fragmentation technique in different liquid media

Applied Surface Science 289 (2014) 462–471

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Optical and magnetic properties of Fe2 O3 nanoparticles synthesized by laser ablation/fragmentation technique in different liquid media B.K. Pandey a,∗ , A.K. Shahi a , Jyoti Shah b , R.K. Kotnala b , Ram Gopal a,∗ a b

Laser Spectroscopy and Nanomaterials Lab, Department of Physics (UGC-CAS), University of Allahabad, Allahabad 211002, India National Physical Laboratory (NPL), New Delhi, India

a r t i c l e

i n f o

Article history: Received 15 October 2013 Received in revised form 22 October 2013 Accepted 3 November 2013 Available online 13 November 2013 Keywords: Laser ablated Fe2 O3 Particle ablation medium Structure analysis Optical property Magnetic property

a b s t r a c t Iron oxide (Fe2 O3 ) bulk powder have been ablated/fragmented in different liquid medium by Nd:YAG laser beam using 1064 nm wavelength. Sodium dodecyl sulfate (SDS), cetyltrimethyl ammonium bromide (CTAB) and double distilled water (DDW) are used as liquid medium. Crystalline size, lattice strain, phase and structure of ablated particles have been investigated using synchrotron X-ray diffraction. Optical band gap energy of as purchased Fe2 O3 found 1.92 eV that increased to 2.03 eV after ablation in CTAB determined by UV–vis absorption spectroscopy. Magnetic properties have been analyzed by hysteresis loops using vibrating sample magnetometer (VSM). Crystalline sizes have been found in the range of 29.23–16.54 nm and coercivity tailored in the range of 206.91–298.36 Oe using laser ablation. Saturation magnetization and remanence have been found in the range of 0.013–3.41 emu/g and 0.0023–.0.51 emu/g respectively. Particle shape and size have been examined by scanning electron microscopy (SEM). CTAB (cationic) and SDS (anionic) surfactants are used as capping agent. CTAB produces phase transformation in ablated iron oxide (Fe2 O3 ). Crystallinity and crystalline size of ablated particles in DDW increased due to presence of rich oxygen in it due to oxidation. Ablated Fe2 O3 nanoparticles have been widely used experimentally for numerous in vivo applications such as MRI contrast enhancement agent, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, drug delivery and cell separation. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Laser induced ablation/fragmentation in liquid media is one of the powerful techniques used for synthesis of nanoparticles with special anisotropy which is otherwise difficult to produce by conventional methods [1–3]. The size and shape of the nanoparticles can be controlled by changing the laser wavelength [4,5] laser fluence [6,7] liquid environment [8,9] and surfactants [10–12]. Iron can form several oxides of different stoichiometry and crystalline structure depending upon synthesis technique. These oxides are wustite (FeO), magnetite (␥-Fe3 O4 ), hematite (␣-Fe2 O3 ), and magnetite (␥-Fe2 O3 ). Hematite is thermodynamically most stable phase of Fe2 O3 , and is the subject of this work. Hematite (␣Fe2 O3 ) has been considered as promising material since ␣-Fe2 O3 is inexpensive, abundant, nontoxic, and stable in most alkaline electrolytes [13–15]. In order to prepare homogenous nano-particles of iron oxide, researchers have employed in different routes to facilitate single-phase iron oxide nano-particles such as sol–gel processes [16], microemulsion [17], combustion [18], solvothermal

∗ Corresponding authors. Tel.: +91 532 2460764; fax: +91 532 2460993. E-mail addresses: [email protected], [email protected] (B.K. Pandey), [email protected] (R. Gopal). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.009

