Influence of chemical properties of liquid environment on the physical characteristics of laser ablation produced tungsten nanostructures
Subject Areas : Journal of Theoretical and Applied Physics
Zohreh Famili
1
,
Davoud Dorranian
2
,
Amir Hossein Sari
3
1 - Laser lab., Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
2 - Laser lab., Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
3 - Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
Keywords:
Abstract :
Influence of chemical properties of liquid environment on the physical characteristics of laser ablation produced tungsten nanostructures
Zohreh Famili, Davoud Dorranian*, Amir Hossein Sari
Laser lab., Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
Abstract
Impacts of the liquid environment on the characteristics of pulsed laser ablation (PLA) synthesized tungsten (W) nanostructures have been investigated. High purity W target was irradiated by the fundamental wavelength of a Q-switched Nd:YAG laser of 7 ns pulse width and 1 J/cm2 laser fluence in different liquid environments including distilled water, ethanol, acetone, and cetrimonium bromide (CTAB) solutions. Structural, chemical, and optical properties of W nanoparticles (NPs) were characterized by different spectroscopic and imaging techniques. FTIR spectra indicate the formation of a bond between W and O in the synthesized NPs, and XRD patterns confirm producing W and WO3 composite NPs in all liquid environments. The excitonic/plasmonic absorption peak of W/WO3 NPs were occurred in the absorption spectra of all samples. The largest particles with the lowest adhesion were synthesized in acetone solution, and adding CTAB surfactant to distilled water reduced the adhesion of NPs as is depicted by FESEM. TEM images confirm the formation of core-shell W/WO3 nanostructures in distilled water. The PL spectra present band-to-band transitions and oxygen vacancies of WO3.
Keyword: W-WO3 composite nanoparticles; liquid environment; laser ablation; core-shell
*Corresponding author: DavoudDorranian;
Email: doran@srbiau.ac.ir
Tel: +98 21 44869654
Fax: +98 21 44869640
1. Introduction
Recently, nanostructures have been utilized in widespread domains of sciences and technologies. Among them, W and its oxide nanostructures have unique electrical, gasochromic, and photoelectrochromic properties so they can be used in infrared switching devices, photovoltaic organic solar cells, photocatalysis, gas and chemical sensors, biosensors, supercapacitors, information storage media, optical modulators, and the like [1-4].
The PLA of metal targets in liquid is known as a facile, fast, green, and cheap method for producing not only pure metal nanostructures but also metal oxides, carbides, and alloys. In addition, in this method, there are many parameters concerning the source and the liquid environment for controlling the size, shape, and composition of the products [5-7]. Numerous studies have shown that the properties of NPs produced by PLA of a metal target in liquid environments strongly depend on the characteristics of the target material, the liquid environment, and the laser beam [8-10].
In this work, the PLA of W target in several liquids is performed, then the effect of these liquids on the properties of products such as size, shape, and composition has been studied. Liquid environments used in the ablation process were distilled water and ethanol, acetone, and CTAB aqueous solutions. The choice of these liquids was due to their different properties such as chemical composition, density, viscosity, and polarity.
Water (H2O) and ethanol (CH3-CH2-OH) are polar protic solvents, and there is a hydrogen bond between molecules in water and ethanol due to the presence of the OH functional group. Of course, the hydrogen bond between ethanol molecules is weaker than water. The hydrogen bond causes exclusive properties such as high specific heat capacity, latent heat of vaporization, and surface tension in the solvent. In addition to create attraction between solvent molecules, the hydrogen bond is responsible for the attraction between solvent molecules and other molecules or surfaces. Acetone (CH3-CO-CH3) is an aprotic polar solvent whose forces between its molecules are dipole-dipole. The dipole moment of ethanol, water, and acetone are 1.69 D, 1.85 D, and 2.88 D, respectively. To be stronger dipole moment of acetone is due to the presence of a double bond of oxygen to carbon in its carbonyl group C = O [4, 11-13].
