CHAPTER 4. PREPARATION AND CHARACTERIZATION OF Cr-DOPED, Co-DOPED AND Fe-DOPED NIO NANOPARTICLES

59 CHAPTER 4 PREPARATION AND CHARACTERIZATION OF Cr-DOPED, Co-DOPED AND Fe-DOPED NIO NANOPARTICLES 4.1 INTRODUCTION Extensive research has been carried out on transition metal ion doped semiconductors because of their vast technological applications in magneto electronic, opto-magneto-electronic and spintronic devices (Mishra et al. 2012). Dilute magnetic semiconductor (DMS) is a kind of novel semi conductor, which is formed using magnetic transition metal ions or rare earth metal ions which randomly replace the non-magnetic cations in semiconductors and make the semiconductor to exhibit magnetic properties (Ohno et al. 1992). Such semiconductors are potential candidates for application in spin-controlled devices. The doping of transition metal ions into NiO lattice modifies the electronic and magnetic properties. Dilute magnetic systems have attracted much attention because of their complex spin order (Ausous & Elliot 1983). The antiferromagnetic behaviour of NiO can be tuned by replacing Ni by transition metal ions. Jianfei Wang et al. (2005) have reported that Fe-doped NiO nanoparticles reveal room temperature ferromagnetism. Manna & De (2009) have reported that size reduction in NiO nanoparticles plays an important role in ferromagnetic phase due to an uncompensated spin sublattice.

60 Douvalis et al. 2007 investigated the structural and magnetic properties of 2 at% Fe-doped NiO samples. Nowotny Janusz & Mieczysław Rekas (1984) reported that Cr-doped NiO and Fe-doped NiO are p-type materials, which exhibit ferromagnetism at room temperature (Kiattisak Noipa et al. 2014). The basic crystal structure of NiO (bunsenite) is being different from that of FeO (Wustite), Fe doping in NiO matrix is expected to create considerable strain in the latter. The associated modification in Fe-doped NiO should therefore influence its magnetic properties. Cr-doed, Co-doped and Fe-doped NiO are found to exhibit room temperature ferromagnetism (Mishra et al. 2012). In general, ferromagnetic responses of transition metal doped semiconductor oxides are mainly due to the exterior and delocalized 3d electrons in transition metals which induce strong direct exchange interactions. Transition metal ion doped semiconductor oxides have received a great interest because of their unique optical properties and promising applications in optoelectronic devices (Manna & De 2009). The magnetic studies carried out on the prepared samples (Chapter 3, Section 3.3.6) shows that the only sample prepared using 0.1 M concentration of nickel nitrate exhibits good magnetic properties. So, in the present work, Cr-doped NiO, Co-doped NiO and Fe-doped NiO nanocrystalline materials have been prepared using 0.1M nickel nitrate by chemical precipitation method and their structural, optical and magnetic properties are studied.

61 4.2 EXPERIMENTAL DETAILS 4.2.1 Preparation of Cr-doped NiO Nanoparticles NiO nanoparticles doped with 4%, 6% and 8% chromium have been prepared by chemical precipitation method. The 4% weight of dopant material chromic nitrate(cr(no3)3.9h2o) is taken according to the stoichiometry, dissolved in 0.1M nickel nitrate solution and stirred for one hour at room temperature. Then aqueous solution of 0.1M NaOH is added drop wise to the above mixture. Finally the mixture is allowed to precipitate and the precipitate is filtered, centrifuged at 3000 rpm for 5 minutes and washed with distilled water several times. The collected precipitate is allowed to dry for 12 hours and then annealed at 350 ºC for 2 hours. The dried powder is grinded in an agate mortar to obtain 4% Cr-doped NiO nanoparticles. 6% Cr-doped and 8% Cr-doped NiO nanoparticles have been prepared using the same procedure. 4.2.2 Preparation of Co-doped NiO Nanoparticles 4%, 6% and 8% Co-doped NiO nanoparticles have been prepared by chemical precipitation method. The 4% weight of cobalt nitrate (Co (NO3)2.6H2O) is taken according to the stoichiometry, dissolved in 0.1M nickel nitrate solution and stirred for one hour at room temperature. Then aqueous solution of 0.1M NaOH is added drop wise to the above mixture. Finally the mixture is allowed to precipitate and the precipitate is filtered, centrifuged at 3000 rpm for 5 minutes and washed with distilled water several times. The collected precipitate is allowed to dry for 12 hours and then annealed at 350 ºC for 2 hours. The dried powder is grinded in an agate mortar to obtain 4% Co-doped NiO nanoparticles. Using the same procedure, 6% Co-doped and 8% Co-doped NiO nanoparticles have also been prepared.

