Appl. Sci. Lett.1(1) 2015, 8-13 Applied Science Letters RESEARCH ARTICLE www.appslett.com An interdisiplinary peer-reviewed interational journal Investigation of physicochemical properties of Ag doped ZnO nanoparticles prepared by chemical route Sethuraman Gayathri 1 , Oriparambil Sivaraman Nirmal Ghosh 1 , S. Sathishkumar 1 , P. Sudhakara 2 , J. Jayaramudu 3 , S. S. Ray 3 and Annamraju Kasi Viswanath 1 1 Nanophotonics and Nanoelectronics Research Laboratory, Centre for Nanoscience and Technology, Madanjeet School of Green Energy Technologies, Pondicherry University, Kalapet – 605014, India, 2 CSIR – Central Leather Research Institute, Regional Centre for Extension and Development, Leather Complex, Jalandhar, Punjab – 144021, India, 3 National Centre for Nanostructured Materials, Materials Science and Manufacturing, Counsil for Scientific and Industrial Research, Pretoria – 0001, South Africa (Received 22 October, 2014; accepted 23 October, 2014; published 01 January, 2015) In this report we have demonstrated the synthesis of silver doped and pure ZnO nanoparticles using facile chemical precipitation method. The crystal structure, optical and magnetic properties of the synthesized nanocrystals were determined from XRD, UV-Vis, PL and VSM respectively. A small variation was found in the particle size after calcination at high temperature 400 o C. The UV-Vis spectra clearly showed a blue shift in optical bandgap due to silver doping and it is observed that the optical absorption is maximum for 3w% silver doped ZnO nanocrystals. SEM-EDAX and TEM-SAED were performed to analyse morphology, chemical composition and size of the nanocrystals. The doping of Ag into ZnO was confirmed from FTIR results. To investigate the effect of Ag ion impregnation into the ZnO lattice, laser raman spectra was recorded at room temperature. It is suggested that the ZnO lattice experiences lattice stress arised from the doping of silver ions. The enhancement in room temperature ferromagnetism in ZnO:Ag was achieved Keywords: Ferromagnetism; zinc oxide; nanoparticles; Raman spectroscopy INTRODUCTION Recent development in nanoscience and technology resulted emerging field of band gap engineered metaloxide nanostructures [1-4]. Wide variety of nano metaloxides such as ZnO, TiO2 and SnO2 have been explored for various applications in photocatalysis, electronics, photonics and magnetism [5-10]. Among these ZnO exhibts unique electronic, magnetic, optical and photocatalytic properties. ZnO has a wide band gap of 3.37 eV and large exciton binding energy around 60 meV [11]. Due to the high electron mobility, tunable magnetic properties and transparency, ZnO has become a key material for applications in potential areas including laser diodes, solar cells, gas sensors, dilute magnetic semiconductors, spintronics and optoelectronic devices [12-16]. The band gap engineering approach to modify the crystal defects and electronic structure of ZnO using noble metal has attracted significant attention in heterogeneous photocatalysis. 17 Synthesis of Ag doped ZnO nanostructures with modified surface properties have been already reported in literature. 18-21 It is found that the Ag doping facilitates the interfacial charge transfer process for effective utilization of conduction band electrons for enhanced photocatalytic activity [22, 23]. In this regard to understand the effect of Ag doping and identify the detailed mechanism of lattice defect formation in ZnO nanocrystals we have carried out a systematic investigation on physicochemical characteristics of the pristine and Ag doped ZnO nanocrystals. Zinc oxide nanomaterials are non-toxic with wide band gap has been identified as a potential semiconducting material for exhibiting room temperature ferromagnetism when it is doped with transition metals. Doping of transition metals into zinc oxide nanoparticles are considered to be an effective method to fine-tune the energy level structure of the host which can be further improved by the different concentrations of the dopants leading refining the optical properties [24, 25]. In Ag doped ZnO, the diffusion of Ag into ZnO can cause variation in its lattice structure as Ag and Cu are known to be fast diffusing compounds in the semiconductor materials and also it effects the corresponding interelated physical properties. Ag could be a good candidate to improve luminescence efficiency and the mechanism for enhancement of emission was not yet understood [26].The influences of silver doping into ZnO on the absorption, visible emission and the corresponding transitions of electron and excitons in ZnO host lattice need to be studied.The observed ferromagnetism Appl. Sci. Lett.1(1) 2015, 8-13 www.appslett.com Copy right 2014 Asian Scientific Publishers 8
