NiOx Core

DOI: 10.1002/cphc.201601181

Articles

Laser-Assisted Synthesis of Colloidal Ni/NiOx Core/Shell Nanoparticles in Water and Alcoholic Solvents Niusha Lasemi,[a] Ulrich Pacher,[a] Christian Rentenberger,[b] Oscar Bomat&-Miguel,[a] and Wolfgang Kautek*[a] The nanosecond-pulse laser-assisted generation of Ni/NiOx core/shell nanoparticles (NPs) in water and alcoholic fluids can yield colloidal solutions without surfactants. The size distribution can be controlled by the nature of the alcohol, the number of laser pulses and the laser fluence. The incubation of the nickel target ablation in liquid contact shows a dependence on the carbon number of the respective alcohol. The laser-generated NPs consist of crystalline nickel cores with face-centred cubic patterns and stacking fault defects surrounded by nickel oxide shells. The solvent butanol, in contrast to ethanol and

isopropanol, yields a narrow, nearly unimodal, size distribution. The majority of NPs have low size distributions, with medians in the range of 10–20 nm. These can be related to a metal ablation plume interacting with a supercritical liquid that decelerates the ejected material in a low-density metal–water mixing region. NPs in the range above 30 nm result in a minority distribution tail that strongly depends on the fluid nature, the pulse number and the fluence. This coarse NP set may be correlated with the rupture of a superheated molten-metal layer into larger entities.

1. Introduction Laser-assisted generation of metal nanoparticles (NPs) in liquids promise high material purity that cannot be reached by conventional manufacturing routes for colloidal solutions, since no chemical precursors are required.[1] Size, shape and surface chemistry of the particles can be controlled by the laser parameters and the nature of the fluid. The complex mechanism involves a series of steps extended over many orders of magnitude in time involving, for example, ablation, plasma expansion inside a gas bubble, the penetration of condensed nanosized phases into the liquid and secondary beam– colloid interactions.[2] A nickel colloidal solution can be produced conventionally.[3] The aim of the present study was to laser-generate pure nickel colloidal aqueous solutions with controlled size distribution (polydispersity), shell nature and crystallography. Nickel NPs are of interest because they can serve as a catalyst to control the growth of helical coiled carbon fibres (CCFs).[4] Furthermore, nickel NPs supported on carbon can be used as a lowcost catalyst for hydrolysis of ammonia borane.[3b] Nickel NPs can also serve as a catalyst for hydrogenation of p-nitrophenol to form p-aminophenol, which has a variety of industrial applications.[5] Interestingly, nickel–copper alloys show high catalytic

reactivity for methane decomposition.[6] Nickel NPs can also act as nanoantibiotics to prevent antibiotic resistance.[7] Incubation represents a dominating phenomenon in industrial laser machining, in contrast to fundamental single-pulse studies, for which the target composition is defined. There have been few attempts to quantify the influence of the target material conversion in a multi-pulse experiment by a phenomenological and a physical model.[8] No incubation investigations exist so far for solid–liquid interfaces. Laser generation of metallic NPs mostly result in core–shell structures.[9] The nature of the shells is controlled by the solvents. Water and ethanol, for example, lead to oxide shells,[10] whereas solvents with a higher carbon content, such as butanol or toluene, result in carbonaceous shells, for example, graphite.[9, 11] Herein, the incubation behaviour of nickel was studied in air and various liquid media with nanosecond laser pulses at l = 532 nm. The influence of the liquid nature, laser fluence and number of pulses on the polydispersity and crystallinity of the nickel NPs was evaluated.

[a] N. Lasemi, U. Pacher, Dr. O. Bomat&-Miguel, Prof. Dr. W. Kautek Department of Physical Chemistry, University of Vienna W-hringer Strasse 42, A-1090 Vienna (Austria) E-mail: [email protected]

2. Results and Discussion The squared diameter of the ablated zone D2 is related to the Gaussian beam radius (w0), the pulse energy (E0) and the threshold pulse energy (Eth), with the assumption of a Gaussian beam profile[8a,b] [Eq. (1)].