[19], hydrothermal [20] and precursor [21]. In the absence of any surface coating, magnetic iron oxide particles have hydrophobic surfaces with a large surface area to volume ratio. Due to hydrophobic interactions between the particles, these particles agglomerate and form large clusters, resulting in increased particle size. These clusters exhibit strong magnetic dipole–dipole attractions between them and show ferromagnetic behavior. When two large-particle clusters approach one another, each of them comes into the magnetic field of the neighbor. Besides the arousal of attractive forces between the particles, each particle is in the magnetic field of the neighbor and gets further magnetized. To avoid further agglomeration cationic and anionic surfactants are subject of present study used. In the present communication we have studied optical as well as magnetic properties of as purchased iron oxide (Fe2 O3 ) bulk powder and ablated NPs in different liquid medium using pulse laser ablation/fragmentation technique. Here it is shown that optical band gap and magnetic properties (coercivity, saturation magnetization, and remanence) and phase of as purchased iron oxide (Fe2 O3 ) bulk powder has been tailored. Samples R2, R3, and R4 were prepared in DDW SDS, and CTAB, respectively, while R1 (Fe2 O3 ) bulk powder purchased commercially. Liquid phase pulse laser ablation (LP-PLA) which involves the firing of laser pulses on the surface of solid/powder target immersed into liquid media. In

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beam, high level of polarization, low emittance, i.e. the product of source cross section and solid angle of emission is small, (3) large tunability in wavelength by monochromatization, (4) pulsed light emission (pulse durations may be below one nanosecond in 3rd generation sources and close to picoseconds in 4th generation sources) which allows ultra-fast time-resolved studies. Iron oxygen bonding is confirmed by FT-IR (ATR) spectrum using thermo scientific. The magnetic measurement has been carried out by a VSM (Lakeshore, USA) at room temperature.

3. Results and discussion 3.1. X-ray diffraction

Fig. 1. Experimental setup for laser ablation/fragmentation of Fe2 O3 bulk powder.

laser pulses, front part of pulse creates vapors on the target surface, which are irradiated by tail part of same pulse. Zeng et al. has already reported the mechanism of laser ablation/irradiation in liquid media [22,23]. This process causes photo ionization and the generation of dense high-temperature and high pressure laser plasma plume, which expands perpendicular to the target surface into the liquid. This expanding plume interacts with the surrounding liquid, creating cavitations bubbles, and collapse, give rise to very high temperature and pressure however; these conditions are much localized and exist in nanoscale range.

2. Experimental The experimental setup of laser ablation/fragmentation is shown in Fig. 1. [24–26]. High purity (99.9%, Qualigens, India) powder of iron oxide (Fe2 O3 ) (particle size 1–5 ␮m) was dispersed in a test tube containing 50 ml aqueous solution SDS and CTAB. The suspension was continuously stirred using magnetic stirrer. Nd:YAG laser (Spectra Physics, USA) operated at 1064 nm wavelength, 40 mJ/pulse energy, and 10 Hz repetition rate was focused to a spot size of 2 mm at the center of the glass tube using a lens for 1 h. After ablation/irradiation of 1–5 ␮m powder of Fe2 O3 , colloidal suspension of Fe2 O3 NPs in SDS and CTAB was centrifuged at 5000 rpm. Obtained colloidal solution was dried and used for further characterization. UV–vis absorption spectrum of synthesized colloidal solution of nanoparticles was recorded using Perkin Elmer Lambda 35, double beam spectrophotometer. X-ray diffraction pattern of as synthesized samples were recorded using Indus-2, beam line-12 synchrotron radiation source ˚ Synchrotron radiation is inherhaving wavelength  = 0.7306 A. ently advantageous to laboratory sources for several reasons, the main advantage are as follows. (1) High brightness and high intensity, many orders of magnitude more than with X-rays from X-ray tubes, (2) high collimation, i.e. small angular divergence of the