Transcript
Appl. Sci. Lett.1(1) 2015, 8-13
Applied Science Letters
RESEARCH ARTICLE
www.appslett.com An interdisiplinary peer-reviewed interational journal
Investigation of physicochemical properties of Ag doped ZnO nanoparticles prepared by chemical route
Sethuraman Gayathri1, Oriparambil Sivaraman Nirmal Ghosh1, S. Sathishkumar1, P. Sudhakara2, J. Jayaramudu3, S. S. Ray3 and Annamraju Kasi Viswanath1
1Nanophotonics and Nanoelectronics Research Laboratory, Centre for Nanoscience and Technology, Madanjeet School of Green Energy Technologies, Pondicherry University, Kalapet – 605014, India,
2CSIR – Central Leather Research Institute, Regional Centre for Extension and Development, Leather Complex, Jalandhar, Punjab – 144021, India,
3National Centre for Nanostructured Materials, Materials Science and Manufacturing, Counsil for Scientific and Industrial Research, Pretoria – 0001, South Africa
In this report we have demonstrated the synthesis of silver doped and pure ZnO nanoparticles using facile chemical precipitation method. The crystal structure, optical and magnetic properties of the synthesized nanocrystals were determined from XRD, UV-Vis, PL and VSM respectively. A small variation was found in the particle size after calcination at high temperature 400oC. The UV-Vis spectra clearly showed a blue shift in optical bandgap due to silver doping and it is observed that the optical absorption is maximum for 3w% silver doped ZnO nanocrystals. SEM-EDAX and TEM-SAED were performed to analyse morphology, chemical composition and size of the nanocrystals. The doping of Ag into ZnO was confirmed from FTIR results. To investigate the effect of Ag ion impregnation into the ZnO lattice, laser raman spectra was recorded at room temperature. It is suggested that the ZnO lattice experiences lattice stress arised from the doping of silver ions. The enhancement in room temperature ferromagnetism in ZnO:Ag was achieved
Recent development in nanoscience and technology resulted emerging field of band gap engineered metaloxide nanostructures [1-4]. Wide variety of nano metaloxides such as ZnO, TiO2 and SnO2 have been explored for various applications in photocatalysis, electronics, photonics and magnetism [5-10]. Among these ZnO exhibts unique electronic, magnetic, optical and photocatalytic properties. ZnO has a wide band gap of 3.37 eV and large exciton binding energy around 60 meV [11]. Due to the high electron mobility, tunable magnetic properties and transparency, ZnO has become a key material for applications in potential areas including laser diodes, solar cells, gas sensors, dilute magnetic semiconductors, spintronics and optoelectronic devices [12-16]. The band gap engineering approach to modify the crystal defects and electronic structure of ZnO using noble metal has attracted significant attention in heterogeneous photocatalysis.17Synthesis of Ag doped ZnO nanostructures with modified surface properties have been already reported in literature.18-21It is found that the Ag doping facilitates the interfacial charge transfer process for effective utilization of conduction band electrons for enhanced photocatalytic
activity [22, 23]. In this regard to understand the effect of Ag doping and identify the detailed mechanism of lattice defect formation in ZnO nanocrystals we have carried out a systematic investigation on physicochemical characteristics of the pristine and Ag doped ZnO nanocrystals. Zinc oxide nanomaterials are non-toxic with wide band gap has been identified as a potential semiconducting material for exhibiting room temperature ferromagnetism when it is doped with transition metals. Doping of transition metals into zinc oxide nanoparticles are considered to be an effective method to fine-tune the energy level structure of the host which can be further improved by the different concentrations of the dopants leading refining the optical properties [24, 25]. In Ag doped ZnO, the diffusion of Ag into ZnO can cause variation in its lattice structure as Ag and Cu are known to be fast diffusing compounds in the semiconductor materials and also it effects the corresponding interelated physical properties. Ag could be a good candidate to improve luminescence efficiency and the mechanism for enhancement of emission was not yet understood [26].The influences of silver doping into ZnO on the absorption, visible emission and the corresponding transitions of electron and excitons in ZnO host lattice need to be studied.The observed ferromagnetism