Adding surfactant to ablation liquid has a significant effect on the size, adhesion, and composition of NPs produced by PLA in a liquid environment. Surfactants are organic compounds containing two parts, a non-polar part which consists of a long hydrocarbon chain (hydrophobic tail), and a polar part which is generally ionic (hydrophilic head). Surfactants are soluble in both water and organic solvents due to their dual nature. One part of the surfactants are soluble, and the other part is insoluble. The molecules of surfactants preferentially orient so that the soluble part bonds to the liquid and the insoluble to the solid surface. These materials change the surface energy and tend to accumulate at the interface between the two environments. CTAB with the chemical formula C19H42BrN is a cationic surfactant with a positively charged ionic hydrophilic head and a non-polar hydrophobic tail [14-16].
In the PLA of a target in a liquid environment, some properties of liquid such as density, viscosity, thermal conductivity, specific heat capacity, control the size, and morphology of the products by affecting the evolution, expansion, temperature reduction, and collapse time of plasma plume and cavitation bubbles. Other properties of the liquid, such as its nature of polarity and dispersion affect the adhesion and aggregation of NPs [2,6,16-18]. Therefore, some effective physical and chemical parameters of used liquids in the PLA of the W target in this study are introduced in Table 1. The data in the Table are taken from references 19-21. Of course, in ethanol/acetone solution, these parameters are affected by the ratio of ethanol/acetone to water. Also, the mentioned parameters change in aqueous CTAB solution compared to distilled water due to the molarity of the solution.
2. Experimental Setup
Synthesis of colloidal NPs was performed by the PLA of a W plate (99.9%) with 2 mm thickness in distilled water, ethanol and acetone diluted with distilled water (ethanol/acetone: water in a 2:1 ratio), and aqueous CTAB solution with the concentration of 0.027 M. Just before the experiment, in order to remove organic compounds, the W plate and the containers were rinsed with ethanol, acetone, and distilled water using an ultrasonic cleaner. The cleaned plate was located at the bottom of a glass vessel containing 30 ml of each ablation liquid environment. The height of the liquid above the surface of the W plate was approximately 8 mm. The W target was irradiated vertically with the fundamental wavelength (1064 nm) of a Q-switched Nd:YAG laser operating at 5 Hz repetition rate with a pulse width of 7 ns. Laser beam was focused on the surface of the target in liquid medium by means of 80 mm focal length lens. 3000 laser pulses at 1.0 J/cm2 laser fluence (before the lens) were employed to irradiated the W target in each of the mentioned liquids. During laser irradiation, the ablation container and target were moved in different directions on the horizontal surface to ensure uniform ablation and prevent texturing effects. Colloidal samples prepared by PLA of a W plate in distilled water, ethanol, acetone, and CTAB solutions have been labeled as samples 1-4 respectively.
The various analytical techniques were utilized for the characterization of the nanostructures prepared in different liquids. Morphological studies and elemental identification were performed using a field emission scanning electron microscope (FESEM; TESCAN MIRA3) coupled with energy dispersive spectrometry (EDS). The size distribution of products was studied using a transmission electron microscope (TEM, Zeiss-EM10C-100 kV). The Fourier transform infrared (FTIR) spectroscopy was performed using Perkin–Elmer FTIR spectrometer (Spectrum Two) in the spectral range of 450–4000 cm-1. In order to study the structural properties of the products, the X-ray diffraction (XRD) patterns were taken of colloidal solutions dried on silicon substrates using PANalytical-X’Pert PRO X-ray diffractometer with Cu Kα source (λ =0.1514060 nm). The optical properties of colloidal suspensions were evaluated by a UV–Vis–NIR spectrometer (PerkinElmer Lambda 950) at room temperature in the range 200–1100 nm. The photoluminescence (PL) spectra of the samples were obtained using a Fluorescence Spectrophotometer F-4500.
