American Journal of Applied Sciences (Special Issue): 25-29, 2005 ISSN 1546-9239 © 2005 Science Publications Photopyroelectric Spectroscopy of MnO Doped Ceramic ZnO at Different Sintering Temperatures B.Z. Azmi, Zahid Rizwan, M. Hashim, A.H. Shaari, W.M.M. Yunus and E. Saion Photoacoustic Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia 43400 UPM Serdang, Selangor D.E., Malaysia. Abstract: The band-gap energy, Eg, of ZnO doped with 0.1 to 2.0 mol% of MnO and sintered at different isothermal sintering temperatures is studied by photopyroelectric spectroscopy in the wavelength range of 300 to 800 nm. This energy is estimated from the plot (ρhυ)2 versus hυ and is about 3.0 eV for the samples sintered at 850°C at all doping levels. Samples sintered at higher temperatures (1050 and 1300°C), the value of Eg decreases with the MnO mol% and beyond 1 mol%, Eg becomes constant at about 2.0 eV. The increase in steepness factor (σA, in A-region and σB, in Bregion) is related to sintering temperature at low doping level. The dielectric constant, ranging from 300 to 3100, increases with the increase in sintering temperature and decreases with the increase in frequency indicating the formation of insulating layer near the grain boundaries. The X-ray diffractrometry shows that the crystal structure of ZnO doped with different MnO mol% at all sintering temperatures remains to be of hexagonal type but a small peak is found related to the new phase ZnMn2O4 only at 1050°C sintering temperature and 2 mol% of MnO doping level. The grain size ranges from 2 - 30 µm, increases with the MnO mol% and sintering temperature. The density is decreased from 95.5 to 87% with the increase of sintering temperature and doping level. Key words: Photopyroelectric, band-gap energy, ceramic ZnO present the photopyroelectric spectroscopy of MnO doped ceramic ZnO at various sintering temperatures. INTRODUCTION The ZnO based ceramic semi-conductors are widely used as gas sensors[1], piezoelectric transducers, electrode for solar cells, phosphors, transparent conducting films[2] and varistors. Regarding ZnO varistor, it posses high-energy absorption capability against various surges and is extensively used as protective device. It has super-fast response to overvoltage transients as they sense and clamp transients in nano-second speed, repeatedly in thousands of times without being destroyed[2]. The varistors are typically fabricated by sintering of ZnO with other metal oxides of small amounts such as Bi2O3, Co3O4, Cr2O3, MnO, Sb2O3, Al2O3, BaO, Y2O3, Pr6O11 etc. These additives are the main tools that are used to improve the nonlinear response and the stability of ZnO varistor[3]. Much have been done in I-V studies on ZnO based varistor by previous workers[3,4] and there are few investigations have been carried out on optical absorption measurements ceramic ZnO doped with metal oxides because of the difficulty of conventional transmitting methods. It is necessary to get information on optical absorption of ceramic ZnO doped with metal oxides for investigation on electronic states of ceramic ZnO and doped impurities at different levels during sintering process that can help the understanding of the electrical behaviour of doped ZnO. In this study we MATERIALS AND METHODS Sample preparation: ZnO (99.9% purity, Alfa Aesar) was doped with MnO (99.5% purity) varied as 0.1, 0.4, 0.7, 1.0, 1.3, 1.6, 2.0 mol% (mole percentage). The 30 g product at each mol% was mixed with ethanol and was stirred for 24 h. The slurry was filtered and dried in air for 48 h. The mixture was ground to make a powder of fine particles before pre-sintering. The powder of each mol% was divided into three parts and was pre-sintered at temperatures 800, 1000 and 1250°C for 2 h at the rate of 10°C min¯1. Then each sample was ground and polyvinyl alcohol (1 wt %) was mixed as a binder to