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Investigation of physicochemical …… nanoparticles prepared by chemical route 9 RESEARCH ARTICLE
in undoped nanoparticles of oxides such as ZnO, MgO and CeO2 could be due to the intrinsic defects and it could be a surface phenomenon as proposed by A. Sundaresan and Rao [27].The effects of Ag doping into ZnO on the PL, Raman scattering and the effect of point defects such as oxygen vacancies, cation vacancies on the ferromagnetic properties of ZnO and silver doped ZnO systems were studied in details.
EXPERIMENTAL
To synthesize pristine ZnO nanoparticles using a facile precipitation method was adopted. In a typical synthesis process, 0.1M stock solutions of Zn(NO3)2.6H2O and NaOH were prepared separately in 50 ml of deionized water. Further the completely dissolved NaOH aqueous solution was added dropwise into the 50ml of Zn(NO3)2.6H2O stock solution to obtain the white coloured zinc hydroxide precipitate. The attained precipitate was washed five times to remove the residue and centrifuged at 3000 rpm to get final product. Then it was dried in hot air oven at 120oC for 2 hours and throughly ground and calined at 400 oC for 3 hours to obtain zinc oxide nanoparticles. To prepare silver doped ZnO nanoparticles, the required amount of AgNO3 precursor was added into the prepared stock solution of Zn(NO3)2.6H2O and followed the above same procedure.
The structural characteristics of the prepared nanoparticles were investigated using X-ray diffractometer (Rigaku Ultima IV) with Cu Kα radiation source (λ= 1.541Å) in the 2θ range from 20o to 80o with the scanning rate of 5o/minute. The surface morphology and chemical compositions of the prepared nanoparticles were examined using scanning electron microscope (SEM-Hitachi S3400N) with energy dispersive X-ray (EDAX) setup (Thermoscientific). The shape and size of the nanoparticles were determined by high resolution field emission transmission electron microscope using JEOL JEM 2100F TEM operating at 200 kV. The nanocrystals used for the analysis were prepared by dispersing the sample into ethanol and dripped over a copper specimen grid coated with a permeable carbon film and allowed to dry. The FTIR spectra were recorded using FTIR spectrometer (Thermo Nickolet-6700). The optical measurements were performed on Perkin Elmer Lambda 650s spectrometer and Horiba-Fluoromax IV. Magnetic properties of the samples were investigated using Lake shore (Model 7404) Vibrating Sample Magnetometer (VSM). Laser raman spctroscopic measurements were carried out on a Renishaw inVia Raman microscope with spectrometer attachment. RESULTS AND DISCUSSION Fig. 1 shows the X-ray diffractogram of pristine and silver doped ZnO nanoparticles. From the XRD results, it is confirmed that the prepared nanoparticles exhibit a hexagonal wurtzite structure. All the diffraction peaks are indexed as shown in Fig. 1 and it is matching well with the standard JCPDS card no(80-0075). The obtained diffraction peaks reveal that the hexagonal wurtzite structure is retained even after the silver doping into ZnO crystal lattice. It is clearly observed that the diffraction peaks become sharper and stronger due to the calcination at high temperature which suggest that the crystallite quality of the nanoparticles is improved and the particle size is increased. The impurity peaks are observed for Ag doped ZnO nanoparticles due to the effect of calcination. Fig. 2 shows shift in X-ray diffraction peaks due to internal stress in the lattice and it was induced by the Ag ion doping in the ZnO nanocrystals [28]. When ZnO is doped with Ag+, the peak is shifted towards lower angle when compared to that of pure
ZnO. This occurs because of the ionic radii mismatch of the elements Zn2+(0.088 nm) and Ag+(0.122 nm) [29].