-Structural properties
Figure 1 shows the image of prepared colloidal samples. Color of samples can be affected by several parameters such as the composition of NPs, the ablation environment, and the amount and the size of NPs [3]. According to the values obtained for the size of NPs in the following sections, no clear relationship was found between the color of the suspensions and the size of NPs. The discrepancy in the color of the samples may be related to the chemical composition of the ablation environments. The color of sample 1 is light gray. Analyses show that this sample contains both W and WO3 NPs. According to other reports, depending on the size of NPs, color of W and WO3 NPs suspensions are light gray to dark color [22, 23]. The colors of the colloidal samples 2 and 3 were light gray and light orangish gray respectively. Samples 2 and 3 were darker than sample 1 due to the appearance of carbon in them. Carbon atoms may be released from ethanol and acetone molecules at high plasma temperatures due to high laser fluence [4, 5, 24]. Sample 4 is colorless colloid, which is expected due to colorlessness of the suspension of CTAB powder in water.
The EDS analysis was performed to investigate the elemental composition of the samples. Figure 2 shows the EDS spectra of samples prepared in different liquid environments. The existence of W and O elements is evident in the spectra of all the samples. The Au signals observed in the EDS spectra are related to the gold coating of samples before EDS analysis [25]. Other elements observed in the spectra are related to the molecules of the used liquids.
XRD spectra of NPs prepared by PLA of W target in different liquid environments are shown in Figure 3. In order to perform this measurement, a few drops of each of the colloidal samples were dried on silicon wafer substrates of 10 mm×10 mm at room temperature. In the XRD spectra, the broad peak observed at about 69° is attributed to the silicon substrate. According to the PDF card of 01-088-2339, the observed peaks at 38.37°, 44.60°, and 77.99° corresponding to (111), (200), and (311) Bragg planes, respectively, confirm the formation of the cubic crystal structure of W with Fm-2m (225) space group and lattice parameter as a=4.060 Å. The identified structure peaks, which indicates the formation of W metal NPs, are different from the W metal target peaks. W target peaks are observed at 40.26°, 58.28°, 73.20°, and 87.02° assigned to (110), (200), (211) and (220) Bragg planes of cubic W with Im-3m (229) space group, and the lattice parameter as a=3.165 Å (PDF: 00-004-0806). That means the ablation process of the W target was led to formation of W metal NPs with different lattice parameters from the W target. The intensity of these peaks in samples 1 and 4 is larger than other samples, which can indicate a higher degree of crystallinity in these samples. Also, in samples 3 and 4, the appeared peak at 40.26° matches with one of the peaks related to the cubic structure of the W target (PDF: 00-004-0806). In the sample 4, the peaks observed at 17.04°, 20.64°, 24.03°, and 27.86° corresponding to the Bragg planes (1 0 3), (0 0 12), (0 0 14), and (0 0 16), respectively, are related to the crystal structure of CTAB (C19H42BrN) (PDF: 00-034-1556).
According to PDF card of 2008-033-1387, the XRD peaks at 13.96°, 24.33°, 44.37°, and 77.70°, indexed to (100), (110), (211), and (402) Bragg planes can be ascribed to the formation of the WO3 hexagonal phase in these samples [26]. The formation of the WO3 structure in sample 4 is more significant than other samples, because the number of WO3 peaks in the diffraction pattern of this sample is larger than other samples. The peaks of the hexagonal WO3 structure in sample 3 are weaker, which could be due to the higher dipole moment of acetone compared to ethanol and water. Other reports indicate that the degree of crystallinity of WO3 NPs in polar liquids is lower than in dispersant liquids [27, 28].
Since the peaks identifying the hexagonal WO3 structure overlap with the peaks attributed to W and CTAB crystalline structures, FTIR analysis was utilized to ensure the formation of WO3 nanostructure in the samples.
The FTIR spectra of samples prepared by PLA of W target in different environments are shown in Figure 4. The spectra were recorded in the range of 400–4000 cm-1. The strong and broad absorption peaks in the range of 3000–3600 cm-1 are ascribed to the stretching modes of OH groups. The medium and broad peaks in the range of 1620–1660 cm-1 and low-intensity peaks located at about 1385 cm−1 are due to bending modes of water molecules [29, 30]. The bands in the region 600–780 cm−1 correspond to the stretching modes of bridging oxygen, ν(O–W–O), which confirm the formation of WO3 NPs in different environments [31, 32]. The difference in peak positions of the stretching mode of ν(O–W–O) in different samples is ascribed to the lengthening of some bonds due to oxygen vacancies or defects in the WO3 structure. It can be seen that the characteristic absorption peak of vibration mode of ν(O–W–O), produced in other liquid environments rather than the distilled water environment were shifted to longer wavelengths [33, 34].