[b] Prof. Dr. C. Rentenberger Faculty of Physics, University of Vienna Boltzmanngasse 5, A-1090 Vienna (Austria) The ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/cphc.201601181. An invited contribution to a Special Issue on Nanoparticles with Lasers

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D2 ¼ 2 w 0 lnðE 0 =E th Þ

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ð1Þ

T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Articles An average w0 was considered to calculate the peak fluence F0 [Eq. (2)]: F 0 ¼ 2 E 0 =pw 0 2

Table 2. Incubation behaviour.

ð2Þ

Equation (1) can be rewritten in terms of the fluence, F0, and the threshold fluence, Fth [Eq. (3)]: D2 ¼ 2 w 0 lnðF 0 =F th Þ

Fluid

Incubation coefficient (x)

air water ethanol isopropanol butanol

0.96 0.52 0.77 0.70 0.52

ð3Þ

The results of nickel ablation are depicted for various contact phases, such as air, water, ethanol, isopropanol and butanol. The D2 versus logF0 relationship [Eq. (3)] for the case of water is presented in Figure 1. The threshold results according to Equation (1) for all fluids are collected in Table 1.

Figure 1. Squared diameter of the ablation area versus pulse fluence (D2 versus F0) in water. ^: N = 50, &: N = 100, *: N = 200, ~: N = 500, *: N = 1000.

Table 1. Threshold fluence (Fth) of nickel targets in various media and with different numbers of pulses (N, shown in parentheses).

Medium

Fth(50) [J cm@2]

Fth(100) [J cm@2]

Fth(200) [J cm@2]

Fth(500) [J cm@2]

Fth(1000) [J cm@2]

air water ethanol isopropanol butanol

1.06 5.92 5.66 8.40 6.29

0.84 4.11 4.39 7.10 4.66

0.85 2.67 3.60 5.91 3.26

0.72 2.02 3.10 4.45 2.37

0.97 1.36 2.75 3.33 1.39

The incubation behaviour, which is the threshold fluence as a function of the pulse number, Fth(N), is given by Equation (4):[8a,b] ð4Þ

F th ðNÞ ¼ F th ð1ÞNx@1

with a material-dependent incubation coefficient, x. If x = 1, no incubation occurs, and x < 1 means finite incubation. The results for all fluids are summarised in Table 2. ChemPhysChem 2017, 18, 1118 – 1124

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Classical heat accumulation due to repetitive pulsing does not take place at a rate of 20 Hz. This phenomenon only comes into play in the multi-kHz range on metals[12] and above 100 kHz on dielectrics.[13] Water leads to a higher incubation in respect to air. Possible reaction products, such as oxide formation, would not affect the radiation absorption. Therefore, the possibility of increasing roughening with cavitation under water may be considered as a cause for increasing radiation coupling with increasing N. The stronger incubation under water versus ethanol may be correlated with the drastically higher surface tension (g) of water, and thus, stronger cavitation impact, leading to more morphological alterations. Finite incubation of the metal in air does not show any correlation with the optical properties of the substrate, in contrast to dielectric materials.[8c, 14] This may be correlated with the formation of increasing thermally insulating solid phases that exhibit nanobubbles.[15] Further research on this matter is underway. Moreover, an increase of incubation (decreased x) with increasing number of carbon atoms from three to four in the solvent molecule could be observed (Table 2). A correlation with the surface tension or density is negligible (see Table 7, below), so that mechanical causes related to bubble collapse seem improbable. Solvents with a higher number of carbon atoms[9, 11] generally show pyrolysis on hot metal surfaces. One can assume that each additional laser pulse leads to more carbonaceous products on the metal target; thus increasing the absorption at the metallic substrate interface, resulting in what can be understood as incubation. Similar incubation coefficients of water and butanol seem to be coincidental because possible mechanisms based on modification layers should exhibit different chemistry (e.g. oxygen versus carbon). TEM was used for the size distribution evaluation of the NPs. A representative image and a respective histogram are shown for ethanol (Figure 2). Curve fitting of Ni NPs in ethanol (N = 1000 and F0 = 663 J cm@2) led to median and mean values that indicated three modes (Table 3). The most abundant, including all data, exhibited a median of around 17 nm. Minor modes could be separated in the tail at higher sizes. EDX analysis of a NP shows mainly Ni (Figure 3 A). The supporting matrix is dominated by the siloxane glue (Si and O; Figure 3 B). The selected-area electron diffraction (SAED) pattern with a contrast aperture at position 3 A and the related dark-field image of a NP laser-synthesised in ethanol is depicted in Figure 4. It is characterised by a [222] face-centred cubic (fcc) pattern. The dark-field image of one nickel NP in position 3 A

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Figure 2. TEM image and size distribution (number-weighted) of laser-synthesised NPs (ethanol, N = 1000 and F0 = 663 J cm@2).