The average crystallite size of the ZnO nanoparticles with various dopant concentrations of Ag have been calculated from XRD using Scherrer’s formula. The obatined values are presented in table 1. The decrease in the crystallite size with the increase of dopant concentration is attributed to the internal stress generated in the lattice upon silver doping.
Figure 1 Powder X-ray diffractogram of pristine and Ag doped ZnO nanoparticles heat treated at 400 oC
Surface morphology of the synthesized samples was identified from SEM electron micrograph. Fig. 3 shows the SEM micrographs of Ag doped ZnO nanoparticles. The synthesized nanoparticles are found to be spherically shaped.
The SEM images suggest that the synthesized ZnO: Ag nanoparticles are uniformly distributed at nano regime. The chemical composition of the Ag doped ZnO is obtained from EDAX
Table 1. Particle size variation of undoped and Ag doped ZnO nanocrystals
% of silver doping
Crystallite size of the samples dried at
120 0C(nm)
Crystallite size of the samples dried at 400
0C(nm) 0 34 41 1 30 38 2 27 32 3 21 23
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spectrum depicted in the Fig. 4. The elemental composition from the EDAX spectrum confirmed the presence of Zn, O and Ag in the prepared sample.
Figure 2 Shows the shift in X-ray diffraction peaks due to the lattice stress induced by Ag ion doping in ZnO nanocrystals.
Figure 3(a, b, c, d) SEM micrographs of ZnO:Ag nanoparticles
Figure 4 Energy dispersive X-ray spectum of ZnO:Ag nanoparticles
Figure 5 TEM images of (5 a, b, c) ZnO:Ag nanoparticles and (5 d) SAED pattern of ZnO:Ag nanoparticles Fig. 5(a, b, c) depicts the typical high resolution TEM electron micrographs of ZnO:Ag nanospheres. The selected area electron diffraction pattern given in Fig. 5(d) shows that the ZnO:Ag nanoparticles are polycrystalline in nature.
Figure 6 FTIR spectra of (a) ZnOnanoparticles and (b) Ag doped ZnO nanoparticles
The FTIR spectra of pristine and 3w% Ag doped ZnO samples are shown in Fig. 6. Pure ZnO and Ag doped ZnO possess wurtzite structure and are further supported by FTIR spectra as shown in Fig. 6. The FTIR spectra of ZnO and ZnO:Ag nanoparticles were recorded in the range 500 – 4000 cm-1. The characteristic stretching mode of Zn-O and Ag-O bonds are assigned to the significant bands at 479 and 538 cm-1. The peak at 1036 and 1029 cm-1 can be attributed to aromatic C=C stretching mode. The absorption bands at 3459 and 3478 cm-1arise due to the stretching mode of O-H group which reveals the existence of small amount of water absorbed by the ZnO nanostructure. The peaks located at 2358 and 2368 cm-1 is due to the atmospheric CO2 present in the instrument. Stretching
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Investigation of physicochemical …… nanoparticles prepared by chemical route 11 RESEARCH ARTICLE
modes of C-C and C=O are observed at 1036 and 1029 cm-1and 1396, 1631 cm-1and 1409, 1619cm-1respectively. The band at 2915 and 2928 cm-1attributed to C-H stretching vibrations. The peak shift observed around 479 cm-1to higher wave number 538 cm-1indicates the Ag doping into the ZnO host lattice [30].
