Rhenium/rhenium oxide nanoparticles production using femtosecond pulsed laser ablation in liquid

In this study, rhenium/rhenium oxide nanoparticles (Re / ReO3 NPs) have been produced for the first time in ultrapure water by using Femtosecond Pulsed Laser Ablation in Liquid (fsPLAL) method. X-Ray Diffraction (XRD) measurements and results obtained for NPs show the existence of well-crystallized peaks and preferred phases. Re NPs have hexagonal structure while ReO3 NPs have the perovskite-like cubic crystal structures. The Re / ReO3 ratio is also determined to be 53 / 47 with ~ 20 nm crystallite size, while pure ReO3 crystallite sizes were measured to be ~ 25 nm. The TEM results have shown that the produced particles have a spherical shape, and particle sizes changes between ~ 20 nm and ~ 60 nm. The crystallite size is similar due to XRD results. Obtained nanoparticles exhibit promising applications for photonic devices with broad bandgap values which have measured to be 4.71 eV for Re / ReO3 NPs mixture and 4.36 eV for pure ReO3 NPs.

by using gamma radiation [21]. ReS NPs were synthesized as magnetic nanoparticles [22,23] for the contrast agent. In comparison with other transition metal oxides, ReO 3 has some attractive properties such as having a perovskite-like cubic structure and higher conductivity than metallic rhenium [24]. The resistivity of the single-crystal bulk ReO 3 is about (8.95 ± 0.03) × 10 -6 Ω•cm at 300 K [25]. The optical properties of ReO 3 Nano-Crystals (NCs) were studied by Biswas and Rao [26]. The colloidal ReO 3 NCs exhibits very high enhanced localized surface plasmon resonance (LSPR) in the visible range of spectrum from 488 nm to 534 nm that is comparable to gold NPs LSPR range, and ReO 3 NPs show some metallic behaviours [27]. In a recent study, 8% ReO 3 NPs doped in 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC) were used as a p-doped hole injection layer (HIL) in transparent organic light emitting diodes (OLED) [28]. Surface plasmon resonance (SPR) nature of ReO 3 NPs has been reported; it has been shown that SPR peaks of NPs lies in the visible range, and it can be tuned over the relevant spectral range by controlling the size of NPs. When particle size tuned from 8.5 nm to 32 nm, SPR peak is also tuned from 490 nm to 540 nm [26]. Recently, researchers are very interested in the production of complex NPs from bulk materials using facile and single step production methods that allow choosing parameters. In this study, the single step facile production of the transition metal/metal oxide (Re / ReO 3 ) NPs was performed by using fsPLAL method and characterizations together with the interpretation of NPs obtained.

Materials and methods
In this work, we have used a Ti:Sapphire femtosecond laser system (Quantronix, Integra-C-3.5, NY, USA) pumped by Kerr-Lens mode-locked Ti:Sapphire laser (Quantronix, Ti-Light, NY, USA) (with 330 mW pulse power). Parameters are given as follow: amplifier laser delivers mode-locked chirped laser pulses with up to 3.5 W per pulse at a wavelength of 810 nm and with 90 fs pulse duration, and 1-3 kHz repetition rate with 8 mm beam diameter. Detailed system parameters were described elsewhere [29].
The laser output was adjusted and controlled by using an oscilloscope (WaveRunner 64 Xi, four channel digital-storage oscilloscope, LeCroy Corporation, NY, USA) triggered with a fast photodiode (Alphalas, UPD-35-UVIR-D, Germany). 0.125 mm thick 99.99% pure rhenium (Goodfellow) target translated to the focal point by using a motorized lab jack (MLJ050, ThorLabs, Newton NJ, USA). We have used micromachining system to scan rhenium target and set laser pulse power to 800mW. Re NPs were produced in ultrapure deionized water (18.3 MΩ•cm) (Millipore, USA) with 1 cm thick water layer above sample (8.04 cm 3 ).
The experimental setup is shown in Figure 1(a) and PLAL method briefly introduced as depicted in Figure 1(b). In PLAL method, NPs were produced by courtesy of PLAL method from Re plate sinked in liquid applying pulsed fs laser beams. Target located above the target holder when the laser beam enters the water vertically. If laser beam comes horizontal direction, one can use rotating cylindrical or fixed flat surfaced targets. In this work, vertical laser beams were used to ablate target material inside ultrapure water. As a usual case of interaction between fs laser pulses and target, resonance laser ablation (RLA) process is dominated by photoionization processes (single or multiphoton processes) and following locally induced space-charge separation fields and electron-ion collisions cause the formation of cavitation bubble [30]. There is two step in PLAL method, first one is plasma production and the last one is the continued growth of the NPs after collapsing of the plasma [31].
Laser-irradiated 10 × 10 mm 2 surface area of target was put inside the container and 10 mm height ultrapure water added. Marking systems laser input was power reduced to 800 mW, pulses were used at 1 kHz repetition rate and wavelength was 800 nm. Scanning parameters were set to 3 mm•s -1 for scanning speed, 250 μm line gap between each scan, working area set to 6 × 6 mm 2 and each line scanned twice.

