Nickel nanoparticles were dispersed uniformly in a graphene oxide solution, using a laser ablation technique with different ablation times that ranged from 5 to 20 minutes. The results indicate that the nickel nanoparticle sizes inside the graphene oxide decreased, and the volume fraction for the nickel nanoparticles in the graphene oxide increased with an increasing ablation time. Further, using Fourier Transform Infrared Spectroscopy, the nickel nanoparticles in the graphene oxide demonstrate greater stability from possible agglomeration when the nanoparticle was capped with oxygen from the carboxyl group of the graphene oxide. The thermal effusivity of the graphene oxide and nickel nanoparticle graphene oxide composite was measured using a photoacoustic technique. The concentration of graphene oxide shifted from 0.05 mg/L to 2 mg/L, and the thermal effusivity increased from 0.153 W·s1/2·cm−2·K−1 to 0.326 W·s1/2·cm−2·K−1. In addition, the thermal effusivity of the nickel nanoparticles graphene oxide composite increased with an increase in the volume fraction of nickel nanoparticles from 0.1612 W·s1/2·cm−2·K−1 to 0.228 W·s1/2·cm−2·K−1.

1. Introduction

Nickel nanoparticles (Ni-NPs) have many important applications as catalysts, conducting and magnetic materials [1], and an electrode layer in multilayer ceramic capacitors [2, 3] and have both unique properties and potential applications in a variety of fields, including electronics [4], magnetic [5], energy technology [6], and biomedicine [7]. Ni-NPs can be synthesized using many methods, including photolytic reduction [8], radiolytic reduction [9], sonochemical [10], solvent extraction reduction [11], microemulsion technique [12], polyol method [13], and microwave irradiation [14].

Graphene Oxide (GO) is obtained from the oxidation of graphite crystals, a single-atomic-layered material. It can dissolve and disperse in a variety of solutions including water. GO has more applications for use in composites materials [15], solar cells [16], medicine [17, 18], antibacterial materials [19], and inorganic optoelectronic devices [20]. The GO molecular structure includes hydroxyl (OH–) and epoxy (–COO) groups at the basal plane. Further, it also contains carboxyl groups (–COO) at the edge of the molecular structure [21, 22].

Metal nanoparticles can improve and modify the physical properties of GO [20] as can be seen in [20] where the alteration of light scattering was achieved allowing an improved absorption of light in the solar cell. Further, Benayad et al. improved the resistivity of GO by the use of metal nanoparticles [23].

A Ni-NPs-GO composite was used as a catalyst for fabrication of carbon nanotube [24] to determine the concentration of glucose in human blood [25], remove and detect aromatic compounds like benzene and toluene in environmental circumstances [26], and develop an electrochemical electrode for sensing carbohydrates [27].

The application of the laser ablation technique offers a novel unique tool for nanofabrication [28]. When the laser ablation is used to produce the metal nanoparticles in an aqueous solution, the nanoparticles are released inside the solution, and nanofluid is formed [29, 30]. Metal nanoparticles preparations are relatively difficult to create because they are easily oxidized. The properties of nanoparticles using laser ablation are unique, and they are not reproducible by any other method [31, 32]. For example, the fabrication of Ni-NPs in liquid does not require any chemical reduction agent [33], and dimension control is easier than with chemical methods.

The thermal properties are significant to characterize the nanofluid. Thermal effusivity explains the aptness of nanofluid to be able to exchange heat or thermal energy with the environment and measure the thermal impedance of fluids. The relationship between the thermal effusivity () and the thermal conductivity () is given as [34] where and are the density of the sample and specific heat. A photoacoustic (PA) application is used to measure the thermal properties of the nanomaterial, for example, thermal effusivity.

In this study, Ni-NPs was synthesized in GO using laser ablation. The size, shape, distribution, and concentration of Ni-NPs in GO were obtained through transmission electron microscopy (TEM) and atomic absorption spectroscopy (AAS), and the binding of Ni-NPs to GO was analyzed using a Fourier Transform Infrared Spectroscopy (FT-IR). Moreover, the thermal effusivity of the GO and Ni-NPs-GO composite was measured using an open cell photoacoustic technique.