FTIR spectra in compliment with the X ray diffraction patterns clearly show the formation of W and WO3 NPs in all samples. Metal W NPs have formed during the cooling of the plasma plume. After the collapse of the plasma plume and the dispersion of NPs in the liquid environment, chemical and physical interactions between the ablated W NPs and water molecules in aqueous environments have formed a WO3 layer on the metal W NPs.
Controlling the kind of synthesized species of tungsten nanostructures in the pulsed laser ablation process of W target in liquid environments was reported frequently. In some studies, the production of WO3 NPs with different phases and compounds has been reported [11]. The production of W metal NPs has been confirmed in several reports [4]. Some studies also suggest the formation of W-WO3 composite NPs with a structure of core-shell [18, 35].
Formation of W-WO3 nanocomposite can be describe in three steps. In the first step, after the interaction between the laser pulse and the W target in the solid-liquid interface, W plasma was rapidly produced with high temperature and high pressure without any liquid molecule. In the second step, ultrasonic and adiabatic expansion of W plasma in cavitation bubbles leaded to cooling the W plasma plume region and hence the formation of W NPs. In our work, since the pulse repetition rate and the pulse width were 5 Hz and 7 ns, respectively, the distance between two sequential pulses was 0.2 s, which was much longer than the lifetime of the plasma plume. Therefore, the next laser pulse had no interaction with the previous plasma plume. In the third step, after extinguishing the plasma, the synthesized W NPs were encountered to liquid molecules (and surfactant molecules, if any) resulting in some chemical reactions and coating effects between them [28, 36-39].
- Size and morphology
The morphology and size distribution of produced NPs were investigated by FESEM and TEM images. FESEM images of samples are presented Figure 5. These figures show the formation of strongly adhesive NPs in sample 1, nanosurfaces in sample 2, non-adhesive NPs in sample 3, and adhesive NPs with a wrinkle in sample 4.
TEM images of NPs are presented in Figure 6. TEM images show the formation nearly spherical NPs in sample 1and spherical NPs in samples 2-4. Also, the aggregation of NPs in the distilled water environment is more than other environments, and in the acetone aqueous solution is the lowest.
Size distribution histograms of NPs, extracted from TEM images are shown in Figure 7. The average size of NPs in samples 1-4 are 49.54, 66.27, 79.24, and 71.01 nm, respectively. In addition, the standard deviation size of the NPs in samples 1-4 are 27.19, 26.67, 32.39, and 35.52 nm, respectively (Figure 7, Table 2). According to these results, NPs have the largest average size and standard deviation in acetone solution.
The surface charge of metal oxide NPs is different in various liquid environments. Depending on whether the pH of the liquid is greater than or equal to or less than the isoelectric point of NPs, the surface charge of NPs will be negative, zero, and positive, respectively. The surface charge of WO3 NPs in various liquids used in our experiment was negative because according to Table 1, the pH of these solutions (about 6.0-7.33) is higher than the isoelectric point of WO3 (about 0.2-0.5) [40]. As mentioned in Section 1, the dipole moment of acetone molecules is higher than water and ethanol (Table 1). In sample 3, due to the high dipole moment of the acetone molecules, at the interface between the WO3 NPs and the surrounding liquid, strongly electrical double layers were formed, causing the effectively electrical repulsion forces between the WO3 NPs. Hence, the aggregation and adhesion of NPs produced in acetone solution were smaller than other liquids. On the other hand, in samples 1 and 2, the repulsion between NPs due to the formation of charged double layers competed with the attraction due to hydrogen bonds between NPs and water/ethanol molecules [2, 6]. In distilled water environment, the attraction forces due to hydrogen bonds overcame the repulsion forces due to the formation of charged double layers, leaded to an intense adhesion between NPs.
