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Tailoring of pyramid cobalt doped nickel oxide nanostructures by composite-hydroxide-mediated approach
Materials Chemistry and Physics 239 (2020) 122036
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Tailoring of pyramid cobalt doped nickel oxide nanostructures by composite-hydroxide-mediated approach Aurang Zeb a, Muhammad Arfan a, b, c, Tauseef Shahid a, b, c, *, Taha Bin Masood a, Abdul Ghafar Wattoo b, c, d, Zhenlun Song b, c, Moazzam Shahzad a, Shehzad Munir Ansari a a
Department of Applied Physics, Federal Urdu University of Arts, Science and Technology, Islamabad, 44000, Pakistan CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China c University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing, 100049, China d Department of Physics, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan, 64200, Pakistan b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Composite-hydroxide-mediated approach is an efficient method to syn thesize nanomaterial. � Doping effect on the optical properties like band gap. � New cost effective method to the development of novel nanomaterial at low temperature. � Morphological structures for application in optoelectronics and sensor based devices.
A R T I C L E I N F O
A B S T R A C T
Keywords: SEM Co doped NiO Nanomaterials CHM approach Optical properties and FT-IR
Nickel oxide nanostructures have been synthesized successfully by composite hydroxide mediated (CHM) approach at 200 � C for various cobalt concentrations (0%, 5%, 10% and 15%). Incorporation of cobalt was explored by employing X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning elec tron microscopy (SEM, and ultraviolet visible spectroscopy. An influence of increase in cobalt doping on average crystallite size was found from 31 to 56 nm. The increase in crystallite size is due to the introduction of point defects and oxygen vacancies caused by the incorporation of Co ions in NiO lattice. The influence of defects is also seen from variation in lattice parameters and X-ray density. The effect of incorporation of increasing cobalt contents was seen from SEM micrographs. The spherical characteristics of pure NiO nanostructures changed to sharp edge pyramids with smooth surfaces. The growth of pyramids is evident with increase in side length 421–691 nm for various cobalt concentrations (5%, 10% and 15%). FT-IR confirms the purity of product while the band gap was estimated by UV–Vis spectroscopy which was found from 5.59 to 1.50 eV.
* Corresponding author. CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. E-mail address: [email protected] (T. Shahid). https://doi.org/10.1016/j.matchemphys.2019.122036 Received 6 May 2019; Received in revised form 15 August 2019; Accepted 18 August 2019 Available online 19 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
A. Zeb et al.
Materials Chemistry and Physics 239 (2020) 122036
1. Introduction Recently, efforts have been made for the study of nanoscale materials to get the dream of reduced sized electronic devices. For this purpose, study of nanomaterials have got a special focus from industrial point of view; as the development of this field will produce remarkable impact on daily life. A number of groups have done significant work to synthesize nanostructures of transition metal oxides with different morphology and size for different valuable applications [1]. Among these transition metal oxides, nickel oxide is a promising ptype semiconductor with wide band gap energy (3.6 eV–4.0 eV). In addition, nickel oxide has great importance due to its intensive appli cations in the fields such as gas sensors, solar cells, electrochemical supercapacitors, optoelectronics devices, solid-oxide fuel cells and photodetectors [2–7]. With the expanding industry and modern tech nology properties of pure nickel oxide nanomaterial are not sufficient for subject applications. Recently, the researchers are interested to acquire the effective performance and elaboration of NiO applications by mixing or doping with different metals (Li, Cu, K, P, Y and other elements) to enhance the novel structural, magnetic, morphological, electro-chromic, electrical and optical properties of newly produced nanostructures [8–13]. Promising results of doping of nickel oxide with cobalt aim at the peculiar structural, morphological and optical properties. Both nickel and cobalt oxides have cubic structure and have low lattice mismatch. Furthermore, the ionic radii of Niþ2 and Coþ2 are nearly similar. For this reason, cobalt is a better doping substitute than other metals. Researchers have developed a number of methods, reported in literature, for the synthesis of nanocrystalline NiO for valuable appli cations [14]. Some of these methods are included as thermal decom position [15], sol–gel, solvothermal and sonochemical [16–18]. Practically, these methods have their own charm but there are some limitations, which limited their use, as reported in the literature, like control over stoichiometry, composition and crystal structure. Com posite hydroxide mediated approach appeals as appropriate substitute with the advantages such as single step, convenient, economical, nontoxic, ecofriendly and low synthesis temperature route for the preparation of oxide materials of various types of structures [19]. Up till now different morphologies of NiO nanostructures including micro spheres, flowerlike, nanowires, nanotubes, polyhedral nanoparticles, nanotowers and nanosheets have been reported with different applica tions [20–28]. In the present work, cobalt doped NiO have been prepared by CHM approach to build feasibility for the synthesis of pure and cobalt doped NiO with better properties. Interesting pyramid type nanocrystals have been obtained in a certain doping range and showed a doping concen tration dependent morphology of the nanostructures. We have estab lished that un-doped NiO nanomaterial showed no well defined morphological structures.