62 4.2.3 Preparation of Fe-doped NiO Nanoparticles 4%, 6% and 8% Fe-doped NiO nanoparticles have been prepared by chemical precipitation method. The 4% weight of ferric nitrate (Fe (NO3)3.9H2O) is taken according to the stoichiometry, dissolved in 0.1M nickel nitrate solution and stirred for one hour at room temperature. Then aqueous solution of 0.1M NaOH is added drop wise to the above mixture. Finally the mixture is changed to precipitate and the precipitate was filtered, centrifuged at 3000 rpm for 5 minutes and washed with distilled water several times. The collected precipitate is allowed to dry for 12 hours and then annealed at 350 ºC for 2 hours. The dried powder is grinded in an agate mortar to obtain 4% Fe-doped NiO nanoparticles. Using the same procedure, 6% Fe-doped and 8% Fe-doped NiO nanoparticles have also been prepared. The flow chart showing the procedure used for preparing Cr-doped, Co-doped and Fe-doped NiO nanoparticles by chemical method is shown in Figure 4.1.

63 Ni(NO3)2.6H2O + Distilled water Cr(NO3)3.9H2O (or) Co(NO3)2.6H2O(or) Fe(NO3)3.9H2O + Distilled water Stirring for 1 hour Stirring for 1 hour Mixture solution Addition of NaOH solution to mixture solution Precipitate Centrifused and precipitate washed with distilled water Precipitate dried at 70 C for 12 hours Annealed at 350 C for 2 hours Cr (or) Co (or) Fe-doped NiO Nanoparticles Figure 4.1 Flow chart showing the preparation of Cr/Co/Fe-doped NiO nanoparticles

64 4.3 CHARACTERIZATION OF Cr-DOPED, Co-DOPED AND Fe-DOPED NIO NANOPARTICLES 4.3.1 X-ray Diffraction Analysis Figure 4.2 shows the X-ray diffraction pattern of the prepared Cr-doped NiO nanoparticles. The addition of Cr-ion is observed to broaden the peak of NiO and no impurity phase is observed in Cr-doped NiO nanoparticles. It is observed that the diffraction peaks of the Cr-doped NiO shows a small shift towards higher 2θ values when compared to that of the undoped NiO which indicates that Cr ions have got accomodated at Ni site without changing the FCC structure. In Cr-doped NiO samples, the lattice parameters are observed to decrease slightly with increase in Cr 2+ concentration as shown in Table 4.1. Doping of Cr in NiO does not lead to any structural phase transformation but introduces a lattice contraction. The grain size has been estimated by using the Scherrer s equation, K D (4.1) cos where D is the grain size, K is a constant taken to be 0.94, λ is the wave length of X-rays, β is the full width at half maximum intensity and θ is the Bragg s angle. The grain size of 4%, 6% and 8% Cr-doped NiO nanoparticles is determined using the above equation and are shown in Table 4.1. The doping of Cr ions reduces the grain size of NiO nanoparticles, as the incorporation of Cr ion creates micro strain in the NiO lattice and affects the grain growth (Granqvist 1995). The reduced grain size indicates that the growth of host lattice is restricted by Cr 2+ -ion. The microstructural disorder is due to the difference in the ionic radii of Ni 2+ (0.69 Å) and Cr 2+ ion

65 (0.89 Å) (Moura et al. 2012). The increase of doping concentration of chromium ions occupy the interstitial voids and create stress between the Ni 2+ ion and Cr 2+ ion which increase the grain size and are shown in Table 4.1. 8% Cr-doped NiO Intensity (a.u) (111) (200) 6% Cr-doped NiO (220) 4% Cr-doped NiO 10 20 30 40 50 60 70 80 2 Theta (Degree) Figure 4.2 XRD pattern of 4%, 6% and 8% Cr-doped NiO nanoparticles Figure 4.3 shows the XRD diffraction pattern of cobalt doped NiO nanoparticles; the addition of cobalt ions also broadens the Bragg s peak and reduces the peak intensity. No characteristic peaks of impurity phase are observed in the X-ray diffraction pattern of the cobalt doped samples. The structural parametesr of cobalt doped NiO is given in Table 4.2. The grain size of the cobalt doped NiO nanoparticles is smaller when compared to that of the undoped NiO nanoparticles.

