1 Introduction

Pulsed laser ablation in liquids is an auspicious green technology to synthesize highly pure nanomaterials for various applications (i.e., biomedicine, energy conversion, catalysis) [1,2,3]. The laser-assisted assembly of colloidal nanomaterials in liquids involves numerous procedures taking place in the bulk, interfacial region and in the molten layer of material during an extended time period from femtoseconds up to milliseconds [4,5,6,7,8,9,10,11,12,13,14,15]. Furthermore, physicochemical processes take place in timescales similar to the natural oscillation times of atoms and molecules from femtosecond (10−15 s) to picosecond regime (10−12 s) [16]. Ultrafast laser systems with their unique characteristics such as short pulse durations and high peak intensities have attracted more attention for both fundamental and practical research and applications [17,18,19].

Ultrashort pulse laser materials processing led to a thermal non-equilibrium condition between electrons and lattice that was previously described by two-temperature model (TTM) [20, 21]. Additionally, high-intensity ultrashort laser ablation in liquids presented a localized synthesis technique that may lead to higher ablation efficiency [22], as the whole laser energy contributed to photon–electron interactions due to insignificant lattice motion and heat diffusion [23].

Furthermore, as pulse duration is much shorter than the electron-lattice relaxation time (10−10 to 10−12 s) [16, 24], the heat affected zone (HAZ) can proficiently be suppressed [15, 25]. In fact, the heat accumulation matter is insignificant for laser frequency repetition rates frep ≤ 200 kHz [26].

Femtosecond laser ablation of Au target in deionized water revealed two suggested mechanisms; cold ablation at relatively low laser fluences which led to monodispersed colloidal solutions and plasma-induced heating of the target that triggered the formation of larger particles and polydispersities at higher fluences [6]. The elemental analysis of laser-synthesized core–shell Fe/Au nanoparticles in liquids disclosed the chemical interactions of the particle surface with liquid media [27]. In fact, the nonlinear energy absorption may initiate the photoionization of solvent due to the interaction of ultrashort laser pulses with liquid media [28]. Laser-induced photoionization processes can be responsible for laser energy losses due to the interaction of laser light with liquid media particularly for laser pulses ≤ 150 fs [29]. Laser-induced optical breakdown can occur in a transparent dielectric environment when the number of free electron density goes beyond its critical value of 1018–1020 cm−3 [30, 31]. The time-of-flight mass spectroscopy confirmed the direct photolysis of acetone molecules by femtosecond laser pulses (@800, 90 fs, 1 kHz) and the formation of a mixture of polyynes with variable chain lengths and graphitic species [32]. In this regard, the non-equilibrium nature of femtosecond laser pulses, nonlinear processes, local thermalization, and the type of liquid media surrounding the target are significant factors to manipulate the shape, size distribution, and internal phase composition of nanoparticles [27, 32, 33].

Designing nanomaterials with enhanced physicochemical properties is of importance. Bimetallic nanomaterials can reveal interesting properties in contrast to single metallic nanoparticles [34]. Bimetallic alloy nanomaterials have shown precious properties in a variety of applications, Ag/Cu nano alloys were employed as highly active catalysts [35]; Au/Fe [34] and Ag/Au [36] due to the enhancement of optical and magnetic behaviors have shown a potential for medical applications. Pt/Au nano alloys were used to produce fuel cell electro-catalysts [37]. A wide variety of Ni-based alloys were used in medical sciences, especially in dentistry and implantology due to their biocompatibility, good mechanical properties, and chemical resistivity [38]. Moreover, Ni/Au alloys have shown high potential in catalysis; increasing O2 reactivity [39], and hydrogenation of aromatic nitro compounds [40, 41]. The mechanism of femtosecond laser generation of nanomaterials in liquid was explained by atomistic modeling of laser ablation of Ag film in water [42]. The rapid cooling and condensation of primary nanoparticles initiated the formation of primary particles with the average size of 10 nm. Secondary nanoparticles around 50 nm were formed due to the destruction of superheated molten metal layer at plume–liquid interface [42]. In this work, the single step eco-friendly synthesis of Ni/Au bimetallic oxide nanoparticles (BONs) by femtosecond laser ablation in various alcoholic solvents is presented. Furthermore, the effect of solvent nature on the chemical composition, crystallinity, and size distribution of Ni/Au BONs is explained.

