation at the set-point. All experiments have been made with 20 g of EG in closed pressure vessels under autogenic vapor pressure of the reaction mixture. Although the pressure has not been measured in the reactors, it increased with the synthesis temperature but never exceeded 25 bar (security limit). After the desired reaction time, the reaction mixture was cooled down to 60 °C prior to opening the autoclaves. The solid product was separated from the suspension by magnetic field and the liquid phase was recovered. The recovered solid was dispersed in water and placed into an ultrasonic bath to remove any remaining EG. The washed solid was dried in air at 80 °C for 2 h. 2.2. Characterization The recovered liquids were used for ATR-FTIR analysis (Nicolet NEXUS). The resulting particles were analyzed by FESEM (Hitachi 4800) operated at 5 kV and an X-ray analysis was performed using Panalytical X’Pert Pro diffractometer using Cu-Ka radiation (wavelength of 1.5418 nm) with 40 mA current and 40 kV voltage. Recorded XRD patterns were compared to the references in an ICDD X-ray diffraction database PDF2. To further support XRD studies, FT-IR spectroscopy (Nicolet NEXUS) was also used. The TEM studies were made on a JEOL JEM-2100F transmission electron microscope equipped with a slow-scan CCD camera and an accelerating voltage of the electron beam 200 kV. The preparation of samples for these analysis involved sonication in ethanol for 5 min and deposition on a copper grid. 3. Results 3.1. Effect of temperature XRD patterns of the derived powders using MW assisted heating in the temperature range 160–260 °C during one hour in presence of H6PtCl66H2O are shown in Fig. 1. Traces of the Ni(OH)2 precursor were detected in the temperature range 200 °C–220 °C as displayed by the X-ray diffraction lines at 2 19.01°, 32.96°, 38.47°, 51.74°, 58.81° and 62.68° (ICDD Ref. 01-1047). On the other hand, the peaks assigned to Ni0 are hardly detected below 200 °C (Fig. 1a–b), thus indicating that the reduction process leading to the formation of Ni0 requires temperatures higher than 200 °C. Pure Ni0 was the only product observed for the sample heated above 220 °C (i.e. at 260 °C) corresponding to the metallic nickel with the main diffraction lines at 2 44.42°, 54.85° and 76.27° (ICDD Ref. 04-0850). FT-IR analysis (results not shown) was also used to check the complete conversion of the hydroxide precursor. The IR absorption band at about 650 cm1, attributed to the dNi–O–H vibration, was used as a fingerprint for detecting the presence of unconverted

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*#

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*

#

(e)

(d) (c)

(b) (a) 10

20

30

40

50

60

70

80

2θ, ° Fig. 1. XRD patterns of reduced Ni(OH)2 in EG using MW-assisted heating for 60 min in the presence of Pt catalyst at: (a) 160 °C, (b) 180 °C, (c) 200 °C, (d) 210 °C, (e) 220 °C and (f) 260 °C (# – diffraction lines corresponding to cubic Ni0 – pattern ICDD 04-0850 and  to Ni(OH)2 – ICDD 01-1047).

precursor. Once no absorption band at 650 cm1 was detected (e.g. for the reaction at 260 °C) the Ni(OH)2 conversion was considered as completed (100% conversion). SEM micrographs of the particles obtained after reduction are shown in Fig. 2a–d. The morphology of the starting Ni(OH)2 precursor (Fig. 2a) corresponds to long needle-like particles. Upon reduction at 210 °C, the particle shape changed (Fig. 2b), though it still remains quite similar to the starting precursor. This result correlates well with the XRD data, a clear indication that this is a mixed material containing both the Ni(OH)2 precursor (i.e. elongated shape) and the Ni0 nanoparticles (i.e. round shape). However, further temperature increase to 260 °C leads to the formation of Ni0 nanoparticles as displayed in Fig. 2c and d and correlating to the formation of Ni0 nanoparticles according to the XRD analysis data (Fig. 1f). Interestingly, a longer dwell time (Fig. 2d) resulted in the formation of larger nanoparticles in the range 50–60 nm as compared to less than 20 nm nanoparticles for the short dwell time (Fig. 2c). Nevertheless, the calculation of crystallite sizes using the Scherrer equation gave the same results of 20 nm for both short and long dwell time. These results demonstrate that the synthesis time strongly influences the particle size whilst maintaining an average crystallite sizes to 20 nm. A longer dwell time led to the particle aggregation which is classically reflected by the effect of solid state sintering as reported elsewhere [23]. Complementary