Fig. 1. X-ray diffraction pattern of NiO nano material prepared different doping concentration of cobalt.
stopped and the beaker was taken out from the furnace and cooled naturally to room temperature. The product was washed by distilled water, filtered and dried. For doping purposes 5%, 10% and 15% of Co (NO3)2⋅6H2O were also added at same conditions along with hydroxide mixture and Ni(NO3)2⋅6H2O till final products were obtained, as described above. The chemical reactions can be expressed as follow: NiðNO3 Þ2 :6H2 O þ 2NaOH→NiðOHÞ2 þ 2NaNO3 þ 6H2 O⇒Ni2þ þ 2OH →NiðOHÞ2 Nickel hydroxide is not chemically stable at high temperature therefore forms NiO and water molecule. NiðOH2 Þ → NiO↓ þ H2 O 2.2. Characterization For structural analysis XRD was performed trough X-pert high pro diffractometer operated at 40 kV, 30 mA, with a step size 0.01 and Cu Kα radiation (λ ¼ 1.5406 Å). Scanning electron microscopy (SEM) was performed to observe the surface morphology by using JEOL (JSM 5910) operated at 20 kV. PerkinElmer spectrometer was used to investigate FTIR spectrum in the range of 400 to 4000 cm 1. To measure and analyze optical bandgap, UV–visible absorption data was used in spectral range of 200–700 nm.
2. Experimental methods
3. Results and discussion
2.1. Synthesis of NiO nanomaterial
3.1. XRD analysis
Raw materials including (Ni(NO3)2⋅6H2O, Co(NO3)2⋅6H2O, NaOH and KOH) used for this experiment were of analytical grade and also used without further purification. Composite hydroxide mediated approach was used to synthesize pure and cobalt doped nanostructures as used successfully for the preparation of different transition metal oxide nanostructure in previous reports [29–31]. In this typical syn thesis, 20 g of mixed hydroxide of NaOH and KOH in the ratio of 51.5% and 48.5% were added in a Teflon Beaker, respectively. Then, the Teflon Beaker was put in a preheated furnace at 200� C till all the hydroxides turned into molten state, then this molten mixture was taken out from the furnace. The molten hydroxides and Ni(NO3)2⋅6H2O were stirred well by a Teflon bar till a uniform mixture was attained. The beaker was put back in the furnace for further reaction. After 24 h the heating was
The X-ray diffraction patterns of as prepared samples of Ni1-xCoxO (X ¼ 0%, 5%, 10% and 15%) nanostructures are presented in Fig. 1. These patterns clearly show polycrystalline cubic phase NiO Table 1 Structural and optical parameters of Ni1-xCoxO calculated by XRD and absorp tion spectroscopy.
2
Cobalt concentration
a (Å)
Vcell (Å3)
Density (g. cm-3)
Crystallite size (nm)
Band gap (eV)
0% 5% 10% 15%
4.169 4.177 4.186 4.158
72.45 72.87 73.34 71.88
6.84 6.80 6.76 6.90
31 36 45 56
5.59 2.38 1.94 1.50
A. Zeb et al.
Materials Chemistry and Physics 239 (2020) 122036
Fig. 2. SEM images of as prepared cobalt doped NiO nanostructures at 200 � C (a) Pure NiO (b) 5% doped (c) 10% doped (d) 15% doped.