66 Table 4.1 Structural parameters of Cr-doped NiO nanoparticles S.No Samples 2 θ(200) d-spacing of (200) plane (Å) Lattice constant " a" (Ǻ) Grain size (nm) 1 Un-doped NiO 43.24 2.085 4.17 10.33 2 4% Cr-doped NiO 43.35 2.085 4.17 9.54 3 6% Cr-doped NiO 43.36 2.071 4.14 9.61 4 8% Cr-doped NiO 43.39 2.060 4.12 9.67 The grain size of 4%, 6% and 8% cobalt doped NiO nanoparticles are 9.71 nm, 9.92 nm and 10.11 nm respectively. The reduction in grain size with cobalt doping in the seed matrix indicates the restriction of seed lattice growth on cobalt doping. The broadening and reduced intensity of peaks show that the doping of cobalt ions increases the micro strain due to the disorder in crystalline structure. The micro structural disorder arises from the difference in the ionic radii of Ni 2+ (0.69 Å) and Co 2+ (0.745 Å) ions. The increase in weight percentage of cobalt nitrate to nickel nitrate increases the grain size of end product. Figure 4.4 shows the X-ray diffraction pattern of the prepared Fe-doped NiO nanoparticles. It is observed that the diffraction peaks of the Fe-doped NiO show a small shift towards higher 2θ values when compared to that of undoped NiO. The diffraction pattern reveals that the Fe-doped NiO also exhibits FCC structure. In Fe-doped NiO samples, the lattice parameters are observed to decrease slightly with increase in Fe concentration as shown in Table 4.3. Doping of Fe into NiO does not lead to any structural phase transformation.

67 8% Co-doped NiO 6% Co-doped NiO Intensity (a.u) (111) (200) (220) 4% Co-doped NiO 10 20 30 40 50 60 70 80 2 Theta (Degree) Figure 4.3 XRD pattern of 4%, 6% and 8% Co-doped NiO nanoparticles. Table 4.2 Structural parameters of Co-doped NiO nanoparticles S.No Samples 2 θ(200) d-spacing of (200) plane (Å) Lattice constant " a" (Ǻ) Grain size (nm) 1 Un-doped NiO 43.24 2.085 4.17 10.33 2 4% Co-doped NiO 43.30 2.095 4.19 9.71 3 6% Co-doped NiO 43.32 2.085 4.17 9.92 4 8% Co-doped NiO 43.35 2.075 4.15 10.11 When compare to undoped NiO, the grain size of Fe-doped NiO is reduced because of ionic radii difference between Ni 2+ (0.69 Ǻ) and Fe 2+

68 (0.74 Ǻ) (Moura et al. 2012). The grain size of Fe-doped NiO is found to increase with increase of Fe-ion concentration (Table 4.3). The grain size of 4% Fe, 6% Fe and 8% Fe-doped NiO is found to be 8.01 nm, 8.11 nm and 8.52 nm respectively. 8% Fe-doped NiO Intensity (a.u) (111) (200) 6% Fe-doped NiO (220) 4% Fe-doped NiO 10 20 30 40 50 60 70 80 2 Theta (Degree) Figure 4.4 XRD pattern of 4%, 6% and 8% Fe-doped NiO nanoparticles Table 4.3 Structural parameters of Fe-doped NiO nanoparticles S. No Samples 2 θ(200) d-spacing of (200) plane (Å) Lattice constant " a" (Ǻ) Grain size (nm) 1 Un-doped NiO 43.24 2.085 4.17 10.33 2 4% Fe-doped NiO 43.03 2.086 4.17 8.01 3 6% Fe-doped NiO 43.10 2.070 4.14 8.11 4 8% Fe-doped NiO 43.17 2.064 4.12 8.52

69 4.3.2 Surface Morphology Studies The surface morphology of the prepared samples has been studied using field emission scanning electron microscopy (FESEM). Figure 4.5 shows the FESEM images of the prepared Cr-doped (4% Cr, 6% Cr and 8% Cr) NiO samples. The image clearly shows that the samples have uniform grain size and nanoclusters have formed. The introduction of Cr-ion in NiO reduces the grain size which indicates the increase of strain in the matrix which restricts the lattice growth (Moura et al. 2012). It is well known that irrespective of the preparation method used to obtain nano-oxides, there is compelling evidence that crystallization does not follow a traditional nucleation and growth mechanism, more so in the case of increasing concentration of oxo-hydroxides to form metal oxides (José A. Rodriguez & Marcos Fernández-García, 2007). Figure 4.6 shows the FESEM images of cobalt doped NiO samples. The particles size is nearly uniform. The increase of doping concentration produces a change in the sample micro structure and a more disperse system is obtained. Figure 4.7 shows the FESEM images of Fe-doped NiO samples. The grains are of uniform size and arranged regularly. The introduction of Fe ion concentration produces homogenous distribution of grains.

70 Figure 4.5 FESEM images of (a) 4% Cr-doped NiO nanoparticles (b) 6% Cr-doped NiO nanoparticles and (c) 8% Cr-doped NiO nanoparticles

71 Figure 4.6 FESEM images of (a) 4% Co-doped NiO nanoparticles (b) 6% Co-doped NiO nanoparticles and (c) 8% Co-doped NiO nanoparticles.