2 Experimental

Target materials; nickel (50 × 50 × 0.5 mm) with purity of ≥ 99.5% and gold wire (purity ≥ 99.95%) for physical vapour deposition (PVD) were provided from Alfa Aesar Company. Solvents were ethanol, butanol, and isopropanol (Sigma-Aldrich; p.a.). These platelets were cleaned by ultra-sonication in ethanol. Au was coated on Ni target via PVD [43]. The thickness of Au on Ni was measured by the time-of-flight secondary ion mass spectrometry IONTOF (ToF-SIMS5). The estimated thickness of gold is about 50 nm. Target was positioned at the bottom of the glass cell with the height of 15 mm. An optical window for the horizontal access of laser beam was placed (Fig. 1).

Fig. 1
figure 1

a The schematic of femtosecond Ti:sapphire laser; seed laser (10 fs, 75 MHz, 800 nm, 400 mW) and amplifying system (30 fs, 800 nm, 1 kHz, 800 mW) pumping with Nd:YAG laser and b nanoparticle synthesis setup; the system connected to the motorized XYZ-scanning stage controlled via CNC program

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Horizontal beam delivery was preferred to decrease the optical fluctuations due to vapor bubble formation.

Nanoparticle production was carried out using a femtosecond titanium–sapphire (Ti:sapphire) laser setup with various sections including oscillator, pulse stretcher, amplifier, and pulse compressor. A commercial femtosecond laser system was employed as the seeding laser system, emitting at the wavelength of 800 nm (SPECTRA-PHYSICS®, ≤ 400 mW, pulse duration 10 fs, repetition rate of 75 MHz and beam diameter ca. 6 mm). So-called chirped pulse amplification (CPA) [44] was applied before transferring seed laser pulses to the Pockels cell. CPA technique includes the temporal stretching of ultra-short pulses with a large spectral bandwidth provided by an oscillator. Therefore, the intensity of pulse is considerably decreased to avoid nonlinear optical effects which cause damage to optical components, amplifier and distortion of temporal and spatial pulse profile [45]. Laser pulses from oscillator entering stretching part after passing through the Faraday isolator (FI) to prevent back-reflections from amplifier to oscillator and changing horizontally polarized beam to vertically polarized one. The specific number of beam reflections were operated between two pairs of third-order dispersion pre-compensating mirrors (TOD-1/2 and TOD-3/4). Subsequently, laser beam went through a main pulse stretcher (PS1), reflected by an adaptive secondary mirror (ASM) and guided into the amplifying part. Amplified laser pulses with pulse duration of 30 fs and repetition rate of 1 kHz were generated. Pulse duration was measured by autocorrelation technique [46]. Three iris diaphragms (A1–A3) were used to make a beam path toward prism compressor. Powermeter (OPHIR Photonics) was positioned after diaphragm A3. Maximum output power for system was ≤ 1 W. Focus position in various liquid media was experimentally evaluated by microscopically measuring ablated area on a silicon target (ImageJ software) as a function of the distance of focusing parabolic mirror (focal length 101.6 mm). The Gaussian beam radius of 87 mm was evaluated. The squared diameter of damaged zone (D2) evaluated by optical microscopy is correlated to Gaussian beam radius (w0), pulse energy, (E) and threshold pulse energy (Eth) [47]. Laser beam is supposed to have a Gaussian beam distribution:

$$D^{2} = 2w_{0}^{2} \ln \left( {\frac{E}{{E_{\text{th}} }}} \right).$$
(1)

Pulse energy E and peak fluence F are related by:

$$F = \frac{2E}{{\pi w_{0}^{2} }}.$$
(2)

Equation (3) was employed to calculate final pulse duration after passing through liquids [48]:

$$\Delta t_{\text{out}} = \sqrt {\Delta t^{2} + \left[ {4\ln 2\frac{{({\text{GDD}})}}{\Delta t}} \right]^{2} } ,$$
(3)

where Δtout is final pulse duration and GDD is group delay dispersion. The final pulse duration in various solvents are shown in Table 1.

Table 1 Final pulse duration after passing through liquids
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Ni/Au BONs were analysed using various transmission electron microscopy (TEM) methods. Selected area electron diffraction pattern (SAED), bright and tilted dark-field images of laser-synthesized nanoparticles were obtained by a Philips CM200 TEM (LaB6 cathode, acceleration voltage of 200 kV) equipped with a Gatan Orius CCD camera. The phase identification and crystallographic data analysis of Ni/Au BONs were done by AtomWork database [49]. Number-weighted size distributions were evaluated from at least seven TEM frames (700 × 700 nm) using microscopy software (Gatan, Inc.). Energy-dispersive X-ray spectroscopy (EDX) in connection with TEM was used for chemical composition studies. TEM samples were prepared by placing several droplets of colloidal dispersion (after 5 min ultrasonic irradiation) on a carbon-film-coated cop-per grid followed by solvent evaporation in air at room temperature. High-resolution transmission electron microscopy (HRTEM) was performed with the FEI Titan 80-300 equipped with an imaging Cs-corrector.