nanocryatals which are closely resembled to that of JCPDS # 01-0750197. The peaks appeared at 2θ ¼ 37.13� , 43.12� , 62.66� and 75.36� correspond to (111), (200), (220) and (311) crystalline planes of nickel oxide, respectively. The calculated lattice parameter from XRD data for pure NiO is a ¼ 0.4169 nm. The sample synthesized for 5%, 10% and 15% doping have similar results with a small change which are enlisted in Table 1. An increase in lattice constant for 5% and 10% cobalt doped nickel oxide samples is seen. Where a decrease in lattice constant is also seen for 15% doped sample. This decrease may be due to the compres sion of unit cell which is caused by the incorporation of Coþ2 atoms at Niþ2 sites. The changes in lattice constants indicate incorporation of cobalt ions at nickel lattice sites. Furthermore, there is maximum upshift seen in comparison of 2θ values for plane (200) for 15% cobalt doped NiO. It is found that most of the planes of crystallites are strongly ori ented in these favorable (111), (200) and (220) directions. These nanostructures have crystalline nature as the sharpness and strong intensities of XRD patterns reveal. Also, no peak is found there related to impurity or its related compound. The average crystallite size for as prepared nanomaterial is estimated by Debye-Scherrer formula [32]: Kλ D¼ βcosθ
Here ‘‘NA’’ is the Avogadro constant, ‘‘M’’ is the molar mass, ‘‘Z’’ is the number of molecules per formula unit, and Vcell is volume of unit cell. It can easily be concluded from Table 1 that the calculated lattice parameters have good agreement with standard values. The as prepared un-doped nanostructures has the average crystallite size 31 nm, for 5% doping 36 nm, for 10% doping 45 nm and for 15% doping, the material prepared has the average crystallite size 56 nm. The increase in crys tallite size with increasing concentration of cobalt is seen. The increase in crystallite size may be caused by the strain or stress produced due to the difference in ionic radii of Niþ2 (0.69 Å) and Coþ2 (0.72 Å) ions which interrupts the crystallite growth process. With the increase in Co dopant, the change in FWHM of diffraction peaks also changes strain in host structure. The produced strain and defects cause the change in crystal lattice which causes important effects on structural and optical properties of NiO nanostructures [33]. X-ray density shows an increasing trend for 15% doping, whereas a decrease is seen for other samples. 3.2. SEM analysis
(1)
Fig. 2(a–d) shows surface morphology and microstructure of pre pared NiO nanoparticles and nano-pyramids which are successfully obtained by composite hydroxide system at constant temperature 200 � C. It seems that un-doped prepared sample has approximately spherical morphology. For doped samples, Figure (b–d) shows well developed pyramid shaped highly symmetrical structures with regular shape and smooth surfaces. Edge length of pyramid shaped NiO doped with 5% cobalt is approximately 421 nm. While it is clearly seen from the Figure (c and d) that as prepared nanostructures are pyramids with side lengths approximately 485 nm–691 nm with well-defined pecu liarities for 10% and 15% cobalt doping, respectively. With increasing concentration of cobalt the side length of pyramid structures increases. In the formation of pure and doped nickel oxide, viscosity and higher concentration of Hydroxide ions (OH 1) are the key process factors in growth of NiO which is formed through chemical reaction in which Niþ2
Here β is “full width at half maximum (FWHM)”, K is assumed to be 0.9, λ is wave length used in nm and θ is the Bragg’s angle. The lattice parameter is calculated by the formula: a¼
nλ 2sinθ h2 þ k2 þ l2
�1=2
(2)
The cell volume of product is calculated using the formula Vcell ¼ a3
(3)
The X-ray density is estimated using the following formula [17];
ρX
ray
¼
ZM Vcell NA
(4) 3
A. Zeb et al.
Materials Chemistry and Physics 239 (2020) 122036
Fig. 3. FT-IR spectra of cobalt doped NiO nanostructures prepared at temper ature 200 � C and constant reaction time (24 h).
and OH 1 ions are produced. These oppositely charged ions are responsible for the formation of Ni(OH)2 which leads the formation of NiO and water molecule to form nanocrystalline structures. Further more, intersiting pecularties has been seen with doping effect, when nickel oxide is doped with 5%, 10% and 15% cobalt, the structure changes to payramidical shape. The growth of as synthesized nano structure is the result of multidirectional nuclie aggrigation. It can also be seen from SEM micrographs that some of the nanostructures are in evolving stage but some of the structures are completely developed. CHM is a versatile approach used for simple to complex nanostructures synthesis but the effect of kinematic viscosity of the melts and complex thermodynamics involved is still poorly understood. 3.3. FTIR analysis Fig. 4. (a) UV–vis. absorption spectra of the cobalt doped NiO nanostructures prepared at temperature 200 � C and constant reaction time 24 h. (b) The (αhʋ) 2 vs. hʋ plot of as prepared cobalt doped NiO.