72 Figure 4.7 FESEM images of (a) 4% Fe-doped NiO nanoparticles (b) 6% Fe-doped NiO nanoparticles and (c) 8% Fe-doped NiO nanoparticles

73 4.3.3 HRTEM Analysis High-resolution transmission electron microscopy (HRTEM) was used to investigate the microstructure of Cr-doped, Co-doped and Fe-doped NiO nanoparticles. The HRTEM images help to check out the crystallinity and phase purity of the prepared samples. For HRTEM analysis, the nanoparticles are dissolved in acetone and sonicated for 30 minutes. Table 4.4 Calculated d-spacing value from XRD pattern and HRTEM fringe pattern of Cr-doped NiO samples S.No Samples d-spacing value from XRD pattern Å d-spacing value from HRTEM fringe pattern Å 1 4% Cr-doped NiO 2.085 2.08 2 6% Cr-doped NiO 2.071 2.07 3 8% Cr-doped NiO 2.060 2.06 Figure 4.8 shows the HRTEM images of Cr-doped NiO nanoparticles. The nanoparticles are an agglomeration of smaller particles forming nanoclusters. The images also exhibit lattice fringes and the lattice spacing has been determined using these fringes. Fringes of uniform width have been observed and the image reveals the crystallinity and defect free nature of the sample. The d-spacing was calculated for various concentrations of Cr-doped NiO and it is 2.08 Å, 2.07 Å and 2.06 Å respectively which corresponds to (200) plane. The d-spacing values calculated from these images are in close agreement with the values obtained from X-ray diffraction studies (Table 4.4).

74 Figure 4.8 HRTEM images and fringe pattern of (a) 4% Cr-doped NiO nanoparticles(b) 6% Cr-doped NiO nanoparticles and (c) 8% Cr-doped NiO nanoparticles

75 Figure 4.9 shows the HRTEM images of cobalt doped NiO nanoparticles prepared using 0.1M nickel nitrate and different weight percentage of cobalt nitrate. The HRTEM images show fringe pattern and the d-spacing is found from the fringe pattern. The d-spacing values calculated from the fringe pattern are 2.09 Å, 2.08 Å and 2.07 Å for cobalt doped NiO nanoparticles which correspond to the (200) plane of FCC NiO and are shown in Table 4.5. Table 4.5 Calculated d-spacing value from XRD pattern and HRTEM fringe pattern Co-doped samples d-spacing value S. No Samples d-spacing value from XRD pattern Å from HRTEM fringe pattern Å 1 4% Co-doped NiO 2.095 2.09 2 6% Co-doped NiO 2.085 2.08 3 8% Co-doped NiO 2.075 2.07 Figure 4.10 shows the HRTEM images of Fe-doped NiO nanoparticles. The images exhibit uniform fringe pattern and using the fringe pattern d-spacing values are determined which correspond to the (200) plane and are 2.08Å, 2.07 Å and 2.06 Å respectively and are shown in Table 4.6.

76 Figure 4.9 HRTEM images and fringe pattern of (a) 4% Co-doped NiO nanoparticles (b) 6% Co-doped NiO nanoparticles and (c) 8% Co-doped NiO nanoparticles

77 Table 4.6 Calculated d-spacing value from XRD pattern and HRTEM fringe pattern of Fe-doped samples S. No Samples d-spacing value from XRD pattern Å d-spacing value from HRTEM fringe pattern Å 1 4% Fe-doped NiO 2.086 2.08 2 6% Fe-doped NiO 2.070 2.07 3 8% Fe-doped NiO 2.064 2.06

78 Figure 4.10 HRTEM images and fringe pattern of (a) 4% Fe-doped NiO nanoparticles (b) 6% Fe-doped NiO nanoparticles and (c) 8% Fe-doped NiO nanoparticles

79 4.3.4 Selected Area Electron Diffraction (SAED) Studies The selected area electron diffraction (SAED) pattern is used to study about the crystal properties of a particular region. The observed rings for the samples show that the samples exhibit FCC structure and are nanocrystalline in nature. Figures 4.11(a, b & c) shows the selected area electron diffraction images of Cr-doped, Co-doped and Fe-doped NiO nanoparticles. The presence of rings with discrete spots suggests that the prepared nanoparticles are made of small particles of uniform size. Ring patterns corresponding to planes (111), (200), (220), (311) and (222) are observed in the SAED pattern. Tables 4.7, 4.8 & 4.9 show the d-spacing values calculated from the selected area electron diffraction patterns of nanocrystalline Cr-doped, Co-doped and Fe-doped NiO nanoparticles and they are found to match well with the standard d-spacing values of JCPDS data (01-078-0423, 01-004-0835, 00-001-1239). Figure 4.11a SAED pattern of (a) 4% Cr-doped NiO nanoparticles (b) 6% Cr-doped NiO nanoparticles and ( c) 8% Cr-doped NiO nanoparticles

