Copyright © 2012 by American Scientific Publishers All rights reserved. Printed in the United States of America
Science of Advanced Materials Vol. 4, pp. 1–4, 2012 (www.aspbs.com/sam)
Optical Properties of Bismuth Oxide Nanoparticles Synthesized by Pulsed Laser Ablation in Liquids M. A. Gondal1, 2, ∗ , Tawfik A. Saleh2, 3 , and Q. Drmosh1, 2 1
Laser Research Group, Physics Department Center of Excellence in Nanotechnology (CENT) 3 Chemistry Department King Fahd University of Petroleum and Minerals, Box 5047 Dhahran 31261, Saudi Arabia 2
ABSTRACT
1. INTRODUCTION A lot interest has been sparked by research community during last few years in the synthesis and applications of nanostructured materials because nanoparticles (NPs) exhibit novel properties due to their nano size that are significantly different from those of their bulk counterparts. Bismuth oxide (Bi2 O3 ) has been widely used in many applications such as in fuel cells,1 medical field,2 gas sensing3 and catalysis process.4 Bi2 O3 exists in four crystalline structures: -Bi2 O3 , a monoclinic crystal structure at room temperature, -Bi2 O3 , when heated between 727 C–824 C. In addition, two meta-stable polymorphs of the bismuth oxide also exist: the tetragonal -Bi2 O3 phase and the cubic -Bi2 O3 phase.5 Many methods for synthesizing Bi2 O3 NPs have been proposed. Bi2 O3 has been synthesized via the oxidation of bismuth metal at 800 C or thermal decomposition of bismuth salt solution.6 7 Flame spray pyrolysis8 has been applied to produce nano-sized Bi2 O3 particles. Citrate gel process has been also reported for the preparation of nanocrystalline Bi2 O3 .9 Pulsed Laser Ablation (PLA) technique has been recently introduced for the synthesis of high quality nano ∗
Author to whom correspondence should be addressed. Email:
[email protected] Received: 4 April 2011 Accepted: 18 May 2011
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structured materials in liquid media from a solid target to prepare metal oxides and metal alloys.10–15 PLA posses has unique advantages for synthesise of nano structured particles like high purity, simple, rapid, does not require sophisticated vacuum equipment and does not require chemicals as in wet chemical methods which contaminate the end product and also pollute the environment. In addition, it has been demonstrated that size of synthesized material can be controlled by changing different parameters such as: laser wavelength, laser pulse duration, changing the pH of the solution, adding surfactants and changing the temperature of solution.16–20 These parameters not only change the size and shape of the NPs but at same time could affect their optical and electronic properties. In this work Bi2 O3 NPs were synthesized by irradiating a solid Bismuth immersed in 3% H2 O2 using Nd:YAG operating at 355 nm as an irradiation source.
2. EXPERIMENTAL DETAILS Figure 1 depicts the schematic of our set up for synthesis of NPS using PLA technique that has been described in detail in our previous work.23 For the synthesis of Bi2 O3 NPs using this method, a high purity 60 mg Bismith powder (99.998%) purchased from Aldrich Company was pressed in pallet shape and was immersed in pure 3% H2 O2 The pallet immersed in H2 O3 solution was rotated
1947-2935/2012/4/001/004
doi:10.1166/sam.2012.1310
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Bismuth oxide (Bi2 O3 ) nanostructured particles were synthesized by pulsed laser ablation (PLA) technique by irradiating solid bismuth target in 3% H2 O2 solution using 355 nm laser radiations generated by third harmonic Nd:YAG laser. Synthesized nanoparticles (NPs) were characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), UV–visible absorption, Fourier transform Infra-Red (FTIR) and Raman spectroscopy. The XRD pattern of the prepared sample was indexed as bismuth oxide - Bi2 O3 and particle size was evaluated = 4 nm. The UV–visible spectrum indicated that the absorption edge is blue shifted and the band gap of the prepared sample was estimated = 3.8 eV. The Raman spectrum exhibited peaks at 68, 96, 135, 315, 183, and 470 nm which are the characteristic bands of -Bi2 O3 . The FTIR spectrum showed the absorption band at 829 cm−1 which is attributed to Bi–O–Bi bond, and the strong absorption band recorded at 442 cm−1 is due to the stretching mode of Bi–O. KEYWORDS: Nanoparticles, Pulsed Laser Ablation, Bismuth Oxide, Optical Properties, Nanotechnolgy.