give strength and also to avoid cracks in the final pressed product. The mixture was pressed in a Specac press under a force of 2 tons using zinc stearate as a lubricant to form a disk of 10 mm diameter with 1 mm thickness. Finally the pellets were sintered at 850, 1050 and 1300°C for 1 h in air at heating and cooling rate of 4°C min¯1. The density of the ceramic ZnO doped with MnO was measured by the Archimedes method. The disk from each sample was ground for 2 h and granulated by sieving through a 75-mesh screen for the photopyroelectric (PPE) spectroscopy and XRD analysis. Corresponding Author: B.Z. Azmi, Photoacoustic Laboratory, Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor D.E., Malaysia. 25 Am. J. Applied Sci., (Sp. Issue): 25-29, 2005 0 ZnO + 2mol% MnO Sintered at 1050 C 20 30 40 50 103 200 112 201 004 202 110 Pure ZnO 102 100 002 101 220 Intensity (arb. Unit) ZnMn 2O4 Photopyroelectric measurements: The PPE spectroscopy, a powerful technique from photothermal science is a non-radiative tool[5] to study optical properties of the materials. The method is based on photothermal effect where the pyroelectric (PE) film transducer is used to detect the temperature variation from the light-induced periodic heating in the sample. When there is absorption of incident light, the nonradiative de-excitation processes with in the solid will cause the sample temperature to fluctuate, through heat diffusion to the surrounding PE film. Due to this temperature change, a PE voltage at modulation frequency (ω) is observed in the PE film and is given by V(ω) = pI d < ∆T > /ε ,[6] where p is the PE coefficient, Id is the film thickness, ε is the film dielectric constant and < ∆T > is the average temperature rise in the film. PPE signal amplitude was measured for a range of optical excitation wavelengths 300 to 800 nm using the PPE spectrometer system as described elsewhere[7] to produce a PPE spectrum which is actually an excitation spectrum. For sample treatment prior the PPE measurement, fine powder was again ground in deionised water, then few drops of the each mixture were dropped on an aluminium foil of area 1.5 cm2 and dried in air to form a thin layer of sample on the foil. The foil was placed in contact to a Polyvinylidene Difluoride PPE film transducer[8] using silver conductive grease. A high power 1 kW Xenon arc lamp (Oriel 6921) was used as the light source in the spectrometer system and the light beam was mechanically chopped at 9 Hz. The PPE spectrum data of the sample ZnO doped with MnO at room temperature were accumulated in a personal computer and normalized with respect to carbon black PPE spectrum data to obtain the true sample spectrum. For the determination of band-gap energy, it was assumed that the fundamental absorption edge of ZnOX metal oxide is due to the direct allowed transition. The optical absorption coefficient β varies with the excitation light energy hυ[9] and is given by the expression, (βhυ)2 = C (hυ-Eg) near the band gap, where C is the constant independent of photon energy hυ, and Eg is the direct allowed band-gap energy. The PPE signal intensity ρ is directly proportional to β, hence (ρhυ)2 is related to hυ linearly. From the plot of (ρhυ)2 versus hυ, the value of Eg is obtained by extrapolating the linear fitted regions to (ρhυ)2 = 0 , as shown in Fig. 2. 