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Abstract
We demonstrate the synthesis of gold nanoparticles in tetrahydrofuran using the pulsed laser ablation technique. Both ablation time and solution stirring effect were investigated. At an ablation time of 30 minutes, the average size of synthesized gold nanoparticles significantly reduced from 11 nm to 6 nm. Additionally, the percentage of gold nanoparticles greater than 15 nm reduced as well, from 20.00% to 0.47%. These observations were caused by forced convection flow and shock waves from the rapid laser pulse that fragmented the ablated gold nanoparticles further into smaller sizes.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Tetrahydrofuran (THF) is a volatile organic solvent which can be easily vaporized at room temperature [1]. With an empirical formula of C4H8O [1], this inert water-miscible ether [2] is an ideal medium to solvate both polar and non-polar compounds for various applications such as polymer coatings and adhesives used in pharmaceutical chemistry [3]. Stabilized amphiphilic gold nanoparticle-polymer composites can be prepared using aqueous THF as an eluent without using any reduction agents [4]. On the other hand, THF was also reported as a solvent in the preparation of silver nanoparticles by reduction method using t-BuONA-activated NaH [5].
For metal nanoparticles, gold nanoparticles (Au-NPs) are the most studied owing to their high chemical stability [6]. Au-NPs have promising properties in biology [7], electronics [8] and medicine [9]. Au-NPs have absorption peaks in the visible range, specifically within the green color region, which arise from localized surface plasmon resonance [10]. Hence, the Au-NPs have great potential in the area of photonics [11], microscopy [12] and biological sensor [13]. The uniqueness and potential of Au-NPs have sparked the interest of researches to seek the simplest method of synthesizing Au-NPs. The most common method for synthesizing metal nanoparticles is using chemical reduction and metal precursor [14,15]. Comparatively, synthesis via laser ablation is also a feasible method to produce stable metal nanoparticles and does not require a stabilizing or reduction agent [16].
Past studies reported that laser ablation techniques in organic solvent enable Au-NPs to be synthesized with diameter ranging from 1.8 nm (with toluene) to 10 nm (with n-hexane) [17]. Between the organic solvents, THF is known as a good solvent to obtain stable colloidal solutions of Au-NPs [18]. Direct synthesis of Au-NPs in THF will significantly simplify the fabricating process of thin film incorporating gold nanoparticles and polymer [17], specifically hydrophobic polymer such as polydimethylsiloxane (PDMS) and polyvinyl chloride (PVC). PLA implements free interfering agents making it easy for THF to be removed later via evaporation process [19]. These Au-NPs-polymer nanocomposites have been applied in photonics to enhance thermo dynamics effect [20,21], applicable in sensing and optics filter [22], and as 3D printable material [23] in manufacturing-related applications.
In this paper, the synthesis of Au-NPs in THF is demonstrated using laser ablation technique. In order to study the size reduction, the effect of solvent stirring during the ablation process is also investigated. It is found that the stirring effect reduces the Au-NPs size down to 6 nm. The THF is proven to be a stable solvent for gold synthesis through this PLA technique.
2. Experimental details
The PLA experiment was carried out at room temperature as depicted in Fig. 1 which consists of a second harmonic 532 nm Q-switched Nd:YAG laser, a beaker, a gold plate, a lens with 25 cm focal length, and a magnetic stirrer [20,21,24,25]. In this research work, a pure gold plate (high purity, 99.99%) and THF (anhydrous, ≥ 99.9%, inhibitor-free) were purchased from Sigma Aldrich. The gold plate was attached to a holder and then, it was immersed in 20 mL of THF. Next, it was ablated using a Nd:YAG laser beam with pulse duration of 10 ns and energy of 1200 mJ/pulse. The ablation of gold plate was performed in the range of 7 to 30 minutes with 40 Hz repetition rate. During this process, there were two conditions set for THF solvent in the beaker; stirred and stationary liquid medium. For the former condition, a magnetic stirrer was employed as shown in Fig. 1, with speed set constant at 400 rpm throughout the experiment. During the ablation process, the formation of the plasma plume was monitored through emission of light [15]. The prepared samples were characterized using UV-visible Spectro Photometer (Perkin Elmer, Lambda35), Fourier transform infrared spectroscopy (FT-IR, Thermo Nicolet, Nicolet 6700) and high-resolution transmission electron microscopy (HR-TEM Jeol JSM-7600F), to identify the UV-visible spectrum, bonding effect, morphology and size of the Au-NPs, respectively.
Fig. 1. Setup for synthesis of Au-NPs in THF using a Nd:YAG laser, a lens, a gold plate and stirrer.