In the aqueous CTAB solution, micelle layers were formed around the NPs because the concentration of CTAB surfactant in this environment was 0.027 M, which was higher than the critical micelle concentration (CMC) of CTAB (CMC= 0.0009 M for CTAB). The negatively charged surface of WO3 NPs absorbed the positive head of the CTAB, and the hydrocarbon tail of the CTAB formed a protective micelle layer around the NPs (Fig. 8). This protective surface reduced the adhesion of NPs in the aqueous CTAB solution in comparison with distilled water [16-18].
In the PLA process of a target in a liquid environment, the formation of a larger plasma plume leads to the formation of larger NPs. Also, with slower cooling rate of the plasma plume, the larger NPs will be produced [41]. Numerous factors such as refractive index, density, viscosity, surface tension, specific heat capacity, and thermal conductivity are effective in the formation and cooling rate of the plasma plume [5, 16-21].
Refractive index of liquid environment is one of the effective parameters on the energy and volume of plasma plume. The laser beam reflects from air-liquid interface, and liquid-target interface before heating the target. Amount of the energy that reaches the target depends on the refractive index of liquid environment [39]. As expressed in Table 1, the refractive indexes of four liquids in this research are approximately equal. Therefore, this parameter can not have a perceptible effect on the amount of energy received by the target surface.
However, according to Table 1, at the wavelength of employed laser in the ablation process (1064 nm), acetone and ethanol have the same transmittance ~ 1, while pure water has a transmittance of 0.54559. Therefore, in the ablation process in ethanol and acetone solutions, which have higher transmittances than pure water, more laser energy reaches the target surface and causes more material to be ablated from the target surface. The greater the amount of ablated material, the larger the plasma plume, resulting in larger NPs to be produced [11, 39].
Plasma cooling rate and subsequent condensation also affect the size of colloidal NPs. Faster cooling of the plasma leads to the production of more particles. In a colloid produced by PLA in a liquid, the higher the number of particles resulting from condensation, the smaller their average size [42]. Specific heat capacity and thermal conductivity are among the parameters affecting the cooling rate. In liquids with smaller specific heat capacity and thermal conductivity, and the cooling of the plasma plume is slower, resulting in the formation of larger NPs [39]. The lower specific heat capacity and thermal conductivity of acetone compared to water and ethanol is another reason for formation of larger NPs in the acetone solution.
On the other hand, in the PLA process in liquid, the density, viscosity, and surface tension of the liquid environment has negligible effects on the confinement conditions of the plasma plume. With decreasing these parameters the volume of the plasma plume will increase, resulting in production of larger NPs [36, 39, 43]. The viscosity of acetone is significantly less than water and ethanol, which could be another decisive reason for forming larger NPs in acetone solution. In this case formation of larger NPs in acetone solution than other liquids were due to the higher transmittance and smaller viscosity, density, specific heat capacity, and thermal conductivity of acetone.
Compared to ethanol and acetone, pure water has the lowest transmittance at the wavelength of 1064 nm and the highest surface tension, density, boiling point, specific heat capacity, and thermal conductivity. With decreasing the transmittance, less energy reaches the target surface. The higher surface tension and density create stronger plasma plume confinement condition. And boiling point, specific heat capacity, and thermal conductivity affect the rate of cooling of the plasma plume and cavitation bubbles. Size of synthesized nanoparticles strongly depends to these parameters.
Decreasing the transmittance of liquid environment, and stronger confinement of plasma plume, beside increasing the rate of plasma cooling are the reasons of production of NPs with the smallest average size in distilled water. The formation of larger NPs in the aqueous CTAB solution than distilled water can be attributed to the reduction of surface tension due to the addition of CTAB surfactant. In fact, by adding CTAB surfactant to the water environment, micelles are formed and reduce the surface tension of water [15, 16].
- Optical properties
The absorption spectra of colloidal solutions prepared in different liquid environments were recorded using a UV–Vis–NIR spectrometer in the wavelength range of 200-1100 nm with respect to the corresponding liquid absorbance as the baseline using quartz cells of 1cm × 1cm. In Figure 9, these spectra are shown in the wavelength range of 200-500 nm because they did not represent specific information at wavelengths higher than 500 nm. The observed absorption peaks can be due to the excitonic absorption of WO3 NPs or the surface plasmonic absorption of W NPs [44-46].
