Eur. Phys. J. Special Topics 153, 33–35 (2008) c EDP Sciences, Springer-Verlag 2008 DOI: 10.1140/epjst/e2008-00387-6
THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS
Photopyroelectric spectroscopy of Sb2O3 - ZnO ceramics A. Zakariaa , Z. Rizwan, M. Hashim, A. Halim Shaari, and W. Mohmood Mat Yunus Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia
Abstract. Photopyroelectric spectroscopy is used to study the band-gap energy of the ceramic (ZnO + xSb2 O3 ), x = 0.1−1.5 mol% and the ceramic (ZnO + 0.4 mol% Bi2 O3 + xSb2 O3 ), x = 0−1.5 mol% sintered at isothermal temperature, 1280 ◦ C, for 1 and 2 hours. The wavelength of incident light, modulated at 9 Hz, is kept in the visible range and the photopyroelectric spectrum with reference to doping level is discussed. The band-gap energy is reduced from 3.2 eV, for pure ZnO, to 2.86, 2.83 eV for the samples without Bi2 O3 at 0.1 mol% of Sb2 O3 for 1 and 2 hours of sintering time, respectively. It is reduced to 2.83, 2.80 eV for the samples with Bi2 O3 at 0 mol% of Sb2 O3 for 1 and 2 hours of sintering time, respectively. The steepness factor σA which characterizes the slop of exponential optical absorption is discussed with reference to the doping level. The phase constitution is determined by XRD analysis; microstructure and compositional analysis of the selected areas are analyzed using SEM and EDX.
1 Introduction A white polycrystalline solid material Zinc Oxide (ZnO) crystallizes into a wurtzite structure. It is n-type semiconductor material with a wide energy band-gap 3.2 eV [1]. A complete hexagonal closed-packed (hcp) lattice with oxygen atoms inserted into the zinc hcp-lattice. It is widely used in the manufacturing of paints, rubber products, cosmetics, pharmaceuticals, floor covering, plastics, textiles, ointments, inks, soap, batteries, and also in electrical components such as piezoelectric transducers, phosphors, solar cell electrodes, blue laser diodes, gas sensors and varistor [2,3]. The exact role of many additives in the electronic structure of ZnO varistors is uncertain. ZnO based varistor is formed with other metal oxides of small amounts such as Bi2 O3 , Co3 O4 , Cr2 O3 MnO, Sb2 O3 etc. These additives are the main tools that are used to improve the non-linear response and the stability of ZnO varistor [4]. It is necessary to get information of optical absorption of the ceramic ZnO doped with different metal oxides for the investigation of the electronic states and in this paper by using photopyroelectric (PPE) spectrometer, a powerful non-radiative tool [5] to study optical properties, we discuss the PPE spectroscopy of ZnO doped with Sb2 O3 , and ZnO doped with Sb2 O3 in the presence of 0.4 mol% Bi2 O3 .
2 Materials and methods ZnO (99.9% purity) was doped with Sb2 O3 (99.6% purity) and Bi2 O3 (99.975% purity) according to the scheme (ZnO + xSb2 O3 ), x = 0.1, 0.4, 0.7, 1, 1.5 mol% and the ceramic a
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The European Physical Journal Special Topics • ZnO •
♦ Zn 7Sb 2 O 12 ♥ Bi2 O 3
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Fig. 1. XRD pattern of Sb2 O3 doped ZnO with and without Bi2 O3 .
Fig. 2. SEM micrograph at 1.5 mol% Sb2 O3 with Bi2 O3 for 2 hour sintering time.