Figure 4. SAED pattern with a contrast aperture at position 3 A and the related dark-field image (A) of a laser-synthesised NP (ethanol, N = 1000, F = 663 J cm@2).

Table 3. Curve-fitting results of number-weighted NP size distribution (ethanol, N = 1000 and F0 = 663 J cm@2).

Log-normal curve fitting

Median [nm]

Mean [nm]

Standard deviation

1 2 3

16.9 : 0.5 28.7 : 0.5 44.4 : 0.5

22.6 : 0.5 40.4 : 0.5 45.6 : 0.5

19.97 19.94 10.24

The respective data of NPs generated in butanol are presented in Figures 5 and 6. The size distribution curve was fitted by both Gauss and log-normal fitting curve methods (Table 5). In contrast to ethanol and isopropanol, lower standard deviations, lower polydispersity and nearly unimodal size distributions were observed. The observation that a higher

exhibits a stacking fault (SF) defect typical for fcc systems (Figure 4 A). The SAED patterns (Figure 4), crystallographic data and phase identifiers are summarised in Table 4. Table 4. SAED patterns, crystallographic data and phase identifiers of NPs (ethanol, N = 1000, F0 = 663 J cm@2).

Material

Crystal system

Miller indices

Pearson symbol

Space group

Space group number

Ni Ni NiO NiO2 Ni

cubic cubic – – cubic

[111] [200] [111] [202] [222]

cF4 cF4 mS4 mS6 cF4

Fm3¯m Fm3¯m C2/m P3¯m1 Fm3¯m

225 225 12 12 225

Figure 5. TEM image and size distribution (number-weighted) of laser-synthesised NPs (butanol, N = 1000 and F0 = 663 J cm@2). Pink: log normal. Green: Gauss.

Figure 3. EDX analysis of laser-synthesised NPs (ethanol, N = 1000, F0 = 663 J cm@2). Positions: A) large NP, B) matrix site.

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Figure 6. EDX analysis of laser-synthesised NPs (butanol, N = 1000, F0 = 663 J cm@2). Positions: A) large NP, B) matrix site.

Table 5. Curve-fitting results of number-weighted NP size distribution (butanol, N = 1000 and F0 = 663 J cm@2).

Curve fitting

Median [nm]

Mean [nm]

Standard deviation

log normal Gauss

12.5 : 0.5 11.5 : 0.5

14.5 : 0.5 11.5 : 0.5

8.27 5.27

carbon chain length causes no multimodal size distributions may be due to the hindrance of coalescence, a reduction of particle diffusion and possibly Ostwald ripening processes.[9a, 16] Results of the SAED analysis (Figure 7 and Table 6) show diffraction patterns of both crystalline Ni and NiO2 NPs with vari-

Table 6. SAED pattern results, crystallographic data and phase identifiers of NPs in (butanol, N = 1000, F0 = 663 J cm@2).

Material

Crystal system

Miller indices

Pearson symbol

Space group

Space group number

Ni NiO2 NiO2 NiO2 Ni Ni

cubic – – – cubic cubic

[111] [200] [111] [201] [220] [311]

cF4 mS6 mS6 mS6 cF4 cF4

Fm3¯m P3¯m1 P3¯m1 P3¯m1 Fm3¯m Fm3¯m

225 12 12 12 225 225

ous crystallographic orientations. Dark-field images were achieved by the electron beam tilting technique over the specified yellow ring. The typical contrasts originate from various atomic orientations of the NPs (Figure 7 A–D). The present results in water, ethanol and propanol show at least two modes of NP sizes with a median of the majority of NPs around 10 nm (Figure 2 and Table 3). The deviation of the mean values approximately indicates a second minority mode at several 10 nm sizes. Butanol, however, represents an excepChemPhysChem 2017, 18, 1118 – 1124


Figure 7. Bright-field image and SAED pattern with contrast aperture at different positions (A–D) along the circumference of the largest ring with related dark-field images (butanol, N = 1000 and F0 = 663 J cm@2).

tion with almost only one median (mean around the 10 nm value (Figure 5 and Table 5).