Fig. 3 shows FTIR spectra in the spectral range of 400-4000 cm 1. In order to understand the structure of the prepared nanostructures and effects of synthesizing materials, this technique has its own importance. The characteristic broad starching vibrational band for NiO is found at 600-630 cm 1 [34]. The small stretching vibrations present at 2359 and 2344 cm 1 are found due to the presence of CO2 molecules [35,36]. The intensity of these bands decreases with increasing concentration of co balt. The FTIR investigation marks the signature on the purity and XRD results in the sense of pure phase of NiO.
where ‘α’ is known as absorption coefficient and can be calculated by the following relation:
α ¼ 2:303A=l
(6)
In these relations n is taken as 2 for direct band gap or ½ for indirect band gap nanomaterial, hν denotes photon energy, B is taken as constant known as material constant and the path length is indicated by l and is taken to be 1 cm. Optical band gap can be calculated by extrapolating the linear part of the (αhν)n – hν curve to zero, as shown in Fig. 4 (b). The estimated bandgap energies of pure and doped nanostructures are enlisted in Table 1. The corresponding estimated band gap energy for undoped nickel oxide is 5.59 eV, which is greater than as reported for bulk NiO material in literature. This increase in band gap value shows quantum size effects. The calculated band gap energies for doped NiO shows substantial decrease; which is 2.38 eV for 5%, 1.94 eV for 10% and 1.50 eV for 15% cobalt doped NiO nanomaterials. The increase in absorption, crystallite size and decrease in band gap is already reported [2]. The decrease in band gap with increase in cobalt concentration may be caused by the replacement of Co ions in NiO lattice. The incorpora tion of cobalt ions in NiO introduce new energy levels near valence band edge and cause decrease in band gap energy of doped NiO [33]. The
3.4. UV–visible spectroscopy UV–visible spectroscopy is performed on all the samples to explore optical properties of prepared nanomaterial and to calculate optical bandgap. UV–Vis absorption spectra recorded for pure and doped NiO are presented in Fig. 4 (a). It can easily be seen from the Figure that cobalt doped samples show exponential decrease as wave length in creases. The recorded graphs shows increase in absorption with increase of Co concentration as dopant that may be attributed as surface morphology change of NiO nanostructures. The absorbance of the nanoparticles usually influenced by different factors including grain size, impurity centers, oxygen vacancy, surface roughness, band gap and lattice strain [33]. For band gap energy calculations from UV–vis data well known Tauc’s relation is used [37]; � ðαhυÞn ¼ B hv Eg (5) 4
A. Zeb et al.
Materials Chemistry and Physics 239 (2020) 122036
References
decrease in band gap of nanomaterial is due to the successful incorpo ration of cobalt ions.
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4. Conclusion In summary, we have reported that composite hydroxide mediated approach is an efficient technique to synthesize pure and doped nickel oxide nanomaterial. The nanostructures are crystalline in nature and average crystallite size found is 31-56 nm. Incorporation of cobalt doping in NiO host lattice introduce point defects and oxygen vacancies. The as prepared nanomaterial shows spherical structure for un-dope nanomaterial. While the nanostructure for 5%–15% cobalt doped show pyramid type beautiful morphological peculiarities and seems to depend on cobalt concentration with side length ranging from 421 to 691 nm respectively. FTIR spectroscopy marks signature on the purity and chemical bonding of NiO. The band gape of pure and doped nano structures is estimated in the range of 1.5–5.59 eV. The decrease in band gap is due to the new energy levels created near valence band edge causing decrease in band gap energy. It is evident that optical band gap is considerably reduced with increasing concentration of cobalt doping. The current investigation will be helpful as an effective technique for morphological dependent development of nanostructures for potential applications in sensor based devices. Acknowledgement The authors are greatly thankful for the cooperation provided by Quaid-e-Azam University, Pakistan (XRD), Ningbo Institute of Materials Technology and Engineering, China (UV–visible), Institute of Space Technology, Pakistan (SEM) and Pakistan Institute of Applied Sciences, Pakistan (FTIR) to access characterization facilities for this research work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122036.