60 70 80 0 Position ( 2Theta ) Fig. 1: XRD pattern for pure ZnO and doped ZnO with MnO. o 850 C, Eg=2.96eV 25 o 1300 C, Eg=1.89eV 20 15 o (ρνh) 2 1050 C, Eg=2.06eV 10 5 0 1.5 2.0 2.5 3.0 3.5 4.0 Photon Energy,eV Fig. 2: Band-gap energy of ZnO doped with 1 mol% of MnO at different sintering temperatures. Microstructure examination: The either surface of samples was lapped and ground with SiC paper and polished with 1 µm diamond suspension to a mirror like surface. The polished samples were thermally etched at the temperature 150°C below the sintering temperature for 10 min. The surface microstructure was examined by a scanning electron microscope (SEM). The average grain size (d) was determined by the linear intercept method, given by d = 1.56L/MN , where L is the random line length on the micrograph, M is the magnification of the micrograph and N is the number of grain boundaries intercepted by lines. The X-ray diffraction (XRD) with Cu Kα radiation using PANAalytical (Philips) X’Pert Pro PW1830 was used to identify the crystalline phases. The XRD data were analysed by using X’Pert High Score software for the identification of the crystalline phases. Dielectric dispersion measurements: The frequency dependence of dielectric constant for the MnO doped ZnO ceramic samples at sintering temperatures 850, 1050, 1300°C at all MnO doping level were scanned from 100 Hz to 1 MHz at room temperature[10], using impedance analyzer (Hewlett Packard Model 4192A). 26 Am. J. Applied Sci., (Sp. Issue): 25-29, 2005 transition metal oxides, such as Mn, is involved in the formation of interfacial states and deep bulk traps at RESULTS AND DISCUSSION The XRD pattern in Fig. 1 shows that the crystal structure of pure ZnO and ZnO doped with 2 mol% of MnO at all sintering temperatures 850, 1050, 1300oC remain to be of hexagonal wurtzite-type structure. No extra peak was found at 850 and 1300°C sintering temperatures at all doping levels, indicating the substitution of Mn2+ ions inside the lattice of ZnO and the absence of any free MnO. The presence of a small peak at the diffraction angle of 44.66o for the plane (220) found in the pattern related to the new phase ZnMn2O4 only at 1050°C sintering temperature at 2 mol% of MnO doping level, indicates the significant amount of the new phase is only found at this temperature and it may present at 850, 1300°C sintering temperatures but in a very low quantity which cannot be detected. It is also expected that low quantity that detected only at 1050°C have no effect on the optical absorption spectra. Therefore, it is considered that presence of second phase ZnMn2O4 and an amorphous Mn-rich thin film, in a nanometer scale that is not detected in XRD pattern, is formed at the grain boundaries at 850 and 1300°C sintering temperatures. The presence of a second phase or amorphous film can benefit grain boundary diffusion, thus promoting the grain growth of ZnO at all sintering temperatures. The density was decreased from 95.5% to 92.3% of theoretical density 5.78 g/cm3 [4] from low to high sintering temperatures upto 1mol% and then decreased from 92.3% to 87% with the increase of sintering temperature and doping level, but the decrement is very prominent with sintering temperature. Therefore, it is expected that there is an abnormal grain growth with the increase of doping level and sintering temperature as the abnormal grains were seen in SEM with irregular shapes. The grain size is ranged from 2-5, 3-20 and 830 µm by increasing MnO mol% at 850, 1050, 1300°C sintering temperatures, respectively, which indicates the grain promotion property of MnO. ZnO behaves as a n-type semiconductor, as its conduction band arises from the 4s orbital of Zn is wide enough to favour the effective charge transfer. Electrons are excited from its valence state due to the absorption of light so this process generates the electron-hole pairs, which starts the conduction mechanism. Above the band-gap energy limit, the PE signal is due to non-radiative process and that of below is due to the Urbach tail[11]. The value of Eg about constant 3.0 eV that is very close to that of pure ZnO, Fig. 3, indicates that at all doping levels the probability of the substitution of the Mn2+ ions into the grain boundaries is very low at 850°C sintering temperature. In practice, there are a variety of inter-grain conduction paths that operate in parallel[12], such as through the bulk inter-granular material or through the grain boundary region and are sensitive to the presence of chemical additives. It has been postulated that 3.4 3.2 o 850 C 3.0 2.8 2.6 2.4 Eg (eV) o 1050 C 2.2 2.0 o 1.8 1300 C 1.6 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 MnO doping mol % Fig. 3:Dependence of energy band gap at MnO mol% at different sintering temperatures. 