3 Results and discussion

Laser fluence F was chosen around 1 J cm−2 to decrease energy losses during the synthesis. Laser-induced persistent bubble formation due to the evaporation of solvent [50, 51] can scatter and shield laser energy and subsequently lower productivity. The nonlinear effect; so-called filamentation of ultrashort pulsed laser beam in fluids at higher fluences can also participate in energy losses [52]. Additionally, to have a better productivity, the number of pulses N = 1000 was applied. TEM bright-field image of Ni/Au BONs in ethanol with respective EDX analyses is presented in Fig. 2. The EDX study of the sample showed mainly nickel, oxygen and very low concentration of Au. The EDX spectrum of area (a) marked in the bright-field image is related to the supporting matrix which is dominated by the siloxane glue contaminations (Si and O, Fig. 2a).

Fig. 2
figure 2

TEM bright-field image and the EDX results of femtosecond laser-synthesized (1 kHz, 800 nm) Ni/Au BONs; a matrix area and b the whole area containing nanoparticles (ethanol, N = 1000 and F = 1 J cm−2)

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The high Cu signal is a consequence of stray electrons hitting the Cu support grid. The EDX spectrum of Fig. 2b represents the signal of the imaged sample area containing nanoparticles.

The number-weighted size distribution study of femtosecond laser-generated Ni/Au BONs in ethanol and respective evaluated results are shown in Fig. 3 and Table 2. Bimodal size distribution was observed. The highest frequency of particles presented a median diameter around 15 nm and minor mode could be distinguished in the tail at larger sizes.

Fig. 3
figure 3

The number-weighted particle size distribution of Ni/Au BONs in ethanol (N = 1000 and F = 1 J cm−2)

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Table 2 The fitting results of number-weighted size distribution (Ni/Au BONs in ethanol, N = 1000 and F =1 J cm−2)
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Due to the positively skewed lognormal size distribution, median size with interquartile range (IQR) is presented.

Essentially, Q1 (quartile 1) and Q3 (quartile 3) are correlated to 25% and 75% of the lognormal size distribution graph, respectively; hence, IQR can be measured by substracting Q1 from Q3 value [53].

IQR for lognormal fit function and standard deviation for Gaussian fit function can correlate to the level of polydispersity of colloidal nanoparticles.

The multimodality of laser-synthesized nanoparticles may be related to the cavitation bubble dynamic, growth and collapse. In fact, the primary particles of 8–10 nm came from the expulsion of vaporized atoms during bubble growth as they dispersed inside and outside of cavitation bubble due to the permeability of liquid–bubble interface [54, 55]. Moreover, the agglomeration of primary nanoparticles during bubble collapse may trigger the formation of secondary nanoparticles around 30–60 nm [54,55,56]. Not only the agglomeration but also the rupture of superheated molten metal layer under supercritical liquid may play a role in the formation of secondary nanoparticles [42]. For structural analysis; SAED pattern and dark-field images (a–e) were taken (cf. Fig. 4). The resulting crystallographic data of Ni/Au BONs in ethanol are shown in Table 3. Crystallographic data are obtained by calculating reciprocal lattice distances using PASAD-tools [57] and comparing measured lattice distances with a database [49]. To be precise, all crystal system possibilities related to the calculated lattice distances are given. Dark-field images are obtained using a contrast aperture in the diffraction plane at different positions along the first and second rings (e.g., position A).

Fig. 4
figure 4

The laser-synthesized Ni/Au BONs in ethanol. (N = 1000, F = 1 J cm−2). SAED pattern (upper left) and the dark-field images of the same area using different diffracted electron beams along the 1st and 2nd rings (a–e)

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Table 3 The laser-synthesized Ni/Au BONs in ethanol, selected area diffraction pattern results, corresponding crystallographic data and phase identifiers. (N = 1000, F = 1 J cm−2)
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The presented dark-field images (Fig. 4a–e) correspond to the first and second rings that are related to hexagonal Ni and monoclinic NiO2 with lattice distances of 0.216 and 0.197 nm, respectively (Table 3). In fact, due to the close distance of the first and second rings, the intensity contributions from different structures cannot be avoided.