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Optical Properties of Bismuth Oxide Nanoparticles Synthesized by Pulsed Laser Ablation in Liquids
Fig. 1.
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Schematic of experimental setup applied for the synthesis of Bi2 O3 nanoparticles using pulsed laser ablation in liquids.
with high speed magnetic starrier to avoid a deep crust due to repeated laser pulses on same spot and was irradiated by a Q-switched Nd-YAG laser (Spectra physics Model GCR 100) operating at 355 nm wavelength generated by third harmonic crystal. The laser beam was focused by a lens onto the soild bismith target. The typical diameter of the laser spot on a Bi target was ∼0.08 mm and the typical liquid volume was 10 ml (Fig. 1). After 40 minutes of ablation, a colloidal solution containing Bi2 O3 -NPs was obtained and then characterized. A variety of analytical techniques were applied for the characterization of the Bi2 O3 NPs. X-Ray Diffraction (XRD) (Shimadzu XRD Model 6000) with Cu K irradiation at = 154 Å was employed to determine crystalline phases, average crystalline sizes, Field Emission Scanning Electron Microscope (FESEM, FEI Nova-Nano SEM-600, Netherlands) was applied for morphological studies. In addition, Infrared absorption spectra were measured at room temperature by FTIR spectrometer using the KBr Pellet technique. UV-Vis spectrometer (JASCO V-570) was used to record the UV-Vis absorption spectra. Raman spectrum measurement of the Bi2 O3 NPs was performed at room temperature by a Raman Nicolet 6700 NXR spectrometer.
‘d’ of Bismuth oxide NPs were determined by applying the Scherrer equation to the line broadening of the peak24 d=
k cos
where = wavelength of the X-ray, = FWHM (Full Width at Half Maximum) width of the diffraction peak, diffraction angle and k constant. The average grain size of Bi2 O3 NPs calculated using the above equation at angle 28.24 was 4 nm. The morphology of the synthesized Bi2 O3 NPs was investigated by FE-SEM which is depicted in Figure 3(a) and clearly shows the formation of NPs. Figure 3(b) is an EDX spectrum obtained from the same NPs. Only Bi and O peaks were observed, suggesting that the NPs
3. RESULTS AND DISCUSSION Figure 2 depicts the X-ray diffraction (XRD) pattern of Bi2 O3 NPs prepared by PLA in 3% H2 O2 . The XRD spectra clearly showed the crystalline structure of the nano particles and various peaks of -Bi2 O3 . The Crystalline size 2
Fig. 2. XRD pattern of bismuth oxide nanoparticles prepared by using Nd:YAG laser operating at 355 nm with pulse energy = 110 mJ for 30 min duration. Sci. Adv. Mater., 4, 1–4, 2012
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Optical Properties of Bismuth Oxide Nanoparticles Synthesized by Pulsed Laser Ablation in Liquids
Fig. 4. Absorption spectrum of bismuth oxide nanoparticles prepared by PLA method using pure bismuth metal target in 3% H2 O2 . The inset shows the plot of h 2 versus energy (h ) for the determination of band gap energy (Ebg ).
FE-SEM images (a) the EDX spectra (b) of Bi2 O3 nanoparticles.
synthesized by PLA technique are of high purity. The quantitative EDX analysis showed that the atomic ratio of Bi/O is 0.8065:0.1935, close to 4:1, indicating that the composition of the synthesized samples is indeed of bismuth oxide (Bi2 O3 ). Figure 4 depicts a typical room temperature UV-Vis absorption spectrum of colloidal Bi2 O3 NPs. The spectra showed a characteristic absorption at 234 nm owing to Bi2 O3 . This value has been moderately blue shifted and the reason for this blue-shift absorption could be explained by Brus’ equation.25 According to this equation, the shift could occur due to the decrease in the particles size especially for particles having size of less then 10 nm. The size dependence of the band gaps of several semiconductor particles has been discussed by various researchers.26–28 The value of the band gap for the Bi2 O3 nanostructures was estimated by using Tauc equation:29–31 h = A h − Eg 2 where A is constant, Eg represents the energy band gap, v is the frequency h Plank constant and is the a absorption coefficient. A Plot of h 2 versus energy (h ) is depicted in Figure 4. The tangent drown at absorption edge was extrapolated to energy axis and the intercept gives Sci. Adv. Mater., 4, 1–4, 2012
Fig. 5. Raman spectra of the synthesized Bi2 O3 nanoparticles.