The spectra show that the absorption starts at around 400 nm and increases towards shorter wavelengths. This property conforms to the reported absorption characteristics for WO3. In each spectrum, with decreasing wavelength, we observe one shoulder and one or two peaks. The positions of the shoulders and peaks observed in the spectra of samples prepared in different liquid environments are given in Table 2. The shoulder position matches the excitonic absorption peak position of WO3 [37, 44]. Since the surface plasmonic resonance absorption of the W occurs in the UV spectral region [47], we believe that the peaks observed in the UV–Vis spectra originate from the surface plasmonic resonance absorption of W NPs. Existence of both the W and the WO3 absorption peaks in the absorption spectrum is in good agreement with the peaks XRD pattern. In the spectra, plasmonic and excitonic absorption peaks of W and WO3 have overlapped together. Since the excitonic absorption is weaker than plasmonic absorption, it was appeared as a shoulder.
In the spectra of samples 1 and 4, two plasmonic absorption peaks are observed. One possible explanation for this double peak could be related to the coarseness of some particles due to their aggregation in the two environments [47]. According to particle size distribution histograms, the double peaks in sample 4 can be attributed to the formation of a significant number of particles larger than 100 nm. In sample 1, the number of particles larger than 100 nm is not significant, so the coarseness of some particles can not be a decisive reason for the duality of the peak in the spectrum of this sample. One of the other factors affecting the number of plasmonic peaks in the absorption spectra is the shape of the nanostructures. For spherical particles, a single adsorption peak appears in the adsorption spectra [6], and when the nanostructures shape changes from the sphere to the rod, a single plasmonic peak splits into two peaks [48]. In the FESEM and TEM images of our samples, rod-shaped nanostructures may not observe. Hence it is reasonable to assume that the cause of the duality of the peaks because of their nearness to each other is the quasi-sphericality of NPs. In sample 1, the quasi-sphericality of NPs is more noticeable, which causes two plasmonic peaks close to each other in the absorption spectrum. In samples 2 and 3, a single plasmonic peak is seen because, in these samples, NPs are spherical, and the coarseness of some particles is not noticeable.
The dependence of optical absorption spectra on characteristics of NPs such as size, size distribution, shape, and material causes UV–Vis spectroscopy to be an appropriate approach for investigating the optical properties of NPs. The size of semiconductor NPs affects their electronic structure. With decreasing the size of NPs, the bandgap energy increases. Absorption spectra present a qualitative study of the size and size distribution of NPs. In the absorption spectrum of colloidal NPs, with increasing the size of NPs, the absorption peak is shifted to a larger wavelength and vice versa. In other words, increasing the size of NPs in the samples causes a redshift in the absorption spectrum. The full width at half maximum(FWHM) of the excitonic absorption peak of semiconductor NPs depends directly on the width of the size distribution curve of NPs [5, 27].
On the other hand, according to Mie scattering theory for small particles, the position and number of plasmonic absorption peaks and the shape of the spectra depend on the size, shape, and material of NPs and the dielectric function of the environment surrounding them. Absorption broadening depends on the size distribution and aggregation of NPs. Mie theory shows that increasing the size of NPs causes the redshift of the plasmonic absorption peak and the decrease in the FWHM of the plasmonic absorption and vice versa [2, 6, 48-51].
Furthermore, the existence of various factors affecting the shape, position, width, and number of peaks, and on the other hand, the overlap of excitonic and plasmonic peaks has complicated the analysis of spectra. Therefore, the accurate comparison of spectrum characteristics using our data is difficult. Despite this, the following results can be extracted from the spectra. As mentioned earlier, the excitonic and the plasmonic peaks shift toward larger wavelengths when the average nanoparticle size increases. Therefore, in Figure 9, the redshift of the absorption peak of sample 3 relative to the other samples can be attributed to forming NPs with the maximum average size in this environment. That part of the absorption peak that can be attributed to the plasmonic resonance effects and be independent of the excitonic resonance effects is narrower in sample 3. Because according to Mie theory, the larger the average nanoparticle size is, the narrower FWHM of the peak is.