The observed contrast originates from various atomic orientations of nanoparticles and crystalline defects (i.e., stacking fault SF) common for cubic and hexagonal systems. One can assume, the laser fluence of 1 J cm−2 is adequately high to make longer melting periods and slower resolidification processes. This can trigger the evolution of crystalline structures [15, 58]. Additionally, Ni/Au binary phase diagram showed an immiscible region at temperatures below 500 K [59]. As temperature of plasma plume where nucleation starts for femtosecond pulses are typically between 5000 and 7000 K even at lower energy fluences [10, 60], phase segregation and core–shell formation of Ni/Au were impossible whereas plasma environment was suitable for nanocomposite formations.

The TEM bright-field image of Ni/Au BONs in isopropanol and the corresponding EDX spectrum are presented in Fig. 5. The EDX study of nanoparticles showed mainly nickel, gold and oxygen.

Fig. 5
figure 5

TEM bright-field image and the EDX results of femtosecond laser-synthesized (1 kHz, 800 nm) of Ni/Au BONs (isopropanol, N = 1000 and F = 1 J cm−2)

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The number-weighted size distribution study of Ni/Au BONs in isopropanol demonstrated the decreasing of IQR, median diameter and subsequently polydispersity compared to ethanol solvent (Fig. 6, Table 4).

Fig. 6
figure 6

The number-weighted particle size distribution of Ni/Au BONs in isopropanol (N = 1000 and F = 1 J cm−2)

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Table 4 The fitting results of number-weighted size distribution (Ni/Au BONs in isopropanol, N = 1000 and F =1 J cm−2)
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While there was not a clear graphite shell around laser-synthesized nanoparticles in isopropanol, one could suggest that the steric stabilization of colloidal Ni/Au BONs came from the existence of amorphous carbonaceous networks [61] due to the laser-induced photolysis and pyrolysis of isopropanol solvent.

A bright-field image, the corresponding diffraction pattern and dark-field images of the same area are shown in Fig. 7. Crystallographic data obtained from the analyses of diffraction pattern are listed in Table 5.

Fig. 7
figure 7

The laser-synthesized Ni/Au BONs in isopropanol (N = 1000, F = 1 J cm−2). Bright-field image (upper left), SAED (upper right) and dark-field images at various tilting angles of the electron beam along the 1st and 2nd rings (a–d)

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Table 5 The laser-synthesized Ni/Au BONs in isopropanol, selected area diffraction pattern results, corresponding crystallographic data and phase identifiers (N = 1000, F = 1 J cm−2)
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In analogy to ethanol, crystalline Ni/Au metallic and oxide nanoparticles of various phases are identified (Table 5). Dark-field images (cf. Fig. 7a–d) were taken using various tilting angles of the electron beam and selecting the intensity along the first and second rings (e.g., position A in the diffraction pattern). Due to the closely spaced rings these images display particles that can be assigned to the hexagonal Ni and monoclinic NiO2 structure with lattice distances of 0.216 nm and 0.197 nm, respectively. In addition, the contrast demonstrates that most of the particles show linear features related to planar defects.

In Fig. 8, TEM bright-field image of Ni/Au BONs in butanol and the corresponding EDX spectrum are shown. The EDX study reveals that the nanoparticles contain mainly nickel and low concentrations of gold and oxygen. The oxidation of nanoparticles is decreased in contrast to ethanol medium and in analogy to isopropanol.

Fig. 8
figure 8

TEM bright-field image and the EDX results of femtosecond laser-synthesized (1 kHz, 800 nm) of Ni/Au BONs (butanol, N = 1000 and F = 1 J cm−2)

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Number-weighted size distribution calculated from the bright-field images of Ni/Au BONs in butanol showed smaller median size and lower IQR in contrast to ethanol medium and similar to isopropanol (Fig. 9, Table 6).

Fig. 9
figure 9

The number-weighted particle size distribution of Ni/Au BONs in butanol (N = 1000 and F = 1 J cm−2)

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Table 6 Fitting results of number weighted size distribution (Ni/Au BONs in butanol, N = 1000 and F =1 J cm−2)
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A SAED pattern and dark-field images acquired using intensities along the first and second rings are displayed in Fig. 10. The measured reciprocal lattice distances and respective dark-field images (Fig. 10a–e) of the Ni/Au BONs synthesized in butanol revealed the existence of crystalline Ni/Au metallic and oxide nanoparticles of different phases containing various crystallographic defects (Table 7). The presented dark-field images are related to lattice distances of 0.216 and 0.203 nm that can be correlated to cubic Au and hexagonal Ni, respectively (cf. Table 7).