Figure 7 UV-Vis absorption spectra of pristine and Ag doped ZnO nanoparticles
The UV-Vis Spectra of undoped and silver doped zinc oxide nanoparticles were shown in the Fig. 7. The band gap of the synthesized nanostructures is calculated from the Kubleka-Munk Plot. it can clearly be seen that the maximum absorbance of silver doped ZnO is at visible region due to the electron trapping. As the doping concentration of silver increases the optical band gap is found to be increasing (Ag 1w% - 3.20 eV, Ag 2w% - 3.24 eV, Ag 3w% - 3.27 eV). Thus there is a blue shift in the absorption spectra as the doping concentration of silver increases.26Maximum absorbance was acheived for the 3 wt % doped ZnO nanocrystals.
Fig. 8 shows that the photoluminescence spectra of ZnO and Ag doped ZnO nanocrystals (excitation at 350 nm). All samples emit strongly in the UV region with a band centred at 390 nm, corresponding to the excitonic emission and two distinct intense visible peaks at 450nm and 630nm which are due to the presence of intrinsic defects in ZnO:Ag nanocrystals [31, 32]. The visible emissions at blue and red regions are found to be decreasing with the increase in silver concentration indicating that there is decreased electron-hole recombination in the ZnO:Ag nanoparticles. The Fig. 9 shows the M-H curves recorded at room temperature for the pristine and Ag doped ZnO nanoparticles. Ferromagnetic behavior has been observed for pure and silver doped zinc oxide nanoparticles.
Generally zinc oxide exhibit diamagnetic behavior, but due to the confinement of magnetic domains, the prepared
Figure 8 Photoluminescence spectra of pristine and Ag doped ZnO nanoparticles recorded at room temperature
Figure 9 M-H curves recorded at room temperature for pristine and Ag doped ZnO nanoparticles
samples are showing typical ferromagnetic behavior which is mainly due to the presence of oxygen vacancies found in ZnO:Ag crystal lattice. For Ag doping, the magnetization increases as increase in concentration of dopant but coercivity and remenance is found to be varying with doping concentration and given below in the table 2. Magnetization is found to be increasing on increasing dopant concentration [33]. Ferromagnetic behavior is enhanced on doping as the Ag atoms are rich in free electrons which inturn has its own electric dipole moment. From the XRD results, the crystallite size of the ZnO: Ag nanoparticles decreases as the dopant concentration increases owing to the stress created in the lattice upon doping. As a result of Ag doping, defects are induced in the lattice which plays a key role in the enhancement of the ferromagnetic behavior of ZnO:Ag nanoparticles. To investigate the influence of Ag doping on the Raman scattering of ZnO nanoparticles, room temperature Raman spectra of all samples were recorded using 488nm laser
Table 2. Calculated values of magnetization, coercivity and retentivity for silver doped zinc oxide nanostructures % of silver
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source. Fig. 10 shows the Raman spectra of pristine and Ag doped ZnO nanocrystals.
Figure 10 Raman spectra of pristine and Ag doped ZnO nanoparticles recorded at room temperature
The wurtzite ZnO has eight sets of characteristic optical phonon modes at the center of Brillouin zone (Γ point)
Γ =1A1+2B1+1E1+2E2
Where A1and E1are the two polar branches, which split into longitudinal optical (LO) and transversal optical components with different frequencies due to macroscopic electric fields associated with the LO phonons. The nonpolar E2 modes are related with low frequency mode (E2L) associated to the heavy Zn sublattice and high frequency mode (E2H) involving only oxygen atoms. While A1, E1, and E2 modes are active in Raman and infrared spectra, B1 modes are usually not active in Raman spectra and due to this reason these modes are generally known as silent modes. The room temperature Raman spectrum of the undoped ZnO nanoparticles was taken in the spectral range varying from 50 cm−1 to 800 cm−1. The spectrum consists of four peaks located at about 90, 383, 433, and 580 cm−1, these peaks are the characteristic peaks correspond to the E2L,A1(TO), E2H and A1(LO) fundamental phonon modes of hexagonal ZnO. The Raman peak positioned at about 203 cm−1 is assigned to the 2E2L second order phonon mode. The remaining three Raman peaks located at about 326, 533, and 660 cm−1 are assigned congruently to the 3E2H −E2L, E1 (TO) + E2L, 2(E2H−E2L) multiphonon scattering modes [34].