80 Table 4.7 Calculated d-spacing value of different planes of 4%,6% and 8% Cr-doped NiO nanoparticles from HRTEM SAED pattern Calculated d-spacing values from Reported S.No HRTEM SAED pattern (Å) 4% 6% 8% Cr-doped Cr-doped Cr-doped d-spacing value (JCPDS) Planes (hkl) NiO NiO NiO Å 1 2.442 2.431 3.422 2.412 ( 111 ) 2 2.085 2.071 2.060 2.089 ( 200 ) 3 1.471 1.460 1.451 1.476 ( 220 ) 4 1.264 1.254 1.241 1.259 ( 311 ) 5 1.165 1.154 1.148 1.205 ( 222 ) Figure 4.11b SAED pattern of (a) 4% Co-doped NiO nanoparticles (b) 6% Co-doped nanoparticles NiO and (c) 8% Co-doped NiO nanoparticles

81 Table 4.8 Calculated d-spacing value of different planes of 4%,6% and 8% Co-doped NiO nanoparticles from HRTEM SAED pattern Calculated d-spacing values from Reported S.No HRTEM SAED pattern (Å) 4% 6 % 8% Co-doped Co-doped Co-doped NiO NiO NiO d-spacing value (JCPDS) Å Planes (hkl) 1 2.4147 2.4045 2.4009 2.412 ( 111 ) 2 2.095 2.085 2.075 2.089 ( 200 ) 3 1.479 1.468 1.452 1.476 ( 220 ) 4 1.259 1.245 1.240 1.259 ( 311 ) 5 1.194 1.187 1.180 1.205 ( 222 ) Figure 4.11c SAED pattern of (a) 4% Fe-doped NiO nanoparticles (b) 6% Fe-doped nanoparticles NiO and (c) 8% Fe-doped NiO nanoparticles

82 Table 4.9 Calculated d-spacing value of different planes of 4%,6% and 8% Fe-doped NiO nanoparticles from HRTEM SAED pattern S.No Calculated d-spacing values from HRTEM SAED pattern (Å) 4% Fe-doped NiO 6% Fe-doped NiO 8% Fe-doped NiO Reported d-spacing value (JCPDS) Å Planes (hkl) 1 2.416 2.409 2.401 2.412 ( 111 ) 2 2.086 2.070 2.064 2.089 ( 200 ) 3 1.477 1.465 1.452 1.476 ( 220 ) 4 1.250 1.248 1.2408 1.259 ( 311 ) 5 1.198 1.191 1.188 1.205 ( 222 ) The HRTEM analysis show that the Cr-doped, Co-doped and Fedoped NiO nanoparticles exhibit FCC structure and there is no phase transformation due to doping of Cr, Co and Fe. 4.3.5 Compositional Analysis The energy dispersive X-ray analysis (EDAX) is used to study the chemical composition of the samples. Figure 4.12 shows the energy dispersive X-ray analysis spectrum of Cr-doped NiO nanoparticles. The spectrum indicates the presence of Cr, Ni and O as the main components and the amount of Ni decreases with the increase of Cr concentration. The components C and Cu present in the spectrum originate from the paste and grid used for EDAX study and the images reveal that the prepared samples do not have any impurities. The chemical constituents present in the 4% Cr-doped NiO sample according to the EDAX analysis are, Ni= 47.12 at%,

83 O= 49.12 at% and Cr =3.76 at%. The chemical constituents of 6% Cr-doped NiO sample are Ni=44.95 at%, O=49.10 at % and Cr =5.95 at%. And the chemical constituents of 8% Cr-doped NiO sample are Ni=42.98 at %, O=49.15 at % and Cr =7.87 at %. Figure 4.12 EDAX spectra of (a) 4% Cr-doped NiO nanoparticles (b) 6% Cr-doped nanoparticles NiO and ( c) 8% Cr-doped NiO nanoparticles The EDAX spectrum of Co-doped NiO nanoparticles is shown in Figure 4.13. The atomic percentage of Ni, O and Co elements present in the