5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Tailoring of pyramid cobalt doped nickel oxide nanostructures by composite-hydroxide-mediated approach Aurang Zeb a, Muhammad Arfan a, b, c, Tauseef Shahid a, b, c, *, Taha Bin Masood a, Abdul Ghafar Wattoo b, c, d, Zhenlun Song b, c, Moazzam Shahzad a, Shehzad Munir Ansari a a
Department of Applied Physics, Federal Urdu University of Arts, Science and Technology, Islamabad, 44000, Pakistan CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China c University of Chinese Academy of Sciences, 19 A Yuquan Rd, Shijingshan District, Beijing, 100049, China d Department of Physics, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan, 64200, Pakistan b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Composite-hydroxide-mediated approach is an efficient method to syn thesize nanomaterial. � Doping effect on the optical properties like band gap. � New cost effective method to the development of novel nanomaterial at low temperature. � Morphological structures for application in optoelectronics and sensor based devices.
A R T I C L E I N F O
A B S T R A C T
Keywords: SEM Co doped NiO Nanomaterials CHM approach Optical properties and FT-IR
Nickel oxide nanostructures have been synthesized successfully by composite hydroxide mediated (CHM) approach at 200 � C for various cobalt concentrations (0%, 5%, 10% and 15%). Incorporation of cobalt was explored by employing X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning elec tron microscopy (SEM, and ultraviolet visible spectroscopy. An influence of increase in cobalt doping on average crystallite size was found from 31 to 56 nm. The increase in crystallite size is due to the introduction of point defects and oxygen vacancies caused by the incorporation of Co ions in NiO lattice. The influence of defects is also seen from variation in lattice parameters and X-ray density. The effect of incorporation of increasing cobalt contents was seen from SEM micrographs. The spherical characteristics of pure NiO nanostructures changed to sharp edge pyramids with smooth surfaces. The growth of pyramids is evident with increase in side length 421–691 nm for various cobalt concentrations (5%, 10% and 15%). FT-IR confirms the purity of product while the band gap was estimated by UV–Vis spectroscopy which was found from 5.59 to 1.50 eV.
* Corresponding author. CAS Key Laboratory of Magnetic Materials and Devices, Zhejiang Province Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, 315201, China. E-mail address: [email protected] (T. Shahid). https://doi.org/10.1016/j.matchemphys.2019.122036 Received 6 May 2019; Received in revised form 15 August 2019; Accepted 18 August 2019 Available online 19 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
A. Zeb et al.
Materials Chemistry and Physics 239 (2020) 122036
1. Introduction Recently, efforts have been made for the study of nanoscale materials to get the dream of reduced sized electronic devices. For this purpose, study of nanomaterials have got a special focus from industrial point of view; as the development of this field will produce remarkable impact on daily life. A number of groups have done significant work to synthesize nanostructures of transition metal oxides with different morphology and size for different valuable applications [1]. Among these transition metal oxides, nickel oxide is a promising ptype semiconductor with wide band gap energy (3.6 eV–4.0 eV). In addition, nickel oxide has great importance due to its intensive appli cations in the fields such as gas sensors, solar cells, electrochemical supercapacitors, optoelectronics devices, solid-oxide fuel cells and photodetectors [2–7]. With the expanding industry and modern tech nology properties of pure nickel oxide nanomaterial are not sufficient for subject applications. Recently, the researchers are interested to acquire the effective performance and elaboration of NiO applications by mixing or doping with different metals (Li, Cu, K, P, Y and other elements) to enhance the novel structural, magnetic, morphological, electro-chromic, electrical and optical properties of newly produced nanostructures [8–13]. Promising results of doping of nickel oxide with cobalt aim at the peculiar structural, morphological and optical properties. Both nickel and cobalt oxides have cubic structure and have low lattice mismatch. Furthermore, the ionic radii of Niþ2 and Coþ2 are nearly similar. For this reason, cobalt is a better doping substitute than other metals. Researchers have developed a number of methods, reported in literature, for the synthesis of nanocrystalline NiO for valuable appli cations [14]. Some of these methods are included as thermal decom position [15], sol–gel, solvothermal and sonochemical [16–18]. Practically, these methods have their own charm but there are some limitations, which limited their use, as reported in the literature, like control over stoichiometry, composition and crystal structure. Com posite hydroxide mediated approach appeals as appropriate substitute with the advantages such as single step, convenient, economical, nontoxic, ecofriendly and low synthesis temperature route for the preparation of oxide materials of various types of structures [19]. Up till now different morphologies of NiO nanostructures including micro spheres, flowerlike, nanowires, nanotubes, polyhedral nanoparticles, nanotowers and nanosheets have been reported with different applica tions [20–28]. In the present work, cobalt doped NiO have been prepared by CHM approach to build feasibility for the synthesis of pure and cobalt doped NiO with better properties. Interesting pyramid type nanocrystals have been obtained in a certain doping range and showed a doping concen tration dependent morphology of the nanostructures. We have estab lished that un-doped NiO nanomaterial showed no well defined morphological structures.