The A1 (TO) andA1 (LO) are the polar branches appeared at about 376 cm−1and 578 cm−1 respectively for all the synthesized nanoparticles. The Ag doping in ZnO leads to the broadening of A1 (LO) peak and it is observed that it is shifted about 13 cm−1 towards lower energy. Moreover this shift and broadening in the A1 (LO) phonon mode can be attributed to the scattering influences of the A1 (LO) branch outside the Brillouinzone center. The A1 (LO) phonon mode is conventionally assigned to the oxygen vacancies, zinc interstitials or defect complexes containing oxygen vacancy and zinc interstitial in ZnO. Furthermore, there appeared a broad Raman peak at about 494 cm−1solely for the Ag-doped samples which was due to the interfacial surface phonon mode [35]. The radial effect of Ag atoms can be identified from the peak at 234 cm-1 obtained in the given spectrum.
CONCLUSION
In summary, we have successfully demonstrated the facile synthesis of ZnO and Ag doped ZnO nanocrystals using chemical precipitation method. The XRD analysis showed that the prepared samples are in hexagonal phase and the crystallite size decreases as the dopant concentration is increased due to the internal stress. From the UV-Visible absorption spectra, it is identified that the band gap of the ZnO:Ag nanoparticles is decreasing with increasing doping concentration of Ag. The results obtained from the photoluminescence specta reveal the presence of oxygen vacancies in the bandgap engineeried ZnO:Ag nanocrystals. The intensities of the PL spectra are decreased upon the increase in concentration of Ag doping. This is due to the increase in oxygen vacancies, which was duly confirmed from the magnetic measurements.The synthesized ZnO:Ag nanocrystals exhibit room temperature ferromagnetism as a result of defect formation arise from Ag doping. Further, results obtained from Raman spectra of ZnO:Ag nanocrystals divulges the detailed mechanism of defect formation and oxygen vacancy generation in ZnO nanocrystal lattice by Ag doping.
ACKNOWLEDGMENTS
S. Gayathri and O. S. N. Ghosh was supported by the doctoral research fellowship of the Pondicherry University, India. Analytical instrumentation facility provided by Central Instrumentation Facility and Centre for Nanoscience and Technology, Pondicherry University are thankfully acknowledged.
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BIOGRAPHY
Annamraju Kasi Viswanath is Associate Professor at the Centre for Nanoscience and Technology, Pondicherry University, India. He received his Ph.D. in Physics, from IIT Madras, India in 1976 and became Research associate at the Michigan State University the same year. He spent the year 1979/81 as Research Associate at University of Maine, USA. He had worked in various research labs as Senior Researcher in CRL, Hitachi Limited, Japan; Brainpool Professor at KRISS, Taejon, Korea and Additional Director at CMET, Pune, India. He has authored more than 100 scientific publications. For the first time the size dependence of the bandgaps in molecular level transition metal clusters was showed by him in 1979. Currently he is focusing on engineering functional nanohybrids for various applications including photonics, magnetism, catalysis and biotechnology. Gayathri Sethuraman obtained her Masters degree in 2008 from the Department of Physics, Annamalai University, India. She worked on several different projects in the area of nanobiophotonics and nanomagnetism. In 2013, she joined the group of Dr. A. Kasi Viswanath and currently pursuing her Ph.D degree in Nanoscience and Technology at centre for Nanoscience and Technology, Pondicherry University, India. Her major research interest is to develop lanthanide and zinc oxide based nanomaterials for biophotonics and therapeutic applications.
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