Introduction

Over the last 10 years, synthesis of Ni and Ni oxide NPs has received significant attention owing to the immense potential of these materials in a broad range of applications (Ditlbacher et al. 2000; Boschloo and Hagfeldt 2001; Tartaj et al. 2003; Zbroniec et al. 2004; Bernand-Mantel et al. 2006), which is due to their unique properties including magnetic, catalytic, and electronic properties. The synthesis methods for NPs include well-established processes such as wet-chemistry, chemical vapor deposition, physical vapor deposition, ion sputtering, pyrolysis, sol–gel, or flame synthesis (Gleiter 1989; Swihart 2003; Willard et al. 2004). However, the final products often contain undesirable impurities and normally require secondary treatment.

Laser ablation of a solid target immersed in a liquid has become an increasingly important technique for the fabrication of nanostructured materials (Fojtik and Henglein 1993; Neddersen et al. 1993; Mafune et al. 2000a, b; Tsuji et al. 2003). This method was originally used to produce noble metal NPs that cannot be achieved by wet chemical techniques due to significant surface contamination from residual anions and reducing agents, leading to the need for secondary treatments (Sibbald et al. 1996; Prochazka et al. 1997). Laser ablation in liquid (LAL) leads to various chemical reactions between the ablated species and molecules in liquid, as the ejected species are highly excited both electronically and translationally (Sakka et al. 2000). Indeed, during laser ablation of chemically reactive materials, such as Al, Cu, Ti, Si, chemical reactions and crystalline phase transformations have been observed to form various metal oxide NPs in a single step (Sasaki et al. 2004; Golightly and Castleman 2006; Sasaki et al. 2006). Although a substantial amount of work has been reported on the generation of Ni and NiO NPs by laser ablation in vacuum or gaseous environments (Zbroniec et al. 2004; Zbroniec et al. 2005; Liu et al. 2007), little effort has been made to investigate the laser ablation of Ni under liquid. Recently, Mahfouz et al. (2008) reported on the generation of NiO NPs with an average size of 3–5.3 nm, by PLAL of Ni submerged in water using an Nd:YAG laser. The generation of Ni metallic phase NPs were also reported by PLAL. Kim et al. (2005) generated spherical Ni metallic phase NPs with an average size of 5–6 nm, by irradiation of Ni foils using an Nd:YAG pulsed laser in water and 0.01 M SDS. Jin and Lan (2008) generated Ni and bi-metallic NPs by PLAL in organic solution using Nd:YAG laser, with the average size of Ni NPs being 8 nm. No work has been reported on the generation of Ni oxide NPs using continuous wave (CW) laser ablation in liquid.

We have recently demonstrated the generation of TiO2 NPs by using CW laser ablation in liquid (CWLAL) (Abdolvand et al. 2007; Abdolvand et al. 2008). The detailed mechanism involved in generation of the NPs by CWLAL has been reported elsewhere (Abdolvand et al. 2008).

In this paper, we report on the generation and characterization of NiO NPs by CWLAL. The NPs produced were characterized in terms of size distribution, shape, chemical composition, and phase structure using XRD and various transmission electron microscopic (TEM) techniques.

Experimental procedure

A Ni plate (99.99% in purity, 0.5 mm in thickness) was fixed at the bottom of a glass vessel filled with 8 mL of liquid with a ~3 mm liquid layer over the top surface of the metal plate. The liquid used was either deionized water or aqueous SDS solutions with a concentration of 0.001 M, 0.01 M, and 0.1 M. Laser processes were carried out using a high-power, high-brightness, single-mode YLR-1000-SM 1 kW fiber laser source (IPG Photonics) at a wavelength of 1,070 nm. The output of the fiber laser, here ~250 W, entered the solution from above at normal incident angle and was focused on the target to a spot 40 μm in diameter. The target was irradiated continuously for 1 s. The process was then repeated five times on different locations on the surface of Ni to ablate sufficient material for the subsequent characterization.

After laser ablation, the particles produced formed colloidal solutions, which were then collected by microcentrifuging at 14,800 rpm for approximately 8 min. This operation was repeated a number of times. The colloidal solution containing the particles and SDS was rinsed with deionized water to remove SDS. Microcentrifuging with such a high rotation speed resulted in a small amount of visible sediment, which was collected and dried on a silicon substrate for handling. Since NPs with a diameter less than 5 nm are difficult to precipitate by centrifugation (Mafune et al. 2003), approximately 1 mL of the colloidal solution was gradually dried on the silicon substrate over a long period of time (24 h). The phases of the particles prepared above were measured by XRD using an X-ray diffractometer (Phillips, PW3710), with CuKα radiation. To prepare samples for TEM analysis, a few drops of colloidal solution containing the particles were placed on a carbon-coated copper microgrid. The microgrids were then desiccated at room temperature. The same process was repeated 4 to 5 times, to ensure enough NPs were placed on the grid. The size and shape of the particles were examined using TEM (JEOL 2000), FEI (Tecnai, F30), and high-resolution TEM (HRTEM). Chemical composition was analyzed using HRTEM, high angle annular dark field (HAADF) in scanning-transmission (STEM) mode, and energy-dispersive X-ray spectroscopy (EDX). Crystallinity and phase structures of the NPs were further identified using selected area electron diffraction (SAED) and HRTEM.