TEM analysis was performed in order to further investigate the morphology of the particles (Fig. 2e and f). Both SEM and TEM analysis confirmed the presence of particles with two size populations. The larger particles are from 50 to 70 nm in diameter (Fig. 2e), while the smaller particles, corresponding to their non-aggregated analogues (more precisely crystallites within the larger particles) are 20 nm in size (Fig. 2f). Electron diffraction pattern is presented in Fig. 2f, confirming the presence of a crystalline material. In addition, it has been confirmed that the morphological particle transformation during extended reaction time (by aggregation of Ni0 individual crystallites or solid state sintering) had no influence on the product yield in the condition of complete nickel hydroxide conversion at 260 °C. A more detailed investigation of the influence of temperature and time on the physical properties of Ni particles, morphology and yield will be considered as a next step of the present work. Currently, these nanoparticle suspensions are considered as catalysts in biomass conversion [24]. 3.2. Catalyst activity As complete reduction of nickel hydroxide was achieved at 260 °C, further studies were carried out to investigate the role played by either the catalysts or the seeds in the formation of Ni0 nanoparticles as a function of time from 10 to 60 min. Possible reaction pathways were suggested by Shengming et al. [12] (formation of 2,3-butanedione (diacetyl), and eventually complete oxidation of EG to CO2 and H2O), and by Carroll et al. [25] who discussed the role of hydroxyl ions and the stability of the Ni-glycol complexes in the series: ethylene glycol, propylene glycol, diethylene glycol, tetraethylene glycol and butylene glycol. It was also shown in [25] that the polyol is playing a major role as an in situ capping agent and that various polyol chain length might in turn result in various particle morphologies. FT-IR spectra of the solutions in Fig. 3 reveal that the H6PtCl66H2O catalyst, which rapidly reduces to metallic Pt in the presence of EG under MW irradiation, acts as a catalyst for the oxidation of EG and release of electrons. Herein the formation of diacetyl, as an intermediate product is evidenced by the IR absorption bands at 1723–1645 cm1 (C@O stretching), 1126 cm1 (CH3 in-plane rocking) and 921 cm1 (CACH3 stretching). The oxidation of EG to diacetyl results in two electrons which become available for the reduction of Ni2+ to Ni0. The newly formed diacetal absorption bands are also observed when Ni0 nanoseeds were added in the starting EG solution. However, no changes were found for the samples prepared without any catalyst or nanoseeds. These results clearly indicate that both Pt and Ni0 nanoparticles catalyzed

b

a

600 nm

c

e

600 nm

d

600 nm

157

10 nm

f

600 nm

200 nm

Fig. 2. FESEM (a–d) and TEM (e and f) images of the particles formed in EG and using Pt catalysts: (a) the commercial Ni(OH)2 precursor, (b) the derived reduction products after 60 min at 210 °C, (c) 10 min at 260 °C, (d), (e) and (f) 60 min at 260 °C.