Fig. 1. X-ray diffraction pattern of NiO nano material prepared different doping concentration of cobalt.
stopped and the beaker was taken out from the furnace and cooled naturally to room temperature. The product was washed by distilled water, filtered and dried. For doping purposes 5%, 10% and 15% of Co (NO3)2⋅6H2O were also added at same conditions along with hydroxide mixture and Ni(NO3)2⋅6H2O till final products were obtained, as described above. The chemical reactions can be expressed as follow: NiðNO3 Þ2 :6H2 O þ 2NaOH→NiðOHÞ2 þ 2NaNO3 þ 6H2 O⇒Ni2þ þ 2OH →NiðOHÞ2 Nickel hydroxide is not chemically stable at high temperature therefore forms NiO and water molecule. NiðOH2 Þ → NiO↓ þ H2 O 2.2. Characterization For structural analysis XRD was performed trough X-pert high pro diffractometer operated at 40 kV, 30 mA, with a step size 0.01 and Cu Kα radiation (λ ¼ 1.5406 Å). Scanning electron microscopy (SEM) was performed to observe the surface morphology by using JEOL (JSM 5910) operated at 20 kV. PerkinElmer spectrometer was used to investigate FTIR spectrum in the range of 400 to 4000 cm 1. To measure and analyze optical bandgap, UV–visible absorption data was used in spectral range of 200–700 nm.
2. Experimental methods
3. Results and discussion
2.1. Synthesis of NiO nanomaterial
3.1. XRD analysis
Raw materials including (Ni(NO3)2⋅6H2O, Co(NO3)2⋅6H2O, NaOH and KOH) used for this experiment were of analytical grade and also used without further purification. Composite hydroxide mediated approach was used to synthesize pure and cobalt doped nanostructures as used successfully for the preparation of different transition metal oxide nanostructure in previous reports [29–31]. In this typical syn thesis, 20 g of mixed hydroxide of NaOH and KOH in the ratio of 51.5% and 48.5% were added in a Teflon Beaker, respectively. Then, the Teflon Beaker was put in a preheated furnace at 200� C till all the hydroxides turned into molten state, then this molten mixture was taken out from the furnace. The molten hydroxides and Ni(NO3)2⋅6H2O were stirred well by a Teflon bar till a uniform mixture was attained. The beaker was put back in the furnace for further reaction. After 24 h the heating was
The X-ray diffraction patterns of as prepared samples of Ni1-xCoxO (X ¼ 0%, 5%, 10% and 15%) nanostructures are presented in Fig. 1. These patterns clearly show polycrystalline cubic phase NiO Table 1 Structural and optical parameters of Ni1-xCoxO calculated by XRD and absorp tion spectroscopy.
2
Cobalt concentration
a (Å)
Vcell (Å3)
Density (g. cm-3)
Crystallite size (nm)
Band gap (eV)
0% 5% 10% 15%
4.169 4.177 4.186 4.158
72.45 72.87 73.34 71.88
6.84 6.80 6.76 6.90
31 36 45 56
5.59 2.38 1.94 1.50
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Materials Chemistry and Physics 239 (2020) 122036
Fig. 2. SEM images of as prepared cobalt doped NiO nanostructures at 200 � C (a) Pure NiO (b) 5% doped (c) 10% doped (d) 15% doped.