84 prepared 4%, 6% and 8% Co-doped NiO nanoparticles is as follows: The composition of 4% Co-doped NiO nanoparticles are Ni = 46.94 at %, O =49.10 at % and Co =3.96 at %. The composition of 6% Co-doped NiO nanoparticles are Ni = 44.96 at%, O =49.12 at% and Co = 5.92 at%.the composition of 8% Co-doped NiO nanoparticles are Ni = 42.95 at%, O = 49.08 at% and Co = 7.97 at%. Figure 4.13 EDAX spectra of (a) 4% Co-doped NiO nanoparticles (b) 6% Co-doped nanoparticles NiO and (c) 8% Co-doped NiO nanoparticles

85 The EDAX spectrum of Fe-doped NiO nanoparticles is shown in Figure 4.14. The atomic percentage of Ni, O and Fe elements present in the prepared 4%, 6% and 8% Fe-doped NiO powder is as follows: The composition of 4% Fe-doped NiO nanoparticles are Ni =46.51 at%, O = 49.55 at % and Fe =3.94 at %. The composition of 6% Fe-doped NiO nanoparticles are Ni = 44.47 at%, O =49.57 at% and Fe =5.96 at%. The composition of 8% Fe-doped NiO nanoparticles are Ni = 42.41 at%, O = 49.67 at% and Fe = 7.92 at %. Figure 4.14 EDAX spectra of (a) 4% Fe-doped NiO nanoparticles (b) 6% Fe-doped nanoparticles NiO and (c) 8% Fe-doped NiO nanoparticles

86 4.3.6 Optical Studies Optical properties which are directly related to the size of the nanoparticles can be studied using the absorption spectra. Figure 4.15 shows the optical absorption spectra of Cr-doped NiO nanoparticles. The absorption edge of Cr-doped NiO is found to be at 312.54 nm, 305.44 nm and 301.91 nm respectively. The absorption spectra of Cr-doped NiO nanoparticles show that the absorption edge is slightly shifted towards shorter wavelength (blue shift) when compared to undoped NiO. This shift is due to the Burstein Moss effect, since the absorption edge of a degenerate semiconductor is shifted to shorter wavelengths with increasing carrier concentration (Burstein 1954). This shift towards blue region predicts that there is an increase in band gap value(eg), which is due to the reduction in particle size. The fundamental absorption, which corresponds to the electron transition from the valance band(vb) to the conduction band(cb), can be used to determine the nature and value of the optical band gap. The optical absorption study is used to determine the optical band gap of the nanoparticles, which is the most familiar and simplest method. The absorption coefficient (α) and the incident photon energy (hʋ) are related by the expression (Pancove 1971 ) (αhʋ) = A(hʋ-Eg) n (4.2) where A is a constant, Eg is the optical band gap of the material, ʋ is the frequency of the incident radiation, h is Planck s constant and exponent n is 0.5 for direct band allowed transition. The optical band gap of 4%, 6% and 8% Cr-doped NiO nanoparticles is determined using equation 4.2. Figure 4.16 shows the (αhʋ) 2 versus hʋ plot of Cr-doped NiO nanoparticles. The optical band gap values have been determined by extrapolating the linear portion of the curve to meet the energy axis (hʋ). The

87 band gap has been calculated and is found to be 3.97 ev, 4.06 ev and 4.10 ev for 4%, 6% and 8% Cr-doped NiO respectively and is shown in table 4.10. The obtained optical band gap of Cr-doped NiO nanoparticles is higher than that of NiO due to quantum confinement effect (Mohseni Meybodi et al. 2012). Quantum confinement of both electrons and holes in all the three dimensions leads to an increase in the effective band gap of the material. 5 Absorbance (a.u) 4 3 2 1 a b a) 4% Cr-doped NiO b) 6% Cr-doped NiO c) 8% Cr-doped NiO c 400 500 600 700 800 Wave length (nm) Figure 4.15 Absorption spectra of 4%, 6% and 8% Cr-doped NiO nanoparticles

88 Figure 4.16 Plot of ( h ) 2 vs. photon energy of 4%, 6% and 8% Crdoped NiO nanoparticles Table 4.10 Band gap of 4% Cr-doped NiO, 6% Cr-doped NiO and 8% Cr-doped NiO nanoparticles Absorption Bandgap S.No Samples wavelength (ev) (nm) 1 4% Cr-doped NiO 312.54 3.97 2 6% Cr-doped NiO 305.44 4.06 3 8% Cr-doped NiO 301.91 4.10