Fig. 2(a) and (b) shows XRD pattern of samples R1 and R2 respectively. The peak positions at 2 = 11.377◦ , 2 = 15.543◦ , 2 = 16.771◦ , 2 = 19.042◦ , 2 = 22.869◦ , 2 = 24.877◦ , 2 = 26.395◦ , 2 = 28.434◦ , 2 = 29.085◦ , 2 = 32.328◦ , 2 = 33.709◦ , 2 = 35.749◦ , 2 = 36.586◦ , 2 = 37.717◦ , and 2 = 38.637◦ correspond to the plane [0 1 2], [1 0 4], [1 1 0], [1 1 3], [0 2 4], [1 1 6], [0 1 8], [2 1 4], [3 0 0], [1 0 1 0], [2 2 0], [1 2 8], [0 2 1 0], [1 3 4], and [2 2 6] respectively with, Rhomb-centered, hexagonal lattice parameters, a = 5.035 = b, c = 13.74, ˛ = ˇ =  (JCPDF No. 86-05500). Fig. 2(c) and (d) shows Xray diffraction pattern of samples R3 and R4. Fig. 2(d) shows mix phase of FeO(OH), Fe2 O3 and C2 H4 Fe2 O3 . The peak position is indicated by symbol H at 2 = 9.996◦ corresponds plane [1 1 0], to iron oxide hydroxide (FeO(OH)) with orthorhombic, primitive lattice a = 4.618, b = 9.952, c = 3.023 (JCPDF No. 81-0462). Peak positions are indicated by symbol C at 2 = 12.708◦ , 2 = 13.670◦ , 2 = 13.861◦ , and 2 = 15.804◦ corresponds to plane [0 0 2], [2 1 1], [3 1 1], and [1 2 0] respectively for iron maleate (C2 H4 Fe2 O3 ) anorthic primitive lattice a = 9.659, b = 5.309, c = 7.378 ˛ = 87.72◦ , ˇ = 65.34◦ ,  = 105.35◦ (JCPDF No. 35-1710). The peak positions indicated by symbol O are of iron oxide (Fe2 O3 ). XRD result reveals that CTAB plays dual role i.e. reducing particle size as well as changing phase of Fe2 O3 . The crystalline sizes can be estimated using Scherrer’s formula D=

K ˇ cos 

(1)

where the constant K is taken to be 0.94,  is the wavelength of X-ray used which is synchrotron radiation ( = 0.730 A), and ˇ the full width at half maximum of the diffraction peak corresponding to 2. Crystalline size is calculated using Eq. (1) corresponding to various peaks as shown in Table 1. The crystalline size of sample R2 is greater than sample R1, R3, and R4 while crystalline size of sample R4 is smaller among the three. Here it should be noted that crystalline size is different than particle size, i.e. crystalline size less than particle size. Furthermore Using Williamson–Hall plot, we have calculated the lattice strain and effective particle size using the following relation [27] ˇ cos  1  sin  = + ε  

(2)

where ˇ is full width at half maximum (FWHM) in radians, ε is the effective crystalline size and  is effective strain. Fig. 3(a)–(d) shows the Williamson–Hall plot, i.e. the plot between ˇ cos / versus sin /. Negative slope in the plot indicate the presence of compressive strain whereas the appearance of positive slope indicate the possibility of tensile strain. The intercept on the ˇ cos / axis gives the particle size corresponding to zero strain [28–32]. Table 2 displays the enhancement in strain with the reduction of crystalline

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Fig. 2. X-ray diffraction of Fe2 O3 powder ablated in different liquid medium.

Table 1 Crystalline size calculation by Scherrer’s formula using X-ray diffraction. S. no.

Sample name

Peak position (2)

Peak intensity

cos 

sin 

FWHM (ˇ)

Crystalline size, D (nm)

1.

Purchased Fe2 O3 (R1 )

11.377 15.543 16.771 19.042 22.869 24.877 28.434 32.328

766.66 2706.23 2320 740 1266.23 1706.66 1406.21 1346.23

0.9950 0.9908 0.9893 0.9862 0.9801 0.9765 0.9693 0.9604

0.0991 0.1352 0.1458 0.1654 0.1982 0.2153 0.2455 0.2783

0.002414 0.002356 0.002463 0.002238 0.002300 0.002157 0.002273 0.002129

27.353 28.145 26.963 29.952 29.145 31.191 29.820 32.129

2.

Fe2 O3 in DD water (R2 )

11.377 15.543 16.771 19.042 22.869 24.877 28.434 32.328

1895.95 5561.12 4639.31 1486.54 2615.47 3348.63 2615.40 2673.60

0.9950 0.9908 0.9893 0.9862 0.9801 0.9765 0.9693 0.9604

0.0991 0.1352 0.1458 0.1654 0.1982 0.2153 0.2455 0.2783

0.002332 0.002295 0.002259 0.002217 0.002222 0.002184 0.002150 0.002074

28.314 28.893 29.398 30.700 30.168 31.293 31.525 32.984

3.