Fig. 10
figure 10

The laser-synthesized Ni/Au BONs in butanol. (N = 1000, F = 1 J cm−2). SAED (upper right) and respective dark-field images at various tilting angles of the electron beam along the 1st and 2nd rings (a–e)

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Table 7 The laser-synthesized Ni/Au BONs in butanol, selected area diffraction pattern results, corresponding crystallographic data and phase identifiers (N = 1000, F = 1 J cm−2)
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High-resolution electron microscopy of laser-synthesized Ni/Au BONs in butanol as shown in Fig. 11 discloses that core–shell structures are present. The observed fringes with the lattice distance of 0.334 nm can be correlated to crystalline graphite with a few atomic layer of thickness. In addition, atomically resolved images of the core confirm the presence of particles with hexagonal crystal structure.

Fig. 11
figure 11

The HRTEM of femtosecond laser-synthesized Ni/Au BONs in butanol and presented intensity profile along the arrow (N = 1000, F = 1 J cm−2)

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The plasma plume generated in tens of femtoseconds of the laser pulse [62]. In fact, plasma environment can initiate nanomaterial manipulation, as the high-intensity laser beam at the focused laser spot on the target at the solid–liquid interface can generate a high concentration of reactive species [30, 63, 64]. Additionally, highly intense femtosecond laser pulse interaction with solvents can induce photolysis [30, 32]. The photoionization of butanol solvent can generate carbonaceous products, these carbonaceous species may deposit on primary nanoparticles that were produced at the beginning of the bubble evolution and create core–shell structures. Typically the interaction of laser beam with solvent molecules with higher carbon contents, such as butanol, acetone, n-hexane and toluene, results in the graphitization of synthesized nanoparticles [32, 61, 65,66,67,68].

Furthermore, the interaction of hot metallic surface with liquid media may play a role in pyrolysis of solvent molecules [12] as liquid can reach its supercritical temperature due to the contact with molten metal layer [42]. Moreover, nickel is a proficient catalyst for carbonization [69]. Laser interaction with nickel target in butanol solvent showed an increase in the incubation behavior of nickel target due to the thermal decomposition of solvent molecules, deposition of carbonaceous species on ablated craters, and increased absorptivity [12, 70]. The general thermal decomposition of n-butanol follows the equation (4) [12, 71]:

$${\text{C}}_{ 4} {\text{H}}_{ 10} {\text{O}} \to {\text{CO}} + 2 {\text{CH}}_{ 4} + {\text{H}}_{ 2} + {\text{C}}$$
(4)

The smaller median diameter of Ni/Au BONs in isopropanol and butanol in contrast to ethanol medium indicates the existence of graphite shell. This can bring more colloidal stability and may prevent nanoparticles from further growth, coalescence and Ostwald ripening processes [61, 72].

4 Conclusions

Femtosecond laser ablation of Ni/Au target in various alcoholic solvents resulted in crystalline bimetallic oxide nanoparticles of various crystalline phases containing defects. In ethanol, bimodal crystalline Ni/Au BONs with median size of 15 nm were generated. In contrast, Ni/Au BONs with smaller median size and lower IQR were observed in isopropanol and butanol.

In butanol, high-resolution transmission electron microscopy revealed the encapsulation of Ni/Au BONs with a thin graphitic shell of a few atomic layers. Presumably, direct photolysis of butanol molecules by highly intense femtosecond laser pulses caused butanol disintegration to carbon species. Furthermore, laser-induced creation of supercritical temperature liquid due to the interaction of molten metallic layer of Ni/Au target with solvent may initiate butanol decomposition and generation of graphitic species.

The improved colloidal stability can be explained by two anticipated mechanisms. Firstly, the formation of graphite networks inside colloidal solution causes steric stabilization among nanoparticles. This can reduce particle diffusion, coagulation, Ostwald ripening processes, and agglomerations. Secondly, graphite deposition on laser-synthesized primary and secondary Ni/Au BONs may prevent particles from further diffusional growth.

High-intensity ultrashort laser pulses with moderate pulse repetition rates due to lower heat accumulation and subsequently less energy losses can generate localized material processing, better ablation efficiency, and productivity.