3. Results and discussion
3.1 Stirred tetrahydrofuran
The optical absorption spectra of the Au-NPs in THF with different PLA time ranging from 7 to 30 minutes are shown in Fig. 2. The observed absorption peaks at 535, 539, 541 and 546 nm are referred to the localized surface plasmon resonance properties of Au-NPs clusters [15]. From the figure, it can be observed that the intensity of these peaks are proportional to PLA time while the PLA time is indirectly proportional to the wavelength of the observed peaks. This authenticates the size decrement of Au-NPs with the increment of PLA time [26]. The breaking of bigger cluster of Au-NPs to smaller particles are in good agreement with previous study [27] and the absorption spectra confirmed that the Au-NPs were formed in the THF.
FTIR spectra of pure THF, ablated Au-NPs in THF for 7 and 30 minutes are plotted in Fig. 3(a). The main spectral wavelength peaks observed at 2970 and 2854 cm−1 region are related to the C-H stretching vibrations in THF [28] while the ring deformation mode is represented by peaks at 906 and 1063 cm−1 [2]. The peak at 3545 cm−1 corresponds to the interface of THF with water [2] which is related to the OH of water. The peaks at 1180, 1370 and 1443 cm−1 are related to C-O-C stretching, C-OH stretching, and O-H deformation vibration of C-O stretching, respectively. The peak at 1370 cm−1 represents the skeletal vibration in furans, whereas the 658 cm−1 peak is the feature of C-H deformation vibration out of plane [2]. Figure 3(a) also shows a peak at 1735 cm−1 that is related to the C = O stretching carbonyl group [29]. Peaks at 2970, 2854, 1443, 1370, 1180, 906, and 658 cm−1 are observed for all conditions while 433 and 443 cm−1 are the fingerprints of Au-NPs as shown in Fig. 3(b) [30].
Fig. 3. (a) FTIR spectrum of pure THF, Au-NPs in THF with 7 and 30 min ablation times and (b) their enlarged spectrum between 420 and 460 cm−1 wavenumber range.
The morphology and particle size of Au-NPs were obtained by placing a drop of prepared sample on the surface of a carbon film supported on a copper grid using HR-TEM. Micrographs of the Au-NPs formed inside the THF were analyzed using ImageJ tool software program. In this experiment, ablation time and stirring process played a vital role to control the size of the nanoparticles. Throughout the experiment, the magnetic stirrer rotation speed was set at 400 rpm and the prepared colloidal solutions presented a similar light pinkish color. With longer PLA time, the colloid solution appeared denser as a result to the increment of Au-NPs formation. Figure 4 depicts TEM images of the ablated Au-NPs and their size distribution with varied of PLA time. It is observed that Au-NPs in this method are almost spherical in shape with diameter size of 11.5 ± 6.9, 7.9 ± 6.6, 7.8 ± 6.6, and 6.0 ± 2.6 nm for 7, 10, 15 and 30 minutes of PLA time, respectively. The biggest Au-NP size was produced by the shortest ablation time (7 minutes). Since THF is an organic solvent, the Au-NPs size ablated in this medium were within the range of 5 to 15 nm, hence the obtained results justify the findings as reported in [31].
Fig. 4. HR-TEM image of Au-NPs and its mean size distribution (standard deviation in parentheses) in stirred THF for different times 7 min (a-b), 10 min (c-d), 15 min (e-f) and 30 min (g-h).
3.2 Stationary tetrahydrofuran
The stirring process is an essential technique that efficiently reduces the NPs size [27,28], while the ambient media also play a great role in laser ablation experiment. To investigate this, the experiment was repeated with the absence of magnetic stirrer. Figure 5 shows the TEM images of Au-NPs and their corresponding size distribution histogram. Results on the HR-TEM images in Fig. 5 indicate that the PLA time does not affect the size of Au-NPs in stationary liquid. The ablated Au-NP size is in the range of 11.5 ± 5.6, 11.0 ± 6.8, 14.0 ± 10.8, and 10.0 ± 7.3 nm for 7, 10, 15 and 30 minutes PLA time, respectively. Most of the Au-NPs are spherical even though some parts of the population look elongated and some are aggregated.
Fig. 5. HR-TEM image of Au-NPs and its mean size distribution (standard deviation in parentheses) in stationary THF for different times 7 min (a-b), 10 min (c-d), 15 min (e-f) and 30 min (g-h).