The photoluminescence (PL) spectroscopy is an advantageous method to identify the existence of different types of defects, oxygen vacancies, and lattice distortions in metal oxides [52, 53]. The PL emission spectra of the samples prepared in the different liquid environments are depicted in Figure 11. The PL spectra have been recorded at room temperature under 200 nm excitation wavelength.
Figure 11 shows the UV–Vis emissions centered at 295 and 300 nm in sample 1 and 295 nm in sample 4. UV–Vis emissions are due to the localized state of oxygen vacancies in WO3. Karazhanov et al. applied first-principles pseudopotential calculations and proposed that oxygen vacancies in WO3 are related to three types of defect states: 1) donor-like state within the fundamental bandgap, 2) hyper deep resonant state in the valence band, and 3) high-lying resonant state in the conduction band [54]. Therefore, the UV–Vis emissions observed in the spectra of samples 1 and 4 are due to the localized state of oxygen vacancies corresponding to the high-lying defect state in the conduction band of WO3 NPs. The intensity of UV–Vis emission in sample 4 is lower than in sample 1, which may indicate a decrease in oxygen vacancies or a decrease in the number of WO3 NPs produced in CTAB solution in comparison with distilled water. The spectra of samples 2 and 3 present no UV–Vis emission. It indicates the absence of the localized state of oxygen vacancies corresponding to the high-lying defect state in the conduction band of WO3 NPs.
The PL spectra illustrate four peaks centered at about 423, 436, 446, and 472 nm attributed to the blue emissions of WO3 NPs. The peak observed at 423 nm wavelength has maximum intensity. On the other hand, the blue emission of bulk phase WO3 powder with maximum intensity has been reported at the wavelength of 467 nm [55, 56]. Thus, the blue emissions of synthesized WO3 NPs produced by us have a blue shift relative to the blue emissions of the WO3 bulk phase. According to match the photon energy of 423 nm (2.93 eV) with bandgap energy reported in Ref. [52] for WO3 NPs, and observing blue emissions in the spectrum of all samples, it is reasonable to suggest that the blue emissions are due to band to band transitions [55, 56]. Investigating the PL spectra does not show a clear relationship between the size of NPs produced in different liquid environments with blue emissions. It means that the effect of quantum confinement on the bandgap is weak in our produced NPs [57].
The green emission peak observe at 528 nm was due to surface defects in WO3 NPs [52, 53]. Other researchers have also reported similar UV–Vis, blue, and green emissions in the PL spectra of WO3 nanostructures [52, 53, 55, 56].
4. Conclusion
PLA of a pure W target was performed in distilled water, ethanol, acetone, and CTAB solutions. XRD patterns, FTIR spectra, and TEM images showed the formation of W-WO3 composite NPs in all samples. The shape of NPs produced in all liquid environments was spherical or nearly spherical. NPs produced in the distilled water, ethanol, and CTAB solutions were strongly adhesive that can be related to the hydrogen bond due to the OH functional group between NPs and molecules of water or ethanol. Due to the high dipole moment of the acetone molecules and the formation of electrical double layers around NPs in the acetone solution, no adhesion was observed between the prepared NPs in this solution.
The liquid characteristics such as refractive index, density, viscosity, surface tension, specific heat capacity, boiling point, and thermal conductivity control the size of produced NPs by affecting the cooling rate and confinement of the plasma plume. In the sample prepared in acetone solution, NPs had the largest average size and standard deviation. It is due to the higher transmittance and smaller viscosity, density, specific heat capacity, and thermal conductivity of acetone. NPs produced in distilled water had the smallest average size in comparison with ethanol and acetone, since pure water has lower transmittance and higher surface tension, density, boiling point, specific heat capacity, and thermal conductivity, causing the formation of a smaller plasma plume and faster cooling rate. On the other hand, with adding CTAB surfactant to distilled water the average size of NPs increased. Also adhesion of produced NPs decreased by reducing the surface tension of CTAB solution and a micelle layer was generated around the NPs.