89 Optical absorption spectrum of Co-doped NiO nanoparticles is shown in Figure 4.17. The absorption band edges of Co-doped NiO are absorbed to be present at 310.08 nm, 304.09 nm and 301.15 nm respectively and are shown in Table 4.11. Absorption spectra of Co-doped NiO nanoparticles shows that the absorption edge is slightly shifted towards the lower wavelength when compared to undoped NiO nanoparticles and this shift further move towards smaller wave length with increase in cobalt concentration. Figure 4.18 shows the (αhʋ) 2 versus hʋ plot of Co-doped NiO nanoparticles. The band gap of 4%, 6% and 8% Co-doped NiO nanoparticles is found to be 3.99 ev, 4.07 ev and 4.11 ev respectively. The absorption edge shift towards lower wavelength indicates the increase of optical band gap of NiO on cobalt doping. 5 Absorbance (a.u) 4 3 2 1 a b a) 4% Co-doped NiO b) 6% Co-doped NiO c) 8% Co-doped NiO c 400 600 800 Wave length (nm) Figure 4.17 Absorption spectra of 4%, 6% and 8% Co-doped NiO nanoparticles

90 Energy (ev) Figure 4.18 Plot of ( h ) 2 vs. photon energy of 4%, 6% and 8% Co-doped NiO nanoparticles Table 4.11 Band gap of 4% Co-doped NiO, 6% Co-doped NiO and 8% Co-doped NiO nanoparticles Absorption Bandgap S.No Samples wavelength (ev) (nm) 1 4% Co-doped NiO 310.08 3.99 2 6% Co-doped NiO 304.09 4.07 3 8% Co-doped NiO 301.15 4.11

91 Incorporation of Fe into the crystal lattice of NiO nanocrystalline semiconductors will alter the optical properties of the semiconductor. Optical absorption spectrum of Fe-doped NiO nanoparticles is shown in Figure 4.19. The absorption band edges of Fe-doped NiO are absorbed to be present at 309.06 nm, 302.08 nm and 295.12 nm respectively and are shown in Table 4.12. The absorption edge of the Fe-doped NiO spectra shown in Figure 4.19 exhibits blue shift when compared to undoped NiO nanoparticles. Figure 4.20 shows the (αhʋ) 2 versus hʋ plot of Fe-doped NiO nanoparticles. The band gap values of 4%, 6% and 8% Fe-doped NiO are found to be 4.01 ev, 4.10 ev and 4.20 ev respectively. As the doping concentration of Fe increases the absorption edge shifts towards lower wavelength and indicates that there is an increase in the band gap energy of NiO on Fe doping. 5 4 Absorbance (a.u) 3 2 1 a a) 4% Fe-doped NiO b) 6% Fe-doped NiO c) 8% Fe-doped NiO 350 400 450 500 550 600 b Wave length (nm) c Figure 4.19 Absorption spectra of 4%, 6% and 8% Fe-doped NiO nanoparticles

92 Figure 4.20 Plot of ( h ) 2 vs. photon energy of 4%, 6% and 8% Fe-doped NiO nanoparticles Table 4.12 Band gap of (a) 4% Fe-doped NiO (b) 6% Fe-doped NiO and (c) 8% Fe-doped NiO nanoparticles S.No Absorption Bandgap Samples wavelength (ev) (nm) 1 4% Fe-doped NiO 309.06 4.01 2 6% Fe-doped NiO 302.08 4.10 3 8% Fe-doped NiO 295.12 4.20

93 4.3.7 Magnetic Properties The magnetic properties of Cr-doped, Co-doped and Fe-doped NiO nanoparticles have been studied using vibrating sample magnetometer (VSM) at room temperature. The magnetization versus applied magnetic field plots of chromium doped NiO is shown in Figure 4.21. A well-defined hysteresis loop is observed. The magnetic parameters of Cr-doped NiO such as saturation magnetization (Ms), retentivity (MR) and coercivity (Hc) for different doping concentrations are calculated from the hysteresis curve and given in Table 4.13. The existence of magnetic property is very closely related to the fine grain microstructure of the particles (Deraz 2012b). The narrowed hysteresis curves of Cr-doped NiO have low magnetization (MS) is due to the lattice imperfection and the compensation of spin values. As the doping concentration of Cr increases there is decrease of saturation magnetization due to lack of coupled spins.

94 Figure 4.21 Hysteresis curves of 4%, 6% and 8% Cr-doped NiO nanoparticles Table 4.13 Magnetic properties of Cr-doped NiO nanoparticles Coercivity Magnetization Retentivity S.No Samples (Hc) (MS) (MR) (Oe) 10-6 emu/g 10-6 emu/g 1 2 3 4% Cr-doped NiO 6% Cr-doped NiO 8% Cr-doped NiO 108.77 34.25 16.74 97.56 20.121 13.25 86.32 11.380 11.58