(ZnO + 0.4mol% Bi2 O3 + xSb2 O3 ), x = 0, 0.1, 0.4, 0.7, 1, 1.5 mol%. Pre-sintered powders at 800 ◦ C were pressed at 800 kg cm−2 to form a disk shape samples. Finally the disks were sintered at 1280 ◦ C for 1 and 2 hours in air at the heating and cooling rate of 8◦ C min−1 . The density was measured by geometrical method. The mirror like polished samples was thermally etched for the microstructure analysis using SEM. The average grain size was determined by the grain boundary-crossing method. The disks of each sample were ground to make a fine powder for the PPE spectroscopic and XRD analysis. The XRD data were analyzed by using X’Pert High Score software for the identification of the crystalline phases. The measurement of PPE signal amplitude using the PPE spectrometer system to produce a PPE spectrum has been described elsewhere [5]. In the present system the light beam was a 1 kW Xenon arc lamp that was kept in the range of 300 to 800 nm, mechanically chopped at 9 Hz, and scanned at 2 nm step size. The true PPE spectrum of the sample was obtained by normalizing PPE spectrum of the sample with that of the carbon black. Prior the PPE measurement, the fine powder sample was ground in deionised water and a few drops of each mixture were dropped on the 1.5 cm2 aluminium foil and dried in air to form a thin sample layer about 12 µm thick on the foil. The foil was placed in contact with PE transducer [6] using a very thin-layer of silver conductive grease. In determining the energy band-gap (Eg ), it was assumed that the fundamental absorption edge of doped ZnO is due to the direct allowed transition. The optical absorption coefficient β varies with the excitation light energy hυ [7] and is given by the expression, (βhυ)2 = C(hυ − Eg) near the band gap, where hυ is the photon energy, C is the constant independent of photon energy, and Eg is the direct allowed energy band-gap. 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 region that crosses photon energy axis.
3 Results and discussion For both samples, secondary phase Zn7 Sb2 O12 (spinel) was observed from peaks at angles of 42.0840◦ , 43.7704◦ , 49.2362◦ , 50.8897◦ , 60.9525◦ (ref. code 00-036-1445) at higher doping levels where it is segregated in grain boundaries, Fig. 1. From EDX analysis, Bi was detected at the grain boundaries and nodal points, and some patches of the mixture Zn, C, O can be seen on the grain surfaces. For both of samples, the grain size increases but the density decreases with the increase of sintering time, indicating pores created during sintering. However reversibly, the grain size decreases but the density increases with the increase of dopan Sb2 O3 mol%, thus this indicates that dopan acts as a grain inhibitor. SEM micrograph is shown in Fig. 2. The Eg of the ceramics without Bi2 O3 is reduced from 3.2 eV (pure ZnO) to 2.86, 2.83 eV at 0.1 mol% of Sb2 O3 , Fig. 3, for 1 and 2 hours of sintering time, respectively, due to the growth of interface states by Sb ions in the grain boundaries and at the particle surfaces. Later,
Photoacoustic and Photothermal Phenomena
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Fig. 4. Effect of Sb2 O3 on steepness factor (σA ) without and with Bi2 O3 .
Eg increases slightly to a value 2.89, 2.87 eV with the increase of Sb2 O3 for 1 and 2 hours of sintering time, respectively. The Eg of the ceramics with Bi2 O3 is reduced to a value of 2.83, 2.80 eV at 0 mol% of Sb2 O3 for 1 and 2 hours sintering time, respectively. This decrease in Eg is due to the growth of interface states due to 0.4 mol% Bi2 O3 , even in the absence of Sb2 O3 . Eg cannot be reduced further with the doping level of Sb2 O3 , but in contrast, the value of Eg is increased to a value 2.86, 2.85 eV for 1 and 2 hours sintering time, respectively. This is due to the decrease in the interface states in the band-gap, as the result of decrease in the defect states [8] by Sb ions in grain boundaries and at the particle surfaces. The value of Eg for the samples is about constant for 2 hours sintering time, after 0.4 mol% Sb2 O3 ; this may be due to the presence of liquid phase Bi2 O3 . The steepness factor σA , Fig. 4, which characterizes the slop of exponential optical absorption [8–10] increases with the increase of Sb2 O3 for the ceramics without Bi2 O3 and the ceramics with Bi2 O3 indicating the decrease in the interface states in the grain boundaries or particle surface. Hence, Eg increases due to the decrease of the interface states, Fig. 3. This decrease in the interface states may be due to spinel phase developed.
4 Conclusion Sb2 O3 acts as a grain inhibitor through generated secondary phase Zn7 Sb2 O12 (spinel) that interferes with ZnO grain growth. Secondary phase and Bi2 O3 segregates at the grain boundary and at the nodal points. The energy band-gap of the ceramics is reduced to a maximum value of 2.80 eV and the further doping level of Sb2 O3 slightly increases its value. Thanks to MOSTI for the financial assistance (Grant FRGS No. 01-01-07-139FR) for this research.
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