Fe2 O3 in SDS (R3 )

11.377 15.543 16.771 19.042 22.869 24.877 28.434 32.328

1596.23 5436.25 4647.62 1433.25 2576.02 3421.12 2521.30 2684.56

0.9950 0.9908 0.9893 0.9862 0.9801 0.9765 0.9693 0.9604

0.0991 0.1352 0.1458 0.1654 0.1982 0.2153 0.2455 0.2783

0.002375 0.002330 0.002241 0.002217 0.002355 0.002196 0.002204 0.002103

27.805 28.459 29.634 30.049 28.464 30.638 30.764 32.529

4.

Fe2 O3 in CTAB (R4 )

9.996 11.377 15.543 16.771

4272.72 3118.88 677.13 700.11

0.9961 0.9950 0.9908 0.9893

0.0871 0.0991 0.1352 0.1458

0.004204 0.004938 0.006243 0.006187

15.685 10.672 10.621 10.733

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Fig. 3. Williamson–Hall plots of Fe2 O3 powder as ablated in DDW, SDS, and CTAB, size and strain analysis.

Table 2 Comparative study of optical band gap energy, effective particle size, effective strain from Williamson–Hall plots and magnetic properties of Fe2 O3 powder as synthesized in different liquid medium, from hysteresis loop measurement at room temperature. S. no.

Sample name

Effective crystalline size, ε (nm)

Optical band gap energy (eV)

1. 2. 3. 4.

Purchased Fe2 O3 (R1) Fe2 O3 in water (R2) Fe2 O3 in SDS (R3) Fe2 O3 in CTAB (R4)

27.93 29.23 21.23 16.54

1.92 1.98 2.01 2.03

Optical particle size (nm) – 8.85 7.22 6.53

size, this result confirms the particle size and strain depends on different surfactant media [33]. In sample R2 and R3 strain is reduced while in sample R4 strain increases reasonably in respect to sample R1. This effect is arisen due to the reduced crystalline size and mix phase respectively. The crystalline size measured using both methods Scherrer’s formula and Williamson–Hall plot is well consistent. Fig. 4 shows variation of crystalline size with effective strain, which confirms strain decreases as crystalline size increase except sample R4. This anomaly may cased by lattice distortion due to quantum confinement. 3.2. UV–vis absorption Fig. 5 shows the absorption spectrum of Samples R1, R2, R3 and R4 defined earlier. The maximum absorption peak of sample R1, is near 792 nm, R2, has maximum absorption near 590 nm while onset absorption starts at 895 nm. The absorption edge is red shifted due to improvement in crystallinity and crystalline size as clear from Tables 1 and 2. R3 has maximum absorption near 580 nm and onset absorption starts at 802 nm. The absorption spectrum of sample R3 is blue shifted than R2 due to smaller crystalline size (Tables 1 and 2) and capping of SDS molecules. Sample R4 has broad

Effective strain ()

Hc (Oe)

MS (emu/g)

MR (emu/g)

Remanence ratio, MR /MS

0.0105 0.00178 0.00126 0.032

239.84 209.33 298.36 206.91

0.024 3.41 0.21 0.013

0.0051 0.51 0.041 0.0023

0.212 0.149 0.195 0.176

absorption in the range of 588–793 nm. S Mitra et al. have reported two absorption edges of ␣-Fe2 O3 nanocrystal around 540–560 and 670–680 nm [34]. The broadness in absorption is due to formation of mix phases i.e. FeO(OH), Fe2 O3 and C2 H4 Fe2 O3 . The sample R4 has smallest crystalline size as confirmed from Tables 1 and 2. The absorption band gap Eg of the as synthesized samples can be determined by the Tauc equation: ˛h = (h − Eg )

n

(3)

where h is the photon energy, ˛ is the absorption coefficient and n is either 1/2 for a direct transition or 2 for an indirect transition. The optical band gap of semiconductor can be estimated from the intercept of the extrapolated linear fit for the plotted experimental data of (˛h)n versus incident photon energy h near the absorption edge. The band gap energies of sample R1, R2, R3, and R4 are found to be 1.92, 1.98, 2.01, and 2.03 e V respectively considering indirect transition as shown in Fig. 6 which is well consistent with previously reported value [35,36]. Crystalline size and band gap variation is shown in Fig. 7. Here it is clear that on decreasing crystalline size optical band gap increases (Table 2). Sample R2 shows larger crystalline size than R1, but also higher bandgap energy. It may due to Sample R2 get more oxidized and optical band gap widen. Direct