Results and discussion

Nanoparticle morphological observation

It is known that NPs generated during LAL do vary in size and shape. NPs can be spherical, cubical, hexagonal, etc., depending on the favorable growth plane during the formation. In our work, the particles generated by CWLAL in water and SDS solutions, as shown in the TEM images in Fig. 1a–d, were a mixture of different sizes and shapes. In water, spherical particles dominated, while an increase in the concentration of SDS decreased the number of spherical particles. When the SDS concentration was 0.1 M, most of the particles were tetragonal in shape. This finding indicated that the addition of SDS enhanced the anisotropic growth of the particles. In the literature, the anisotropic shape control of NPs has been discussed in chemical synthesis techniques, based on selective adsorption theory (Kumar and Nann 2006). According to this theory, mono-surfactant or a mixture of surfactants provides selective adsorption of surfactants at different crystallographic faces of the growing crystals, and results in modification of the surface free energy of individual crystallographic faces leading to different crystallographic growth along these planes. The difference in growth along different crystallographic faces is responsible for the shape anisotropy in such a crystal. In addition, it was observed that most populations of NPs smaller than the average values were tetragonal in shape, while most of bigger particles were spherical.

Fig. 1
figure 1

a–d TEM images of NPs generated by CWLAL and e–h Size distribution with inset showing SAED patterns of the corresponding NPs

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The size and size distribution, as shown in Fig. 1e–h and Table 1, evaluated by estimating larger side dimension of over 300 particles from TEM images, indicated that the average size of the NPs decreased with increasing concentration of SDS. This was consistent with most of the observations in PLAL that the addition of SDS decreased the size of NPs. The effect of SDS on the size distribution of NPs has been explained previously; the SDS molecules encapsulated the newly formed NPs and prevent their further growth, which would otherwise result in larger size NPs (Mafune et al. 2000a, b). In our work, the lowest concentration of SDS, 0.001 M, was insufficient, thereby resulting in a similar size of NPs in average to that in water. The concentration of 0.01 M reduced the average size of the NPs down to 10.6 nm, but with further increase in SDS concentration to 0.1 M the average size remained at 10.4 nm. This observation may suggest that there should exist a critical concentration of SDS to provide adequate amount of SDS molecules to completely attach to the NPs, preventing their further growth.

Table 1 Size distribution of the nanoparticles generated by CW laser ablation in liquid
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Another important function of SDS is to act as a surfactant to enable the generation of stable colloidal solution by preventing agglomeration of the NPs. This is achieved by double layers formed around the NPs by SDS molecule with the first layer having an orientation of hydrophilic part inward and hydrophobic part outward and the second layer having an opposite orientation (Mafune et al. 2000a, b). Such an effect of SDS observed in our work was further evident as observed in Fig. 1a–d, demonstrating much less aggregation when the SDS was applied, and the degree of dispersion increased with the increase in the SDS concentration.

Chemical compositional analysis

Formation of noble metallic NPs by PLAL has been widely investigated (Mafune et al. 2000a, b; Mafune et al. 2003; Tsuji et al. 2003). However, when the metal is reactive with the surrounding liquid, such as water, more complicated chemical reactions between the ablated species and the molecules of water are involved to form oxides or hydroxides (Sasaki et al. 2006). In this process, it was expected that the ablated Ni, in the form of ions, atoms, or clusters, would strongly react with the water molecules to form Ni oxides and/or Ni hydroxide molecules, which rapidly were quenched in the liquid solution to form NPs. EDX analysis, using HRTEM on individual NPs with different sizes and different shapes, detected only nickel and oxygen elements exclusive of substrate signals. Table 2 represents the normalized atomic percentage of Ni and O of the individual NP generated in 0.1 M SDS solution. The ratio of Ni and O (1:1.1) was close to 1:1, supporting the formation of NiO. Further analysis showed that the composition remained consistent and also the variation of the size and shape of the NPs did not affect their composition.

Table 2 EDX compositional analysis of the nanoparticles produced in 0.1 M SDS
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High angle annular dark field (HAADF) images in scanning-transmission (STEM) mode for selected NPs are given in Fig. 2a. The line X–X′ shows the scanning line along which the compositional analysis was performed. Since the compositions of Ni and O represented in Fig. 2b were acquired in STEM mode at a very short counting time (5 s.) for each point, significant fluctuations in concentration levels were observed along the scanning lines. In additional, the nature of fine-sized NPs generates limited signals for analysis. Therefore, the compositional profiles shown in the present work can only be suggested for the purpose of initial qualitative comparison. However, homogenous compositional contrast, as shown in Fig. 2a, excluded the formation of core-shell structure in our process, which is in disagreement with the formation of Ni/NiO core-shell NPs reported by several researchers using different techniques (Sakiyama et al. 2004; Seto et al. 2004; Liu et al. 2007). The common feature of those different techniques was a subsequent oxidation of Ni NPs generated by pulsed laser ablation in vacuum, although laser ablation of Ni target in oxygen atmosphere was also reported to form Ni/NiO core-shell structured NPs. In the current case, the formation of NiO NPs was probably the direct result of spontaneous chemical reactions between the ejected Ni species and the surrounding liquid. Thus, we conclude that the CWLAL provided enough energy with sufficient reaction time to complete the chemical reactions between Ni species and O, or H2O, to form homogenous NiO NPs.