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+

++

+

(g) (f)

(e) (d) (c) (b) (a) 2000

1600

800

1200

cm-1 Fig. 3. FT-IR spectra of the solution irradiated by MW at 260 °C for 10 min (a) without additive, (b) with Ni0 nanoparticles, (c) with Pt catalyst, and for 60 min (d) without additive, (e) with Ni0 nanoparticles, (f) with Pt catalyst and (g) EG precursor only (+ corresponds to 2,3-butanedione).

the EG oxidation towards the reduction of nickel hydroxide precursor as reported elsewhere [26,27]. Further insight into the reaction process can be derived from the XRD patterns shown in Fig. 4. The sample without any catalyst or nanoseeds resulted in a lower conversion of Ni(OH)2 to Ni0 nanoparticles as displayed in Fig 4a which mainly correspond to the pattern of the nickel hydroxide precursor. The addition of nanoseeds resulted in an increase of the peak intensity corresponding to Ni0 (Fig. 4b). The XRD lines of nickel hydroxide completely disappeared for the samples prepared with the Pt catalyst (Fig. 4c). Therefore, only the Pt catalyst gave a complete conversion of nickel hydroxide to Ni0 in the first 10 min of reaction. The higher efficiency of the Pt catalyst on the reaction conversion is possibly related to its high dispersability in the sol and easier diffusion to the precursor, in comparison with Ni0 nanoparticles (seeds). However, MW-assisted treatment with a longer exposure time (60 min) delivered interesting outcomes as full conversion of nickel hydroxide to Ni0 was

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(f)

*#

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*

observed irrespective the catalyst type, or the presence of nanoseeds, even without any catalyst or seeds as shown in Fig. 4d and f. The proposed MW-assisted method proved to be successful for the reduction of nickel hydroxide sols without any other additives. To further understand the nickel formation mechanisms, additional tests were carried out by varying nickel hydroxide concentration in EG (0.3, 1.2, 2.4, 3.6 and 4.8 M). The XRD patterns in Fig. 5 show that total conversion of nickel hydroxide to Ni0 was observed for concentrations in the range 0.3–1.2 M. The Ni0 nanoparticle conversion rate for 1.2 M concentration is at least one order of magnitude higher for the MW-assisted process as compared with conventional solvothermal methods (Table 1). However, for concentrations at or in excess of 2.4 M, non-reduced Ni(OH)2 precursor was detected, clearly indicating the inhibiting effect of precursor concentration. For the 2.4 M and 4.8 M Ni(OH)2 suspensions, the EG amount was high enough for reducing the precursor to Ni0 in both cases. Purely stoichiometric considerations cannot explain the limited reaction yield obtained for these highest suspension concentrations. This could be attributed to diffusion limitations at high precursor concentrations, as increasing Ni(OH)2 aggregation hinders EG access to the surface of individual nickel hydroxide particles under static reaction conditions (no stirring was applied as the MW oven was not equipped with stirrers). In all experiments, partial sedimentation of both the hydroxide precursor and derived product(s) has been observed, which increased with the suspension concentration. For the highest suspension concentration, the sedimentation layer was thick enough to limit the EG access and reaction. Such limitation could be possibly overcome by using a mechanical stirring, i.e. an optional modification of the MW reactor design. 3.3. Optimization of ethylene glycol consumption By bearing in mind the need for zero hazardous waste emissions to comply with environmentally benign processes, the recycling of EG was considered. The process of recovery of the unreacted EG was implemented for the optimised sample with the MW-assisted thermal treatment at 260 °C (60 min and 1.2 M). The unreacted EG after separation of the Ni0 nanoparticles was re-used for further three consecutive reduction reactions. XRD patterns of the derived solid products and FT-IR spectra of the associated liquids obtained after each cycle are displayed in

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2θ, ° 10

Fig. 4. XRD patterns of the solid products from Ni(OH)2 reduction with EG either after 10 min reaction: (a) without additive, (b) with Ni0 nanoseeds, (c) with Pt catalyst, or after 60 min reaction, (d) without additive (e) with Ni0 nanoseeds and (f) with Pt catalyst. All samples were exposed to MW irradiation at 260 °C and 300 W (# – diffraction lines corresponding to cubic Ni0 – pattern ICDD 04-0850 and  to Ni(OH)2 – ICDD 01-1047).

20

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80

2θ,° Fig. 5. XRD patterns of the products derived from the reduction of different amounts of Ni(OH)2 in EG: (a) 0.3 M, (b) 1.2 M, (c) 2.4 M, (d) 3.6 M and (e) 4.8 M, after 1 h reaction at 260 °C. (# – diffraction lines corresponding to cubic Ni0 – pattern ICDD 04-0850 and  to Ni(OH)2 – ICDD 01-1047).