nanocryatals which are closely resembled to that of JCPDS # 01-0750197. The peaks appeared at 2θ ¼ 37.13� , 43.12� , 62.66� and 75.36� correspond to (111), (200), (220) and (311) crystalline planes of nickel oxide, respectively. The calculated lattice parameter from XRD data for pure NiO is a ¼ 0.4169 nm. The sample synthesized for 5%, 10% and 15% doping have similar results with a small change which are enlisted in Table 1. An increase in lattice constant for 5% and 10% cobalt doped nickel oxide samples is seen. Where a decrease in lattice constant is also seen for 15% doped sample. This decrease may be due to the compres sion of unit cell which is caused by the incorporation of Coþ2 atoms at Niþ2 sites. The changes in lattice constants indicate incorporation of cobalt ions at nickel lattice sites. Furthermore, there is maximum upshift seen in comparison of 2θ values for plane (200) for 15% cobalt doped NiO. It is found that most of the planes of crystallites are strongly ori ented in these favorable (111), (200) and (220) directions. These nanostructures have crystalline nature as the sharpness and strong intensities of XRD patterns reveal. Also, no peak is found there related to impurity or its related compound. The average crystallite size for as prepared nanomaterial is estimated by Debye-Scherrer formula [32]: Kλ D¼ βcosθ
Here ‘‘NA’’ is the Avogadro constant, ‘‘M’’ is the molar mass, ‘‘Z’’ is the number of molecules per formula unit, and Vcell is volume of unit cell. It can easily be concluded from Table 1 that the calculated lattice parameters have good agreement with standard values. The as prepared un-doped nanostructures has the average crystallite size 31 nm, for 5% doping 36 nm, for 10% doping 45 nm and for 15% doping, the material prepared has the average crystallite size 56 nm. The increase in crys tallite size with increasing concentration of cobalt is seen. The increase in crystallite size may be caused by the strain or stress produced due to the difference in ionic radii of Niþ2 (0.69 Å) and Coþ2 (0.72 Å) ions which interrupts the crystallite growth process. With the increase in Co dopant, the change in FWHM of diffraction peaks also changes strain in host structure. The produced strain and defects cause the change in crystal lattice which causes important effects on structural and optical properties of NiO nanostructures [33]. X-ray density shows an increasing trend for 15% doping, whereas a decrease is seen for other samples. 3.2. SEM analysis
(1)
Fig. 2(a–d) shows surface morphology and microstructure of pre pared NiO nanoparticles and nano-pyramids which are successfully obtained by composite hydroxide system at constant temperature 200 � C. It seems that un-doped prepared sample has approximately spherical morphology. For doped samples, Figure (b–d) shows well developed pyramid shaped highly symmetrical structures with regular shape and smooth surfaces. Edge length of pyramid shaped NiO doped with 5% cobalt is approximately 421 nm. While it is clearly seen from the Figure (c and d) that as prepared nanostructures are pyramids with side lengths approximately 485 nm–691 nm with well-defined pecu liarities for 10% and 15% cobalt doping, respectively. With increasing concentration of cobalt the side length of pyramid structures increases. In the formation of pure and doped nickel oxide, viscosity and higher concentration of Hydroxide ions (OH 1) are the key process factors in growth of NiO which is formed through chemical reaction in which Niþ2
Here β is “full width at half maximum (FWHM)”, K is assumed to be 0.9, λ is wave length used in nm and θ is the Bragg’s angle. The lattice parameter is calculated by the formula: a¼
nλ 2sinθ h2 þ k2 þ l2
�1=2
(2)
The cell volume of product is calculated using the formula Vcell ¼ a3
(3)
The X-ray density is estimated using the following formula [17];
ρX
ray
¼
ZM Vcell NA
(4) 3
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Materials Chemistry and Physics 239 (2020) 122036
Fig. 3. FT-IR spectra of cobalt doped NiO nanostructures prepared at temper ature 200 � C and constant reaction time (24 h).
and OH 1 ions are produced. These oppositely charged ions are responsible for the formation of Ni(OH)2 which leads the formation of NiO and water molecule to form nanocrystalline structures. Further more, intersiting pecularties has been seen with doping effect, when nickel oxide is doped with 5%, 10% and 15% cobalt, the structure changes to payramidical shape. The growth of as synthesized nano structure is the result of multidirectional nuclie aggrigation. It can also be seen from SEM micrographs that some of the nanostructures are in evolving stage but some of the structures are completely developed. CHM is a versatile approach used for simple to complex nanostructures synthesis but the effect of kinematic viscosity of the melts and complex thermodynamics involved is still poorly understood. 3.3. FTIR analysis Fig. 4. (a) UV–vis. absorption spectra of the cobalt doped NiO nanostructures prepared at temperature 200 � C and constant reaction time 24 h. (b) The (αhʋ) 2 vs. hʋ plot of as prepared cobalt doped NiO.