Fig. 2
figure 2

a HAADF image in STEM and b concentration profiles across the line X–X′

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Phase identification

XRD patterns as shown in Fig. 3 clearly revealed a significant degree of crystallinity of the NiO NPs generated in various SDS solutions. The peak intensities of those in water were much weaker, in fact close to amorphous feature. The reason for the amorphous behavior in water may, however, be the result of fewer NPs collected for the measurements and the influence of preferential orientation of the crystals during the sample preparation for XRD, since the SAED patterns, as discussed later, showed clear crystal structures, identified as NiO in all conditions, including water. In addition, the XRD pattern of the NPs in 0.01 M SDS solution revealed an extra shoulder peak at the position of 44.5° (marked with arrow sign), which was matched with f.c.c. Ni〈111〉 (JCPD: 04-0850). However, the absence of any other distinguishable peak at other characteristic positions such as 2θ = 51.8°, expected for f.c.c. Ni〈200〉, implied that the presence of the Ni phase was not significant. For the other conditions, no sign of Ni was observed, indicating a possible completion of chemical reactions between the ejected Ni and the surrounding liquid in the process of NP formation. As shown in Table 3, the rhombohedral phase of NiO has the diffraction planes at the same positions as the cubic phase of NiO. As a result, no information could be extracted from the XRD data to assign the cubic and rhombohedral phases of NiO.

Fig. 3
figure 3

XRD patterns of the NPs generated in different liquid conditions, a water, b 0.001 M SDS, c 0.01 M SDS, and d 0.1 M SDS. The arrow sign shows the 2θ location for pure Ni

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Table 3 XRD standard data of cubic and rhombohedral NiO from JCPDS and calculated d-spacing from SAED patterns
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Table 3 also shows the d-spacing of the NPs calculated from the SAED patterns as shown in Fig. 1. Compared to the standard XRD data in JCPDS, the diffraction rings clearly fitted both the cubic and rhombohedral phases of NiO, confirming the formation of well-crystalline feature of the NiO NPs, independent of liquid conditions, but failed to identify the phase or phases of NiO NPs between the cubic and rhombohedral structures. In addition, it is worth mentioning that the possible presence of Ni NPs in 0.01 M SDS solution, as detected by XRD, was not reflected in the SAED pattern.

To identify the phase structures, HRTEM was carried out at different locations on the microgrid. The observations revealed clearly visible lattice fringes of the NPs in all the liquid conditions further confirming the formation of crystalline NPs. Figure 4 shows typical HRTEM images of NiO NPs generated in water and 0.1 M SDS solution. With the assistance of fast Fourier transformation (FFT) technique on HRTEM images, it was possible to measure the d-spacing as well as the angles between the planes of the crystal structures. Analyses of FFTs of individual particles (inset of Fig. 4) indicated that both cubic and rhombohedral phases of the NiO NPs were evident, independent of the size and shape of the particles. The proportion of the two phases in each condition is yet to be established.

Fig. 4
figure 4

HRTEM image of NiO by CWLAL in a deionized water and b 0.1 M SDS. Insets show the FFT at selected location, marked as Cub for cubical and Rho for rhombohedral phase

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Further HRTEM analyses show the consistent lattice pattern when aligned to the beam at different angles. Figure 5 shows the typical NPs, of different sizes and shapes with clear lattice fringes. This indicates a single crystalline nature of the particles generated by CWLAL, irrespective of the size and shape. In case of short pulsed laser ablation (PLA) of sintered NiO in Argon, bi-crystalline NiO were also formed when 120 pulses were used, due to the continual growth of the particles (Zbroniec et al. 2005). Ultra-short PLA of Ni in oxygen (40 Pa), however, resulted in both single crystalline and polycrystalline NiO NPs depending on the size and shape (Liu et al. 2007).

Fig. 5
figure 5

HRTEM image of single crystalline NiO by CWLAL, showing different sizes and shapes of the nanoparticles with clear lattice fringes a 0.1 M SDS, b 0.001 M SDS, and c deionized water

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Conclusions

Detailed characterization of NiO NPs generated by a high-power, high-brightness CW fiber laser ablation in liquid was performed for the first time. The formation of NiO NPs with well-crystalline structure in both water and various SDS concentrations was demonstrated. The average size of NiO NPs generated in water was 13 nm, while the average size of NiO NPs generated in the SDS solution with concentration higher than 0.01 M was approximately 10 nm. In additional, the shape of the NPs was also observed to be affected by the SDS, showing the change of shapes from more dominated spherical in water to tetragonal with increased SDS concentrations. The NiO NPs consisted of both cubic and rhombohedral crystal phase structures which were single crystal in nature irrespective of the size and shape. This study paves a route toward a new application for high-power, high-brightness CW fiber lasers.