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that are magnetically coupled. In our case the transition in the range of 600 to 750 nm is assigned due to the single d-d transition of Fe3+ + inter valence charge transfer transitions 6 A1 → 4 T2 (4 G) [39]. The optical particle size from band gap is calculated by using effective mass formula given as Eg = E 0 +

n2 2 2 1.8e2 + εR 2 R2

where E0 is the bandgap of the bulk Fe2 O3 and R is the size of the nanocrystallites. In above equation, e is the electron charge, ε is the effective dielectric constant, and is the reduced effective mass of electron and hole of Fe2 O3 . In the above equation, we used ε = 5.7,

= 0.08m0 (m0 is the electron’s rest mass) [40]. For bulk bandgap (E0 ) we took the lowest value we have measured in our experiments. Calculated particle size for sample R2, R3 and R4 are shown in Table 2.

3.3. FTIR spectra of Fe2 O3 Fig. 4. Strain versus crystalline size.

bandgap energy of Fe2 O3 nano wire and thin film are reported in range of 2.14–2.22 e V and 2.21–2.23 e V respectively [37,38]. It is already reported that three types of electronic transitions occur in the optical absorption spectra of Fe3+ substances: (a) the Fe3+ ligand field transition or the d–d transitions, (b) the ligand to metal charge–transfer transitions, and (c) the pair excitations resulting from the simultaneous excitations of two neighboring Fe3+ cation

Fourier transform infrared (FTIR) transmittance spectrum of samples R1, R2, R3 and R4 are shown in Fig. 8(a). Fig 8 shows spectrum in the range of 500–4000 cm−1 and Fig. 8(b) shows in the range of 480–660 cm−1 . The peak positions assigned to 499, 515, 536, and 573 cm−1 are due to Fe O stretching vibration of bulk sample (R1) [41–45]. Intense and wide peak at 497, 547, and 699 cm−1 for sample R2 are again due to Fe O stretching vibration, peak position at 497 is intense and wide it may due to enhancement of crystalline size and percentage of crystallinity. The peak positions for sample R3 are assigned at 511, 540, 563, and 584 are

Fig. 5. UV–vis absorption spectra of ablated Fe2 O3 colloids in different liquid medium.

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Fig. 6. Tau’s plot to determined band gap of Fe2 O3 solution as synthesized in different liquid medium.

Fig. 7. Crystalline size and optical bandgap analysis.

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Fig. 8. FTIR spectra of as synthesized Fe2 O3 nanoparticles in different liquid media.

due to iron–oxygen stretching vibration. Sample R4 shows intense and wide peak position at 495, 522, 565, and 588, are again due to Fe O stretching vibration. Sifting of respective peak positions of samples R1, R2, R3, and R4 may arise due to different crystalline size, percentage of crystallinity and different strain in sample as shown in Table 2. Peak position at 1363, 1520 and 1736 cm−1 are due to H O H bending and deformation vibrations of adsorbed water as shown in Fig. 8(a) [24,26]. An intense peak at 2362 cm−1 is assigned to atmosphere CO2 or nitrogen oxide. The peak positions at 3336, 3643, 3739, and 3862 cm−1 are assigned as H-OH stretching vibrations [26].

crystalline size and coercivity variation is shown in Fig. 10(a) [49]. Sample R4 shows lowest value of coercivity and crystalline size, this result is not consistent with above result; it may due to mix phases of iron oxide NPs. The enhance coercivity may caused by magneto crystalline anisotropy and easy axis of magnetization. Saturation magnetization (MS ) and remanence (MR ) increases due to increase of crystalline size, shown in Fig. 10(b) and (c). [52].