Results of the UV–Vis spectra also presented the excitonic and plasmonic resonance absorption of W and WO3 NPs in all samples. In the spectra of samples prepared in distilled water and CTAB solution, two plasmonic absorption peaks were observed relating to the coarseness of some NPs or the quasi-spherical of NPs. The PL spectra of all samples illustrate the green emission peaks due to surface defects and the blue emission peaks due to band-to-band transitions in WO3 NPs. The UV–Vis emissions observed in the PL spectra of samples prepared in distilled water and CTAB solution were due to the localized state of oxygen vacancies corresponding to the high-lying defect state in the conduction band of WO3 NPs.
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Table 1: Physical and chemical properties of materials of ablation environments[19-21].
CTAB | Pure Acetone | Pure Ethanol | Pure Water | Material |
C19H42BrN | CH3-CO-CH3 | CH3-CH2-OH | H2O | Chemical formula |
|
|
|
| Chemical structure |
- | 56.3 | 78.37 | 100 | Boiling point(℃) |
- | 2.14 | 2.430 | 4.184 | Specific heat capacity(J/gK) |
- | 0.79 | 0.789 | 0.998 | Density(g/cm3) |
- | 0.316 | 1.209 | 0.982 | Viscosity(cP) |
- | 2.191 | 1.69 | 1.85 | Dipole moment(D) |
- | 7 | 7.33 | 7 | PH |
- | 20.7 | 25.3 | 78.4 | Dielectric constant |
1.44 | 1.36135 | 1.36371 | 1.3204 | Refractive index at the wavelength of 1064 nm |
- | ≈1 | ≈1 | 0.54559 | Optical transmittance along 1cm in the liquid |
- | 0.161 | 0,167 | 0.595 | Thermal conductivity (W/m.K) |
- | 25.20 | 22.39 | 71.86 | Surface tension (mN/m) |
Table 2: Introduction of samples, the size distribution of NPs (taken from TEM images,) and plasmon/exciton resonance wavelengths of W and WO3 NPs (taken from UV–vis spectra) in different environments
CTAB Solution (0.0027 M) | Acetone Solution (Acetone:Water in ratio 2:1) | Ethanol Solution (Ethanol:Water in ratio 2:1) | Distilled Water | Liquid Environment |
4 | 3 | 2 | 1 | Sample |
71.01 | 79.24 | 66.27 | 49.54 | Average size of NPs (nm) |
35.52 | 32.39 | 26.67 | 27.19 | Standard deviation(nm) |
212 ,231 | 234 | 216 | 219 , 231 | Plasmon resonance wavelengths(nm) (Peak positions) |
275 | 278 | 279 | 262 | Exciton resonance wavelengths(nm) (Shoulder positions) |
Figure 1: Colloidal samples prepared by PLA of W target in different liquid environments.
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Figure 2: EDS spectra of samples prepared in different liquid environments.
Figure 3: XRD spectra of a) samples prepared in different liquid environments and b)W target used in PLA.
:broad peak attributed to the silicon substrate , : Hexagonal WO3 , : CTAB(C19H42BrN)
: cubic W with Fm-2m( 225) space group ,: cubic W with Im-3m(229) space group
Sample 2 |
Sample 1 |
Sample 4
|
Sample 3
|
Figure 4: FTIR spectra of samples prepared in different liquid environments.
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|
 |
Figure 5: FESEM images of W-WO3 composite NPs prepared in different liquid environments.
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Figure 6: TEM images of W-WO3 composite NPs prepared in different liquid environments.
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Figure 7: Size distribution of W-WO3 composite NPs prepared in different liquid environments. Data taken from TEM images
Figure 8: Formation of micelle layer around W-WO3 composite NPs with core-shell structure.
Figure 9: UV–Vis spectra of W-WO3 composite NPs prepared in different liquid environments.
Figure 10: PL emission spectra of W-WO3 composite NPs prepared in different liquid environments at 200 nm excitation wavelength.