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Table 1 Literature data comparing Ni production methods by solvothermal reduction in either EG or ethanol. The results reported in this work are also reported. MW and CL refer to microwaves and classical heating respectively. PVP and DDA correspond to polyvinyl-pyrrolidone and n-dodecylamine respectively. Ref.

Ni precursor

pH control

a

c d

Solvent Seed or Reducing catalyst agent

PVP –

EG – Ethanol –

Concentration, M

[17] [15]

Ni(OH)2 NiCl26H2O

0.15 0.20

[16] [21]

NiCl26H2O 0.25 Ni(CH3CO2)24H2O 0.0016–0.019

NaOH, Na2CO3 PVP EG – PVP, DDA EG

– Pta

[13]

Ni(OH)2

0.14

H2SO4

PVP

EG

0.0036–0.143 1.2

NaOH –

– –

EG EG

– PtaPdb Agc – Pta Nid

[19] NiSO46H2O This work Ni(OH)2

b

Steric stabilizer

Formula

– NaOH

Synthesis conditions Heating method Duration Product (temperature)

EG MW N2H4H2O CL

5 min 2h

N2H4H2O MW EG CL (195 °C) MW (195–200 °C) EG CL (195 °C)

1 min 2–17 h 45 min 12 h

N2H4H2O CL (100 °C) EG MW (260 °C)

1h 1h 10 min 1h

Ni crystallite size (nm)

Ni/Ni(OH)2 4–12 Ni 150 (at 60 °C) 50 (at 180 °C) Ni 5–79 Ni 20–120 Ni Ni 135 Ni 30 Ni 43–91 Ni 20 Ni 20 Ni 20

Hexachloroplatinic acid. Palladium acetylacetonate. Silver acetate. Nickel nanoparticles.

(A)

(B)

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+

EG

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60

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80 2000

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cm-1

Fig. 6. XRD patterns (A) of the Ni0 nanoparticles derived from Ni(OH)2 reduction, at 260 °C for 60 min, and FT-IR spectra (B) of the recovered liquid phase after reduction: (a) EG after one reaction, (b) the first recycling and (c) second recycling cycles (# – diffraction lines corresponding to cubic Ni0 – pattern ICDD 04-0850 and FT-IR + corresponds to 2,3-butanedione).

Fig. 6A and B, respectively. The results in Fig. 6A clearly show that the recycled EG was still effective as a reducing agent in all the three cycles, leading to the complete conversion of the nickel hydroxide precursor to Ni0 nanoparticles. SEM observations of the Ni0 nanoparticles obtained in recycled EG were similar to those shown in Fig. 2d and did not reveal any significant changes in their size or morphology. FT-IR analysis of the liquid phase recovered after each reduction cycle revealed absorption bands of the 2,3-butanedione with gradually increasing intensities. Simultaneously, the bands attributed to ethylene glycol decreased, thus the redistribution of the intensity of EG and 2,3-butanedione bands was progressively more pronounced after each cycle. However as reported in [12], the as-formed solution remains still an active medium for the nickel hydroxide reduction, as in parallel to ethylene glycol, 2,3-butanedione becomes a source of electrons until its complete oxidation to CO2 and water. A schematic representation of the whole MW-assisted process leading to the formation of Ni0 nanoparticles is shown in Fig. 7. The combination of continuous MW irradiation and high temperature favors a fast local dissociation of Ni(OH)2 to OH and Ni2+ ions (Eq. (1)) by a reversible ongoing process resulting in high concentration of these ions on the surface of nickel hydroxide particles. Due to the formation of these ionic species in the reaction mixture, then MW energy can be transferred via ionic conduction.

Fig. 7. Schematic representation of the production of Ni0 nanoparticle from Ni(OH)2 and EG.