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Articles Time-resolved small-angle X-ray scattering (SAXS) probing of the cavitation bubble dynamics yielded two different NP populations at different stages of bubble evolution.[17] The present findings are in accordance with a recent concept[2b] of two NPgeneration mechanisms in liquid contact. They yield NPs with different characteristic sizes at different stages of the ablation process. In the first, the metal ablation plume interacts with the fluid, leading to the rapid deceleration of the ejected material and the formation of a dense superheated molten-metal layer at the water–plume interface. Water is in a supercritical state and transforms into an expanding upper low-density metal–water mixing region. This serves as a precursor for the formation of a cavitation bubble. The low-density metal–water mixing region is the origin of metal condensation into NPs in the range not much more than 10 nm. The second mechanism is the rupture of a superheated molten-metal layer located below the low-density metal–water mixing region. Complex morphological features are generated due to deceleration of the superheated molten-metal layer by supercritical water. Their disintegration leads to greater NP sizes (several 10 nm) than those particles condensing in the upper low-density metal–water mixing region. The nature of the fluid has a strong influence on the size distribution (Table 7 and Figure 8). A higher carbon number of

number alcohols.[9b, 10a] The formation of such shells could not be directly supported on the basis of bright- and dark-field TEM, SAED patterns, and EDX results, reported herein. Investigation with high-resolution (HR) TEM will have to resolve this issue in the future. The increase of the number of pulses, N, and fluence, F0 (Figure 9), has a drastic influence on the size distribution (Table 8). The median size and size distribution width increased with higher F0 and N. This is in accordance with observations in laser generation of copper oxide[20] and zinc oxide NPs.[21]

Figure 9. Histogram of median size diameters of NPs versus laser fluences in ethanol at N = 1000.

Table 7. The influence of fluids on the size of laser-synthesised NPs (N = 1000 and F0 = 663 J cm@2). The curve fitting is log normal.

Material water ethanol isopropanol butanol

[18]

Table 8. The influence of fluence and number of pulses on the final size of laser-synthesised NPs in ethanol. The first fitting curve is log normal.

[18b, 19]

Median [nm]

Mean [nm]

Standard deviation

Density [kg m@3]

g [mN m@1]

13.8 : 0.5 16.9 : 0.5 13.4 : 0.5 12.5 : 0.5

17.7 : 0.5 22.6 : 0.5 16.8 : 0.5 14.5 : 0.5

14.17 19.97 12.84 8.27

998 789 785 809

72 22.4 21.7 24.6

Figure 8. Histogram of the median size diameters of NPs generated in various fluids (N = 1000, F0 = 663 J cm@2).

the alcohols led to a decreased higher size mode. This may be controlled by increasing solvation and stabilisation of the colloidal particles with higher carbon number alcohols.[16] Another explanation of the stabilisation of smaller sizes may be found in the carbonaceous shell formation of NPs with high carbon ChemPhysChem 2017, 18, 1118 – 1124


F0 [J cm@2]

N

Median [nm]

Mean [nm]

Standard deviation

840 840 663 486 309

100 1000 1000 1000 1000

13.6 : 0.5 20.6 : 0.5 16.9 : 0.5 11.8 : 0.5 10.6 : 0.5

14.6 : 0.5 26.9 : 0.5 22.6 : 0.5 17.1 : 0.5 12.8 : 0.5

5.74 22.74 19.97 17.85 8.71

Increasing fluence generally results in an increase of the large size fraction of NPs (Table 8). This may suggest influences of either primary ablation processes or secondary types, such as particle growth in the solution due to higher NP concentration, and therefore, faster coalescence and Ostwald ripening.[16b] On the other hand, a primary ablation mechanism may explain the results. Only a low-density metal–water mixing region is generated at moderate fluences, yielding small NPs, whereas a superheated molten-metal layer gets disintegrated to form larger NPs at higher fluences.[2b] An analogous explanation may hold for the increase of the median with increasing N (Table 8). A higher N results in an increased NP concentration, and therefore, an elevated probability of coalescence and Ostwald ripening.[9b, 16b] Moreover, increasing N connected with deeper cavities may lead to the expulsion of larger melt splashes.[22]

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Articles 3. Conclusions

ance of any optical fluctuations due to the movement and accumulation of droplets or bubbles.