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Fig. 3.
the value of the band gap. The optical band gap value obtained from Tauc plot (Fig. 4) is 3.8 eV. This value is in agreement with the values reported earlier29–31 for Bi2 O3 synthesized by wet chemical methods. Raman spectroscopy (RS) was used to differentiate between various structures of nano-materials. The four known structures of bismuth oxide, labeled as (, , and -Bi2 O3 have been reported because of their attractive spectroscopic properties. Raman features in the 50–600 cm−1 range have been assigned to -Bi2 O3 and -Bi2 O3 while Raman bands in 50–900 cm−1 range are attributed to -Bi2 O3 and -Bi2 O3 .32 Thus, the Raman analysis was applied to confirm the phase information of the Bi2 O3 . Figure 5 shows the Raman spectrum of the NPs of bismuth oxide, with a frequency region from 50 to 500 cm−1 . As shown in Figure 5, the Raman peaks at 68, 96, 135, 315, 183, and 470 nm are the characteristic bands of -Bi2 O3 . Three sharp bands at 470, 315, and 135 cm−1 are assumed to be due to three unique Bi–O stretches.32 The similarity between the structures of bismuth oxide, -Bi2 O3 and -Bi2 O3 has been reported.33 The Raman band modes at 69 and 93 cm−1 asre assigned to the Eg and
Optical Properties of Bismuth Oxide Nanoparticles Synthesized by Pulsed Laser Ablation in Liquids
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References and Notes
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Fig. 6. FTIR spectra of Bi2 O3 nanoparticles prepared by pulsed laser ablation method.
A1g vibrational modes of Bi in Bi2 O3 .33 34 One can easily conclude from the Raman bands spectra, the presence of the -Bi2 O3 (tetragonal) phase. However, the appearance of the peaks at 69 and 93 cm−1 in the Bi2 O3 system can also be assigned to the presence of -Bi2 O3 30–34 as evident in the XRD studies. In order to corroborate the results concerning the identification of the bismuth oxide phase exhibited in the XRD pattern, FTIR spectroscopy was also applied. Figure 6 depicts the FTIR spectra of Bi2 O3 NPs. In this figure, the vibrational absorption band at around 3400 cm−1 is due to hydroxyl group of water adsorbed on the synthesized NPs. The absorption band at 829 cm−1 is a consequence of Bi–O–Bi bonds. The strong absorption band at 442 cm−1 is due to the stretching mode of Bi–O. This is in agreement with data reported in reference.35 The absorption bands recorded at 442, 470 and 510 cm−1 are attributed to -Bi2 O3 .36 These findings are in agreement with XRD diffractogram results depicted Figure 2.
4. CONCLUSIONS Nano- structured Bi2 O3 NPs were synthesized by PLA method in 3% H2 O2 for the first time. XRD data and FE-SEM image confirm that synthesized -Bi2 O3 with our method were having grain size of 4 nm. The optical absorption spectrum of the NPs exhibited an absorption band at about 234 nm and the band gap estimated was 3.8 eV. The Raman spectra indicated the characteristic bands of -Bi2 O3 at 68, 96, 135, 315, 183, and 470 nm. The FTIR spectra of the synthesized Bi2 O3 NPs confirmed the absorption bands at 829 cm−1 that is attributed to Bi– O–Bi bonds and the strong absorption band recorded at 442 cm−1 is assigned to the stretching mode of Bi–O. Acknowledgment: The support by King Fahd University of Petroleum and Minerals through projects 08NAN93-4 is gratefully acknowledged. 4
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