Fig. 3 shows FTIR spectra in the spectral range of 400-4000 cm 1. In order to understand the structure of the prepared nanostructures and effects of synthesizing materials, this technique has its own importance. The characteristic broad starching vibrational band for NiO is found at 600-630 cm 1 [34]. The small stretching vibrations present at 2359 and 2344 cm 1 are found due to the presence of CO2 molecules [35,36]. The intensity of these bands decreases with increasing concentration of co balt. The FTIR investigation marks the signature on the purity and XRD results in the sense of pure phase of NiO.
where ‘α’ is known as absorption coefficient and can be calculated by the following relation:
α ¼ 2:303A=l
(6)
In these relations n is taken as 2 for direct band gap or ½ for indirect band gap nanomaterial, hν denotes photon energy, B is taken as constant known as material constant and the path length is indicated by l and is taken to be 1 cm. Optical band gap can be calculated by extrapolating the linear part of the (αhν)n – hν curve to zero, as shown in Fig. 4 (b). The estimated bandgap energies of pure and doped nanostructures are enlisted in Table 1. The corresponding estimated band gap energy for undoped nickel oxide is 5.59 eV, which is greater than as reported for bulk NiO material in literature. This increase in band gap value shows quantum size effects. The calculated band gap energies for doped NiO shows substantial decrease; which is 2.38 eV for 5%, 1.94 eV for 10% and 1.50 eV for 15% cobalt doped NiO nanomaterials. The increase in absorption, crystallite size and decrease in band gap is already reported [2]. The decrease in band gap with increase in cobalt concentration may be caused by the replacement of Co ions in NiO lattice. The incorpora tion of cobalt ions in NiO introduce new energy levels near valence band edge and cause decrease in band gap energy of doped NiO [33]. The
3.4. UV–visible spectroscopy UV–visible spectroscopy is performed on all the samples to explore optical properties of prepared nanomaterial and to calculate optical bandgap. UV–Vis absorption spectra recorded for pure and doped NiO are presented in Fig. 4 (a). It can easily be seen from the Figure that cobalt doped samples show exponential decrease as wave length in creases. The recorded graphs shows increase in absorption with increase of Co concentration as dopant that may be attributed as surface morphology change of NiO nanostructures. The absorbance of the nanoparticles usually influenced by different factors including grain size, impurity centers, oxygen vacancy, surface roughness, band gap and lattice strain [33]. For band gap energy calculations from UV–vis data well known Tauc’s relation is used [37]; � ðαhυÞn ¼ B hv Eg (5) 4
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4. Conclusion In summary, we have reported that composite hydroxide mediated approach is an efficient technique to synthesize pure and doped nickel oxide nanomaterial. The nanostructures are crystalline in nature and average crystallite size found is 31-56 nm. Incorporation of cobalt doping in NiO host lattice introduce point defects and oxygen vacancies. The as prepared nanomaterial shows spherical structure for un-dope nanomaterial. While the nanostructure for 5%–15% cobalt doped show pyramid type beautiful morphological peculiarities and seems to depend on cobalt concentration with side length ranging from 421 to 691 nm respectively. FTIR spectroscopy marks signature on the purity and chemical bonding of NiO. The band gape of pure and doped nano structures is estimated in the range of 1.5–5.59 eV. The decrease in band gap is due to the new energy levels created near valence band edge causing decrease in band gap energy. It is evident that optical band gap is considerably reduced with increasing concentration of cobalt doping. The current investigation will be helpful as an effective technique for morphological dependent development of nanostructures for potential applications in sensor based devices. Acknowledgement The authors are greatly thankful for the cooperation provided by Quaid-e-Azam University, Pakistan (XRD), Ningbo Institute of Materials Technology and Engineering, China (UV–visible), Institute of Space Technology, Pakistan (SEM) and Pakistan Institute of Applied Sciences, Pakistan (FTIR) to access characterization facilities for this research work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122036.
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