3.4. Magnetic properties

Positively charged surface of the ablated iron oxide (Fe2 O3 ) surface electrostatically attracts the anionic head (SO4 2− ) of the SDS molecule, making its tail away from the surface [53]. However, anionic surfactant SDS usually does not show a significant effect toward reduction of particle size it may attributed due to weak interaction between ablated iron oxide (Fe2 O3 ) and SDS [54]. So SDS is playing important role for enhancing coercivity due to the surface coating and spin ordering along long axis [55], since it is well known that as particle size decreases magnetization also decreases which is well consistent with present result [55,56]. In aqueous solution, CTAB dissociates into a bromide ion (Br− ) and a cetyltrimethylammonium ion (CTA+ ) such that the surface charge property could be modified [57]. When CTA+ is adsorbed onto the ablated iron oxide (Fe2 O3 ) surface, it behaves as positively charged surface and due to Colombian repulsion it restrict further agglomeration and stabilized, further it cases reduction in particle size of sample R4. Sample R1, R3 and R4 have low value of MS and MR due to magnetically disordered surface layer around the particles. Magnetic properties of Fe2 O3 have been studied previously by many authors [58–62].

Fig. 9 shows the magnetic properties of as synthesized samples R1, R2, R3, and R4 at room temperature. The coercivity and other magnetic properties such as ferromagnetism have a direct relationship with the crystalline shapes and sizes [46]. Sample R1 shows low value of saturation magnetization (MS ) and remanence (MR ), while coercivity 239.84 Oe. Sample R1 shows Maximum remanence ratio (MR /MS ) which is found to be 0.212. The low value of remanence and saturation magnetization are observed for sample R1, due to low degree of crystallinity and spin disordering on surfaces, while in sample R3, and R4 it is due to capping, amorphous and diamagnetic nature of SDS and CTAB [47]. Sample R2 has largest value of remanence (MR = 0.51), and saturation magnetization (MS = 3.41) due to the greater degree of crystallinity, and crystalline size as shown in Table 2. Sample R2 has smaller value of coercivity and sample R3 has larger coercivity as compared to sample R1, it is previously reported that coercivity of hematite increases with decreasing grain size [48–51]. On the other hand, induced larger magnetic coercivity, is attributed to magnetic spin are preferentially aligned along the long axis,

3.5. Effect of anionic and cationic surfactant

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3.6. Scanning electron microscopy The SEM images were recorded by using JEOL SEI having 15.0 kV energy. Fig. 11 show the SEM image of samples R1, R2, R3 and R4 provided with bar scale of 1 ␮m. Particle size of sample R1 and R2 are in the range of 100–200 nm, but sample R2 has uniformity in size due to recrystallization or oxidation in

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presence of water, and R1 has non uniformity toward particle size distribution due to bulk sample. In sample R3 particles are embedded in SDS matrix and having smaller particle size than samples R1 and R2. Particle size of sample R4 surrounded in CTAB environment is in the range of 50–100 nm. Smaller particle size may be attributed due to capping of cationic surfactant CTAB.

Fig. 9. Hysteresis loop characterization of Fe2 O3 powder as synthesized in different liquid medium.

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Fig. 10. Variation of coercivity, magnetization (MR and MS ), and remanence ratio with crystalline size.

Fig. 11. Scanning electron microscopy (SEM) images of sample R1, R2, R3, and R4, at bar scale of 1 ␮m.

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4. Conclusion In present work optical band gap of as commercially purchased Fe2 O3 powder is ablated by laser ablation technique in different liquid media. Williamson–Hall plot and Scherrer formula employed for effective particle size and strain calculation. It has been analyzed that double distilled water have enhanced the crystallinity and crystalline size of Fe2 O3 due to oxidation in the presence of oxygen in it. While surfactant supports to reduce crystalline size due to its capping effect. Mix phases of iron oxide are formed in presence of cationic surfactant CTAB. Optical bandgap and magnetic properties such as coercivity, saturation magnetization, and remanence have been tailored by pulse laser ablation in different aqueous media. Ablated Fe2 O3 nanoparticles can be used for numerous in vivo applications such as MRI contrast enhancement agent, tissue repair, hyperthermia, drug delivery and in cell separation. Acknowledgement Authors are thankful to Dr. A.K. Sinha, RRCAT Indore for providing Synchrotron radiation source for X-ray diffraction and UGC for D. Phil. Fellowship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

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