The movement of ions in the electric field generates heat, which also affects the transfer of energy. The higher the temperature of the reaction mixture, the more efficient the transfer of energy becomes. MW

2þ

þ 2OH

NiðOHÞ2 ! Ni

ð1Þ

ð2Þ 2þ

Ni

0

þ 2e ! Ni

ð3Þ

ð4Þ ð5Þ 

At these conditions, the mobile OH species lead to a high pH (10.25) in the solution around the surface of nickel hydroxide particles [12] and serve as a catalyst for the primary dissociation of ethylene glycol to diacetyl. This reaction leads to a release of 2

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electrons (Eq. (2)), which are subsequently consumed in the reduction of Ni2+ to Ni0 (Eq. (3)). Despite of the high temperature and MW irradiation, the basic catalytic conversion of EG to diacetyl remains a reaction limiting step resulting in a slow formation of metallic nickel within the first 10 min at 260 °C (Fig. 4a). Although this reaction step has slow kinetics, the formation of Ni0 nanoparticles provides favorable environment for further reduction of Ni2+, attributed to the surface catalytic activity of Ni0 nanoparticles which are in essence the same as Ni0 nanoseeds. As the reaction proceeds further with time, the influence of basic catalysis (OH) resulting in EG (HOACH2ACH2AOH) oxidation to 2,3-butanedione (CH3ACOACOACH3) (Eq. (4)) becomes negligible in comparison with surface catalysis. Then surface catalysis on the formed Ni0 nanoparticle becomes the principal driving force for EG oxidation, producing electrons for the reduction of nickel hydroxide. Under optimal reaction conditions (260 °C), the formation of pure Ni0 nanoparticles (Eq. (5)) can be completed within 60 min, as confirmed by the XRD results in Fig. 4d. Batch or semi-continuous batch processes are the considered options for up-scaling Ni0 nanoparticles production by microwave solvothermal synthesis. In addition to the development of adapted technological devices, such as those reported in [28], a key point motivating an up-scaling of the synthesis process is the possibility to obtain almost 100% conversion within 60 min, using high suspension concentration (up to 1.2 M) and the possibility to recycle the EG. This is a strong originality of the present work. 4. Conclusions This work demonstrates a novel environmentally friendly process, without need of pH adjusters (acid or bases) and viscosity controllers, to synthesize Ni0 nanoparticles, based on Ni(OH)2 reduction by ethylene glycol. This process is driven by a fast heating rate and high temperature supplied by MW irradiation, contrary to conventional processes using catalysts. Although the use of catalysts triggers a fast reduction of nickel hydroxide, the MW-assisted reduction dispenses the use of expensive and metal Pt based catalysts. In this MW-assisted process, the reduction of the nickel hydroxide precursor is accelerated by the in situ formation of Ni0 nanoparticles, which serve as catalytic domains like nanoseeds. Hence, the MW reduction is combined with surface catalysis activity of the product. Additionally, temperature plays a key role in the process for completing the reduction of Ni(OH)2 to nickel nanoparticles. Interestingly, the MW-assisted strategy at 260 °C resulted in a complete conversion of highly concentrated suspensions (1.2 M Ni(OH)2) to Ni0 nanoparticles within 60 min. Such concentration is at least one order of magnitude higher than those used in conventional solvothermal method. The recycling of EG as a reaction medium proved to be successful over three consecutive reduction cycles with the production of more Ni0 nanoparticles at each cycle. Finally, the findings in this work lay the foundation for an environmentally friendly method to achieve a scale-up industrial production of Ni0 nanoparticles. Acknowledgements Authors acknowledge both the CNRS (PIE Energy ProgrammeProject #PRC08-2.5-1 MEM-sTiMULHY) and the Languedoc-Roussillon region (ARPE program) for financial support. Didier Cot and Dr. Arie van der Lee, from the IEM technical staff, are sincerely acknowledged for their contribution in FESEM and XRD analysis, respectively. Also the authors acknowledge the facilities, and the scientific and technical assistance, at The University of Queensland, provided by the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy and Microanalysis.

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