The nature of the alcohol, number of laser pulses and fluence controlled the nickel NP size distribution. The incubation of the nickel target ablation in liquid contact showed a dependence on the carbon number, particularly from three to four in the solvent molecule. This may be caused by alcohol pyrolysis on the laser-irradiated target area. The thus-increased interfacial absorption of the generated carbonaceous conversion layer may result in a decreased ablation threshold fluence and a finite incubation phenomenon. TEM images and SAED patterns indicated crystalline nickel cores with fcc patterns and SF defects surrounded by nickel oxide shells. The solvent butanol yielded the narrowest, nearly unimodal, size distribution in contrast to those of ethanol and isopropanol. Clearly, a longer carbon chain length hinders coalescence, particle diffusion, Ostwald ripening processes and a multimodal size distribution. Two major modes of NP sizes with medians of 10–20 nm and above 30 nm were observed in water, ethanol and propanol. Butanol, however, represented an exception with almost only a median around the 10 nm value. This is in accordance with a model correlating these modes with two stages of the ablation process. In the first, yielding NPs around 10 nm, a metal ablation plume interacts with the supercritical fluid, which decelerates the ejected material in a low-density metal–water mixing region. The second stage generates greater sizes and relies on the rupture of a superheated molten nickel layer below the low-density metal–water mixing region. Increasing fluence mainly increased the share of the larger NPs generated by the rupture of superheated molten-metal layers. However, secondary mechanisms, such as NP coalescence and Ostwald ripening, may also be considered.

A Q-switched Nd:YAG laser system was employed by emitting at a wavelength of l = 532 nm (Spectra Physics GCR-130, , 1.2 W, pulse duration 5 ns, repletion rate 20 Hz, beam diameter ca. 5 mm) was employed. The power meter (OPHIR Photonics) was positioned after the polariser (THORLABS). The focus position in air and various liquid media were experimentally evaluated by microscopically measuring the ablation area on a silicon target (Zeiss AxioVision software) as a function of the distance of the focusing planoconvex lens (focal length 92 mm). A depth of focus of 1.5 mm in air was calculated from the Raleigh distance.

Experimental Section Laser ablation of nickel targets (Alfa Aesar; thickness 2 mm; purity 99.5 %) were submersed in various liquids (water, ethanol, butanol and isopropanol; Sigma–Aldrich) in a closed glass vessel with a lateral optical window (Figure 10). The vertical position of the targets, together with a horizontal beam delivery, guaranteed the avoid-

The size distribution and SAED patterns of the laser-synthesised nickel NPs were studied by TEM (Philips CM200 TEM; LaB6 cathode, acceleration voltage of 200 kV). The TEM images and SAED patterns were recorded by a charge-coupled device (CCD) camera. The size distribution was evaluated from at least six TEM frames (700 V 700 nm) by using microscopy software (Gatan, Inc.). The analysing parameters of the SAED patterns were calculated by intensity profile analysis selected area diffraction (PASAD; University of Vienna, C. Gammer)[23] and microscopy software (Gatan, Inc.). Crystallographic data and phase identification of Ni NPs were achieved by using the AtomWork database.[24] Chemical composition investigations were performed by EDX analysis in connection with TEM. The Ni NP samples for the TEM investigation were prepared by placing a droplet of the colloidal dispersion (after 5 min ultrasonic irradiation) on a carbon-coated copper grid followed by solvent evaporation in air at room temperature.

Acknowledgements Partial financial support by the Austrian Research Association (:FG) and the H2020 Action MSCA-IF-EF-ST (no. 656908) is gratefully acknowledged.

Conflict of interest The authors declare no conflict of interest. Keywords: alcohols · colloids · laser chemistry · nanoparticles · nickel

Figure 10. Schematics of the laser setup: 1) motorised XY scanning stage, 2) glass vessel with optical window, 3) plano-convex lens, 4) reflecting mirror, 5) polariser with a half-wave plate, 6) reflecting mirror, 7) Q-switched Nd:YAG laser, 8) power supply, 9) pulse generator, and 10) control computer.

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Manuscript received: October 28, 2016 Revised: December 30, 2016 Accepted Article published: January 2, 2017 Final Article published: February 16, 2017

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