2. Materials and Methods

2.1. Sample Preparation

The preparation of GO followed the previous reports by Huang et al. [35]. A final concentration of 2 mg/L GO in water was used for this study.

Figure 1 shows the schematic diagram for the laser ablation setup. The nickel plate (99.99% purity; Sigma Aldrich, St. Louis, MO) was immersed in 30 mL GO and ablated with a pulsed Q-Switched Nd:YAG laser. A pulse duration of 10 ns and 40 Hz repetition rate at wavelength 532 nm with 1200 mJ power was applied to prepare Ni-NPs in GO solution. During the ablation, a stirrer was used to uniformly disperse the Ni-NPs in GO. The laser beam was focused on the nickel target using a 300 mm focal length lens. The ablation was carried out at room temperature with different duration times: 5, 10, 15, and 20 minutes. The nickel nanofluids were characterized using an atomic absorption spectroscopy (AAS, S series), a transmission electron microscopy (TEM, Hitachi H-7100; Hitachi, Chula Vista, CA), and a Fourier Transform Infrared Spectroscopy (FT-IR).

2.2. Photoacoustic Setup

Figure 2 depicts the photoacoustic setup containing a He-Ne laser (75 mW, 632.8 nm), a chopper, a mirror, a fluid cell, a microphone, a preamplifier, and a lock in amplifier. The chopper frequency was controlled by a computer program that shifted from 21 Hz to 236 Hz.

The bottom of the fluid cell was closed and shut using a 0.017 mm thick aluminum sheet, and the fluid cell was filled with the nanofluid. To measure thermal effusivity of the Ni-NPs-GO composite, the photoacoustic signals were detected using an electret microphone, connected to a preamplifier and lock-in amplifier. The phase and amplitude of the signals were registered as a function of a light beam modulation frequency. Measurements were carried out at room temperature for an empty fluid cell and a fluid cell with Ni-NPs-GO composite. In order to obtain thermal effusivity, the amplitude of the photoacoustic signal was analyzed using the Rosencwaig-Gersho (RG) theory.

3. Results and Discussion

Figure 3 shows the TEM patterns achieved by using the same accelerating voltage of 120 kV, and the patterns explain the shape, particle size, and distribution of Ni-NPs inside the GO. As shown in Figures 3 and 4, the Ni-NPs, which were distributed evenly in the GO, had a spherical shape, and the particle size decreased with an increase of the ablation time. This results is illustrated in Figure 4 which shows that the mean sizes of Ni-NPs in the GO decreased from 15.53 nm to 6.83 nm. In addition, Figure 4 shows the distribution of Ni-NPs’ increasing with a longer ablation time.

The Ni-NPs concentrations were measured by AAS, and the concentrations increased with increases in the ablation time. Volume fractions (V) were calculated by the following equation: where and are the volume of liquid and the volume of the particles (, where and are the mass density of the nickel and the particles mass dispersed in the GO, resp.). Hence, (3) was derived from a modification of (2) as follows: where are the concentrations of the Ni-NPs-GO composite obtained from AAS result.

The pertinent parameters are displayed in Table 1.

The FT-IR spectra were recorded to identify capping of the Ni-NPs in the GO. Figure 5 shows the FT-IR spectrum of the GO where plot (a) did not contain any impurity, and plot (b) showed the spectrum containing Ni-NPs dispersed in GO. Both results exhibit the FT-IR spectra of the GO and the Ni-NPs-GO composite at a frequency range of 4000–300 cm−1. The spectrum in plot (a) shows transmission peaks at 3232, 2829, 1719, 1611, 1401, 1155, 1029, 856, 564, and 412 cm−1. Similarly, transmission peaks for the Ni-NPs in the GO were obtained at 3218, 2914, 1718, 1595, 1411, 1037, 792, 736, 665, 615, 587, 543, 438, and 375 cm−1 for plot (b). The FT-IR spectra of the GO and Ni-NPs/GO hybrid showed the peak at 1719, 1611, and 1401 cm−1 had shifted to 1718, 1595, and 1411 cm−1.