3.3 Discussion on stirring condition in ablation process
Laser radiation during PLA will cause the targeted material to evaporate and detach from the surface, thus forming a hot and dense gas area known as plasma plume [32]. After a while, this area expands due to high kinetic energy [33] until the pressure within plasma and surrounding liquid reaches equilibrium. Then, the ablated Au-NPs gradually cool by collisions with liquid particles and finally condense into clusters [34]. There are two possible states of the condensed plasma plume, a portion of it will be deposited back at the target surface due to the pressure from the confined liquid while another part will be diffused into the surrounding THF region [35]. When stirring is applied, the magnetic stirrer forms a forced mechanical convective flow in the liquid as illustrated in Fig. 6(a). Circulatory motion caused by the stirred rod swiftly diffuse the Au-NPs from the target surface to the bottom layer of liquid due to the forced convection flow [36], and later, these particles will be dispersed back to the radiation region near the target surface. The particles in this region will absorb energy from the laser beam and induce fragmentation [37], as the fluence value needed to fragmentize the particles is dependent on the particle size itself; an estimation of 0.07 J/pulse.cm2 for 10 nm-sized particles using a nanosecond laser [38]. Rapid heating and growth of the plasma plume creates high pressure shock waves that propagate from the laser target spot through THF [33], initiating mechanical deformations at a distance away from the target surface [39]. The shock waves are generated by a characteristic distance (L) as;
where $\rho $ is the medium density, $\tau \; $ is the duration of transient stress wave inversely proportional to pulse laser frequency, c is the speed of sound (c = 1.5 km/s), $\varepsilon $ is the permeability of liquid media (THF) and P is the pressure [35]. Thus, since $\tau $, c, $\varepsilon $ are constant value, and $\rho $ is linearly dependent on laser ablation time, then L is inversely proportional to the P. Hence distance of ablated particles and cluster from the target surface are important in order to continuously tailor the size of NPs during the ablation period.Fig. 6. Schematic diagram showing the Au-NPs diffusion dynamics in (a) stirred THF and (b) stationary THF.
Based on HR-TEM images as depicted in Figs. 4 and 5, PLA time plays a significant role in size manipulation of Au-NPs only if PLA liquid medium is stirred. Table 1 shows a comparison made between the two Au-NPs produced by two different setups. In the stirred liquid condition, a gradual decrement of mean size and its variation value between the shortest and longest time of PLA were observed. At PLA time of 7 minutes, a high degree of particle aggregation was visible as shown in Fig. 4(a), with a mean particle size of 11.5 nm and standard deviation of 6.9 nm. Their size distribution analysis as illustrated in Fig. 4(b) indicates the widespread distribution until 28 nm. In this case, PLA time at 7 minutes produced the highest percentage of Au-NPs bigger than 15.0 nm (28.26%). In contrast to 30 minutes of PLA time, the percentage value dropped to 0.47%. In this work, 15 nm particle size becomes the basis of comparison for the standard maximum spherical size produced in THF as reported in [15].
Table 1. Comparison of mean particles diameter (standard deviation in parentheses) for stirred and stationary THF for 4 different times. The percentage of Au-NPs size which exceed 15 nm is also analyzed.
In stationary solution, heat produced from the plasma plume growth near the target surface creates density difference between the irradiated and non-irradiated THF region. The colloidal particles produced in the plasma plume region behave as Brownian particles [40], which later will diffuse into the surrounding due to thermal convection flow at the end of the each pulse [41]. The random movement of this Au-NPs enable some of the particles to enter the region close to the target surface and get fragmented through the aid of the shock waves generated from the plasma plume growth. Nonetheless, other diffused particles are scattered in the region far from the irradiated target surface, where the laser fluence is insufficient to cause another fragmentation [42,43]. Instead, the heated Au-NPs gain kinetic energy, increasing collision frequency which leads to size growth and larger aggregation of Au-NPs. This results to the formation of Au-NPs in bigger cluster or chain-like structures [34,35]. From Table 1, the reduction of Au-NPs size is not significant, and its distribution is heterogeneous. At 30 minutes of PLA time, the mean diameter was achieved at 10.0 nm with standard deviation of 7.3 nm. Under the same scenario, the percentage of Au-NPs with more than 15.0 nm size was 20.00%. These values were much larger than their counterpart for the stirred condition.
4. Conclusions
The formation of Au-NPs in THF using PLA method was experimentally studied. The localized surface plasmon resonance peaks appeared in the green range of UV-visible spectrum for all ablated Au-NPs. The stirring configuration was investigated to study its effect on the synthesized particles size. The properties of Au-NPs in stirred THF showed average spherical size reduction from 11.5 to 6.0 nm in tandem with PLA time from 7 to 30 minutes. In contrast, the Au-NPs ablated in stationary THF were random in size from 10.0 to 14.0 nm within this time frame. Furthermore, for the case of stirred THF, the percentage of Au-NPs greater than 15.0 nm significantly reduced from 28.26% to 0.47% at 7 and 30 minutes PLA time, respectively. These results strongly suggest that both time and stirring condition of liquid (THF) play a major role in controlling particle size and attaining homogeneous distribution of AuNPs.
Funding
Ministry of Higher Education, Malaysia, Geran Inisiatif Putra Siswazah (GP-IPS/2018/9636000).
Acknowledgement
We gratefully acknowledge the Ministry of Education Malaysia and Universiti Malaysia Pahang for their supports under Skim Latihan Akademik Bumiputera (SLAB).
Disclosures
The authors declare no conflicts of interest.
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