Results and discussion
The crystal structures of Re / ReO 3 nanoparticles produced in this study have been examined using XRD technique (Bruker AXS, Bruker D8 Advance, Germany), and XRD results of powdered NPs are shown in Figure 2. Figure 2(a) shows XRD spectrum for Re / ReO 3 NPs mixture in a ratio of 53 / 47 produced within 30 min. Diffraction peaks of the produced nanocrystals are intense and neat, showing that the Re NPs have hexagonal (space group: P63 / mmc) structure while ReO 3 NPs have the perovskite-like cubic (space group: Pm-3m) structure [32] with effective crystallization and lack of impurities. The crystallite size of the obtained NPs measured from XRD spectra was calculated using Debye-Scherrer equation, size = Kλ/β cosθ, where, K is a dimensionless constant, β is full width at half maximum (FWHM in radian), θ is the diffraction Bragg angle and λ is the wavelength of X-ray in equation [33]. The average crystallite size was calculated due the most intense peaks to be 21 nm for ReO 3 and 22 nm for Re using the Debye-Scherrer equation.
XRD results of the ReO 3 NPs are shown in Figure 2   ReO 3 in literature [34]. All of the samples show semi-broad diffraction peaks and these broad peaks are due to the small size of the ReO 3 nanocrystals [35]. The average crystallite size of ReO 3 NPs was found to be about 25 nm.
TEM (JEOL JEM-2100F, Japan) and SEM (Zeiss LS 10 -Carl Zeiss NTS GmbH, Germany) analyses have been carried out to present some further properties of the NPs as seen in Figure 3. TEM results of Re / ReO 3 and ReO 3 NPs are given in Figure 3(a) and (d), respectively. According to these results, produced particles have a spherical shape and particle sizes changes from ~ 20 nm (similar to that obtained from XRD crystallite sizes) to ~ 60 nm. Furthermore, HR-TEM analyses were carried out to investigate the crystal structure of Re / ReO 3 and ReO 3 NPs, and these results are given in Figure 3(b) and 3(e). As can be clearly seen from Figure 3(b) and 3(e), atoms constituting the lattice fringes are perfectly arranged and it shows that NPs have highly crystalline nature. The inter-planar spacing of Re and ReO 3 NPs were measured as 5.41 and 2.58 Å, respectively. It is seen that these are due to (100) and (110) crystallographic planes, respectively.
Moreover, samples prepared for SEM imaging were first centrifuged at 1200 rpm for 10 min, NPs were sprinkled to double-sided adhesive band stick on the sample stub and then blow away the loose excessive particles and then samples were coated with gold using sputter coater. SEM images of NPs given in Figure 2(c) and 2(f) show that an increase in duration of the NP production causes solvent evaporation during the sample preparation process and then an increase in the aggregation of NPs. A condensation of the solution after evaporation of the liquid by heating may cause the highly aggregated nanoparticles. Furthermore, as can be seen clearly from Figure 3(c) and 3(f), pure ReO 3 have a homogeneous structure than Re / ReO 3 mixture.
An increase in the sample concentration can cause some increase in the number of photons absorbed by the NPs. This difference in the concentration can also result in an increase in the refractive index of the medium and stronger nonlinear effects like white light continuum (WLC) can be generated [36], and then this may cause some fragmentation of the particles to smaller sized fragments, and therefore, suppresses the aggregation [37]. In this study, in addition to WLC generation, some agglomeration after the end of irradiation was observed dramatically increased in time.
UV-Vis absorption spectra (taken using V-670 spectrometer, Jasco Corp., JAPAN) of Re NPs are given in Figure 4(a) and 4(b). It has been observed that, in the Re / ReO 3 NPs mixture, absorption spectra of Re NPs is dominated by UV- Vis spectra. The obtained absorption spectrum of the ReO 3 NPs produced in pure water is shown in Figure 4(b), which presents an absorption peak around 510 nm. It has been observed that the obtained absorption peak is compatible with the literature [26].
Band gap values for Re / ReO 3 NPs produced in this work were calculated from the absorption data by using Tauc-plot (hν-(αhν) 2 ) given in Figure 4 [38][39][40] to be 4.71 eV and 4.36 eV for Re/ReO 3 mixture and pure ReO 3 , respectively. It can be concluded that changes in the Tauc-plot indicate that the Re NPs dominates the UV-Vis spectrum. The band gap energy values obtained are also compatible with the literature [34,41,42]. It was found that the absorption spectra of Re/ReO 3 mixture and pure ReO 3 NPs were varied within the range from ultraviolet region to infrared region in spectrum depending on the synthesis parameters.

Conclusion
In recent years, rhenium NPs have been used for tumor treating therapies and coatings (plastics, metals, textiles) technologies. In addition, magnetic rhenium NPs have been used as a contrast agent and was produced using both methods of pulsed-laser decomposition of ammonium perrhenate or gamma radiation of dirhenium decacarbonyl.
As a summary, in this study, it has been reported that the Re / ReO 3 NPs mixture or ReO 3 NPs were produced for the first time in the literature using the fsPLAL method. The produced NPs were characterized by XRD, SEM, TEM, EDX, and UV-VIS absorption spectroscopy methods. Re / ReO 3 NP mixtures can also be produced changing laser irradiation time and reducing the liquid volume. Pure ReO 3 NPs can be produced in the high laser fluence regime. XRD results show that pure ReO 3 NPs have perovskite-like cubic structure. Produced particles have a spherical shape and particle sizes change from ~ 20 nm to ~ 60 nm due to the TEM results. SEM results present that micron-sized particles can be aggregated due to the evaporation of the liquid by heating when a large volume of water is used. Wide band gap materials are used in very broad application areas in semiconductor devices, optoelectronic devices, photodetectors, catalytic applications, chemical sensors etc. ReO 3 NPs show a broad absorption band (from 350 nm up to 850 nm), and it can be emphasized that this may indeed be useful for sensors, diodes, and solar cell as photonic applications.