1.0 0.9 At 2 mol% MnO Intensity (arb.units) 0.8 0.7 o 1050 C 0.6 o 1300 C 0.5 o 0.4 850 C 0.3 300 400 500 600 700 800 Wavelength (nm) Fig.4: PPE spectra for 2 mol% of MnO different sintering temperatures. At 2 mol% MnO σΑ 0.9 σΒ 0 oC 0.8 σΒ 130 0.7 σΑ 0.6 850 o C 1.0 105 o 0C Ln(PPE Signal Intensity), arb. units 1.1 0.5 0.4 0.3 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Photon Energy (eV) Fig. 5: PPE signal intensity spectra for different sintring temperatures. 27 Am. J. Applied Sci., (Sp. Issue): 25-29, 2005 and also the decrease in the Eg is very low ranging from 0.14 σB (1300 C) At 2 mol% MnO 0.13 o 0.12 Steepness Factor (σA, σΒ) 0.11 0.10 0.09 σB (1050 C) 0.08 o 0.07 0.06 0.05 σA (1050 C) 0.04 o 0.03 σA (1300 C) 0.02 o 0.01 0.0 0.5 1.0 1.5 2.0 MnO mol% Fig.6: Dependence of the steepness factor σA and σB on MnO mol%. 3500 3000 2500 Dielectric Constant grain boundaries, providing large potential barriers to give better nonlinear characteristics when the I-V characteristics of the material was studied as a varistor[3]. The PPE spectra of samples can be seen in Fig. 4 with the PPE signal intensities normalized to that of lower wavelength range. The PPE intensity is higher at high sintering temperatures indicating that the phonon-phonon interaction is greater at the higher wavelength. PPE signal intensities plotted against photon energy (hυ) vary linearly just below the fundamental absorption edge in two regions (denoted as A and B) in accordance with Urbach’s rule (exponential tail)[13]. Therefore, an empirical relation for the dependence of PPE signal intensity (P) on the photon energy and sintering temperature (T) has been fitted to the data according to: P = Po exp[σ(hυ - hυ o )K ] where K = (kT) −1 , k is the Boltzman’s constant, P0 , υ o are the fitting parameters[14] and σ is the steepness factor that characterized by the exponential optical absorption. Hence from these spectra in Fig. 4, the steepness factor is obtain by the above equation and is presented in Fig. 5. Steepness factor cannot be observed at 850 o C sintering temperature. In Fig. 6, the steepness factors σA (region A) and σB (region B) are plotted against MnO mol% of doping for sintering temperatures of 1050 and 1300 o C . It is noted that at low doping about 0.1 mol% for both 1050 and 1300°C sintering temperatures, σB and σA roughly have equal value about 0.08 but later σB increases but σA decreases with doping. At low doping (< 0.4 mol%) with the increase of sintering temperature both σB and σA increase indicating an increase in structural ordering[15]. At higher doping (> 0.4 mol%) with the increase of sintering temperature, σB still increases indicating an increase in structural ordering but σA decreasing indicating a decrease in structural ordering. This suggests that probing at higher photon energy and at higher MnO doping level in itself disturbed the orientation of the polycrystalline structure of doped ZnO. Also, at higher doping (> 1.0 mol%), both σB and σA are about constant with the increase of doping level that indicates MnO mol% plays no further contribution. Dielectric constant of bulk ZnO[16] is less than 10 but for the present doped ceramic ZnO it ranges from 300 to 3100 and decreases with the increase of frequency at all MnO doping levels, as can be seen in Fig. 7. However, for sample sintered at 1300°C and sintered the decrement is very rapid with the rise of frequency. The increase in the dielectric constant at higher temperature indicates the segregation of MnO forming insulating layer in or near the grain boundaries and the decrease in Eg ranging from 3.2-2.0 eV is also observed at higher temperatures indicating the introduction of interfacial states. The dielectric constant has low value at lower sintering temperature At 0.1 mol% MnO 2000 o 1500 1300 C 1000 500 o 1050 C 0 o 850 C 2 3 4 5 6 Log F (Hz) Fig. 7: The frequency dependence of dielectric constant for 0.1 mol% of MnO at different sintering temperatures. 