In Figure 5, the broad peak at 1719 and 1611 cm−1 corresponding to (C=O) of –COOH on the GO shifts to 1718 and 1595 cm−1 due to the formation of –COO after coating with Ni-NPs. The characteristic peak corresponding to the stretching vibration of Ni-O bond shifts to lower wave number that suggests that Ni-NPs are bound to the –COO on the GO surface. In plot 5(b), the broad peaks of 792, 736, 665, 615, 587, 543, 438, and 375 cm−1 are related to Van der Waals forces that are bonding with oxygen on the of GO surface with surface charges of Ni-NPs [36]. Consequently, the Ni-NPs were located on the GO surface and between the GO sheets (see Figure 5(c)).

In accordance with RG theory, the amplitude of the photoacoustic signal is inversely proportional to the modulation frequency [36]. The amplitude of pressure fluctuations () in the presence of Ni-NPs/GO was calculated as where , and are the thermal diffusivity, thermal effusivity, and thickness of the aluminium foil. To measure the thermal effusivity of the GO and Ni-NPs/GO composite, and have to obtain from the Al foil signal as a function of modulation frequency. Adjustable parameters ( and ) must be calculated by fitting the former data to [34] The experiment was carried out with deionized (DI) water and ethylene glycol to calibrate the setup (see Figure 6).

The measurement of thermal effusivity was done for the GO and Ni-NPs/GO composite with different concentration of GO and Ni-NPs, respectively. The photoacoustic signals used for measuring of thermal effusivity are depicted in Figures 7 and 8. , and are 2.36  W·s1/2·cm−2·K−1, 0.0017 cm, and 0.99 cm2/s, respectively, and the results are further sorted in Table 2.

The concentration of the GO solution was shifted from 0.1 mg/L to 2 mg/L, and the thermal effusivity of the GO solution increased when increasing the concentration of the GO from 0.153 W·s1/2·cm−2·K−1 to 0.326 W·s1/2·cm−2·K−1 (Figure 9(a)). The variation in the thermal effusivity of the Ni-NPs/GO composite is shown in Figure 9(b), enhanced by increasing the volume fraction of Ni-NPs from 0.1612 W·s1/2·cm−2·K−1 to 0.228 W·s1/2·cm−2·K−1. The thermal effusivity concept is an exchange of heat with the environment. The effective nanofluid surface expanded by increasing the volume fraction, and the ability of the nanofluid to exchange heat with the environment was enhanced, when the volume fraction increased, causing an increase in the thermal effusivity of the Ni-NPs-GO composite.

4. Conclusions

The Ni-NPs were synthesized in the GO using the laser ablation method. The binding between the carboxyl groups (–COO) on the GO and Ni-NPs occurred and achieved via FT-IR spectroscopy. The particle sizes of 15.53, 12.30, 9.76, and 6.83 nm were obtained in 5, 10, 15, and 20 minutes ablation times, respectively. The GO controls the particle size and shape and prevents agglomeration between the ablated Ni-NPs. Hence, the Ni-NPs stay completely stable for a long time, and with an increase in the ablation time, the particle size reduces, and both absorption and volume fraction increase.

Consequently, laser ablation is an environmentally friendly, simple, and green method for preparation of Ni-NPs in the GO. Moreover, the photoacoustic signals can be registered for a determination of thermal effusivity of the Ni-NPs/GO composite. The results show that with an increasing volume fraction of Ni nanofluid, the thermal effusivity does increase.


The authors acknowledge Universiti Putra Malaysia for the fund from the Research University Grant Scheme (05-02-12-2015RU/05-01-12-1626RU/05-01-12-1627RU) and the postdoctoral fellowship under the Wireless and Photonics Network Research Centre (WiPNet).