95 Magnetization versus applied magnetic field plot of cobalt doped NiO nanoparticles is shown in Figure 4.22. The observed magnetisation and coercivity values are shown in Table 4.14. The cobalt doped NiO nanoparticles show ferromagnetic behaviour at room temperature. The saturation magnetization of the cobalt doped samples is mainly due to the spin coupling effect which is due to smaller particle size. It is reported that the magnetic property of the materials is dependent on the particles size, shape, magnetization and crystallinity. The unusual magnetic behaviour may be due to grain size reduction and breaking of large number of exchange bonds. The presence of small magnetic clusters on the surface and lattice imperfection increases the uncompensated spin values (Anandha Babu et al. 2015). The increase of doping concentration of cobalt reduce the saturation magnetization value. The cobalt doped NiO nanoparticles are widely used in the area of magnetic storage devices (Kaliyan Vallalperuman et al. 2003). Magnetization (emu/g) 0.0008 0.0004 0.0000-0.0004-0.0008 a b c a) 4% Co-doped NiO b) 6% Co-doped NiO c) 8% Co-doped NiO. -15000-10000 -5000 0 5000 10000 15000 Applied field (Oe) Figure 4.22 Hysteresis curves of 4%, 6% and 8% Co-doped NiO nanoparticles

96 Table 4.14 Magnetic properties of Co-doped NiO nanoparticles Coercivity Magnetization Retentivity S.No Samples (Hc) (MS) (MR) (Oe) 10-6 emu/g 10-6 emu/g 1 2 3 4% Co-doped NiO 6% Co-doped NiO 8% Co-doped NiO 463.37 843.47 20.36 120.61 137.38 12.90 116.24 125.47 11.28 The magnetic field dependent magnetization (M-H) curves for the 4%, 6% and 8% of Fe-doped NiO nanoparticles at room temperature are shown in Figure 4.23. The hysteresis curve for 4% Fe-doped NiO indicates the presence of ferromagnetic phase and it decreases with increase in doping concentration. A weak ferromagnetic phase was observed in 6% and 8% Fedoped NiO nanoparticles. The observed magnetisation and coercivity values are shown in Table 4.15. Doping effect increases the grain size and reduce the net magnetization. Doping of Fe into NiO interface creates charge carriers which gives rise to more exchange interaction. The hysteresis phenomenon observed for Fe-doped NiO matches with the earlier results reported by Lin et al. (2006) and the curve is narrow due to uncompensated spin systems (Manna & De 2009). Smaller the crystallite size, the more will be the net magnetization (Moura et al. 2012). The magnetic properties of the materials are closely associated with the dependence of particle size, shape, oxygen deficiency, magnetic direction and crystallinity (Mishra et al. 2012). Table 4.16 shows the magnetic parameters of Cr-doped, Co-doped and Fe-doped NiO nanoparticles. It is found that the 4% Fe-doped NiO nanoparticles have high saturation magnetization value

97 when compared to 6% and 8% Fe-doped NiO nanoparticles. The magnetization value of Fe-doped NiO is more than those of chromium and cobalt doped NiO nanoparticles. As a result of high saturation magnetization values, Fe doped NiO nanomaterials are widely used for the fabrication of spintronic devices (Mishra et al. 2012 & Jianfei Wang et al. 2005). Magnetization (emu/g) 0.0020 0.0015 0.0010 0.0005 0.0000-0.0005-0.0010-0.0015-0.0020 a b c a) 4% Fe-doped NiO b) 6% Fe-doped NiO c) 8% Fe-doped NiO -15000-10000 -5000 0 5000 10000 15000 Applied field (Oe) Figure 4.23 Hysteresis curves of 4%, 6% and 8% Fe-doped NiO nanoparticles

98 Table 4.15 Magnetic properties of Fe-doped NiO nanoparticles Coercivity Magnetization Retentivity S.No Samples (Hc) (MS) (MR) (Oe) 10-6 emu/g 10-6 emu/g 1 2 3 4% Fe-doped NiO 6% Fe-doped NiO 8% Fe-doped NiO 99.419 1901.7 27.46 96.020 332.75 19.01 89.028 194.63 13.93 Table 4.16 Comparing the magnetic properties of Cr-doped, Co-doped and Fe-doped NiO nanoparticles S.No 1 2 3 Magnetic Property Coercivity (Hc) (Oe) Magnetization (MS) 10-6 emu/g Retentivity (MR) 10-6 emu/g Concentration of doped samples Cr-doped NiO nano particles Co-doped NiO nano particles Fe-doped NiO nano particles 4% 108.77 463.37 99.419 6% 97.56 120.61 96.020 8% 86.32 116.24 89.028 4% 34.25 843.47 1901.7 6% 20.121 137.38 332.75 8% 11.380 125.47 194.63 4% 16.74 20.36 27.46 6% 13.25 12.90 19.01 8% 11.58 11.28 13.93