3.2-3.0 eV indicating the possibility of substitution of Mn2+ ions in the grain boundaries. CONCLUSION The XRD results and MnO doping level are correlated with the PPE spectroscopy results indicating that the optimum concentration of MnO in ceramic ZnO is possible upto certain mol% at certain ceramic processing conditions. The PPE spectroscopy of ZnO doped with different mol% of MnO shows the decrease in band-gap energy, increase in steepness factor at high sintering temperatures indicating the formation of interfacial states and structural ordering. The dielectric constant decreases with increasing frequency and has higher values at higher temperatures indicating the formation of microstructure of semiconducting ZnO surrounded by insulating layer. 28 Am. J. Applied Sci., (Sp. Issue): 25-29, 2005 8. ACKNOWLEDGEMENTS The authors would like to thank the Ministry of Science, Technology and Environment of Malaysia for the financial support of this work under IRPA Grant No. 02-02-04-0132-EA001. 9. REFERENCES 10. 1. 2. 3. 4. 5. 6. 7. Joshy Jose and M. Abdul Khadar, 2001. Role of grain boundaries on the electrical conductivity of nanophase zinc oxide. Mater. Sci. Engg.A, 304306: 810-813. Look, D.C., 2001. Recent advances in ZnO materials and devices. Mater. Sci. Engg.B, 80: 383-387. Eda, K., 1989. Zinc Oxide Varistors. IEEE Elect. Insul. Mag., 5: 28-41. Choon-Woo Nahm and Byoung-Chil Shin, 2003. Highly stable nonlinear properties of ZnO-Pr6O11CoO-Cr2O3-Y2O3 based varistor ceramics. Mater. Lett., 57: 1322-1326. Minamide, A., M. Shimaguchi and Y. Tokunaga, 1998. Study on photopyroelectric signal of optically opaque material measured by Polyvinylidene Difluoride film sensor. Jpn. J. Appl. Phys., 37: 3144-3147. Almond, D.P. and P.M. Patel, 1996. Photothermal Science and Techniques. Chapman and Hall, London, pp: 91-99. Mandelis, A., 1984. Frequency-domain photopyroelectric spectroscopy of condensed phases (PPES): A new, simple and powerful spectroscopy technique. Chem. Phys. Lett., 108: 388-392. 11. 12. 13. 14. 15. 16. 29 Minamide, A. and Y. Tokunaga, 1998. Nondestrictive evaluation of defect in optically opaque material by Polyvinylidene Difluoride film sensor. IEEE Ultrasonic Symp., pp: 865-868. Toyoda, T., H. Nakanishi, S. Endo and T. Irie, 1985. Fundamental absorption edge in semiconductor CdInGaS4 at high temperatures. J. Phys. D Appl. Phys., 18: 747-751. Kutty, T.R.N. and S. Ezhilvalavan, 1997. The effect of aluminosilcate on the delayed onset of upturn voltages of zinc oxide varistor ceramics. Mater. Sci. Engg. B, 47: 101-109. Kimura, A., Y. Ohbuchi, T. Kawahara, Y. Okamoto and J. Morimoto, 2001. Photoacoustic spectra of ZnO-CoO alloy semiconductor. Jpn. J. Appl. Phys., 40: 3614-3616. Levinson, L.M. and H.R. Philipp, 1986. Zinc oxide varistors. Am. Ceram. Soc. Bull., 65: 639646. Urbach, F., 1953. The long wavelength edge of photographic sensitivity and electronic absorption of solids. Phys. Rev., 92: 1324-1330. Dow, J.D. and D. Redfield, 1972. Towards a unified theory of Urbach’s Rule and exponential absorption edges. Phys. Rev. B, 5: 594-610. Taro Toyoda and Satoshi Shimamoto, 1998. Photoacoustic spectroscopy of ceramic ZnO doped with Bi2O3. Mater. Sci. Engg. B, 54: 29-32. Ezhilvalavan, S. and T.R.N. Kutty, 1997. Effect of antimony oxide stoichiometry on the nonlinearity of zinc oxide varistor ceramics. Mater. Chem. Phys., 49: 258-269.