Structural, electrical and gas-sensing properties of In 2 O 3 : Ag composite nanoparticle layers

Transcription

1 PRAMANA c Indian Academy of Sciences Vol. 65, No. 5 journal of November 2005 physics pp Structural, electrical and gas-sensing properties of In 2 O 3 : Ag composite nanoparticle layers B R MEHTA and V N SINGH Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi , India brmehta@physics.iitd.ernet.in Abstract. The central objective of this study is to investigate (i) size-dependent properties of In 2 O 3 nanoparticles and (ii) the role of metal additives in enhancing the gas sensing response. For this purpose, In 2 O 3 : Ag composite nanoparticle layers having welldefined individual nanoparticle size and composition have been grown by a two step synthesis method. Thermogravimetric analysis, X-ray diffraction and transmission electron microscopy have been used to study the effect of post-synthesis heat treatment on the size and structure of the nanoparticles. A first-time unambiguous observation of sizedependent lowering of transformation temperature has been explained in terms of lower cohesive energy of surface atoms and increase in surface-to-volume ratio with decrease in nanoparticle size. The gas sensing studies of In 2 O 3 as well as the In 2 O 3 : Ag composite nanoparticle layers have been studied as a function of size and composition. In 2 O 3 : Ag composite nanoparticle layers with 15% silver show a sensitivity of 436 and response time of 6 s for 1000 ppm of ethanol in air. Ag additives form a p-type Ag 2 O, which interact with n-type In 2 O 3 to produce an electron-deficient space charge layer. In the presence of ethanol, interfacial Ag 2O reduces to Ag, creating an accumulation layer in In 2O 3 resulting in increased sensitivity. Keywords. In 2O 3 nanoparticles; Ag nanoparticles; gas sensor; composite nanoparticles. PACS Nos Df; Df; w; Be 1. Introduction Indium oxide (In 2 O 3 ) is an n-type, highly degenerate and wide band gap material having wide applications in flat panel displays, photoelectric devices, windshield defrosting layers, heat-reflecting mirrors, high transparency layers and gas sensor devices. The adsorption of oxygen on an n-type semiconductor produces a space charge layer in the surface region [1]. When exposed to a reducing gas, the surface oxygen concentration decreases to a steady level, releasing the trapped electrons back to the bulk of the semiconductor. The resulting change in electron transport is the basis of gas sensing mechanism. Recently, nanocrystalline or nanostructured layers with porous structures have been used to enhance the sensitivity [2]. In various studies on nanostructured oxide semiconductor materials for gas sensing applications, nanoparticle size is not well defined at all. Parameters, which control 949

2 B R Mehta and V N Singh particle size, also modify thickness, porosity or stoichiometry of the semiconductor layers [3]. It has also been observed that metal additives like Pt, Pd or Ag enhances sensitivity and selectivity but decreases response time and operating temperature [4]. The amount and distribution of the metal (semiconductor metal configuration) is an important parameter to be controlled in order to study the effects of the metal addition on the gas sensing properties and to obtain highest sensitivity [5]. Diverse methodologies (impregnation, sol gel, electroless, etc.) have been used for introducing these additives [6]. In the previously reported studies, metal additives are understood to form a thin catalytic layer or nano/microclusters or take part in doping/alloy formation [7]. In the present study, In 2 O 3 : Ag composite layers have been formed from In 2 O 3 and Ag precursor nanoparticles using a two-step procedure. The central objective of this study is to investigate the dependence of structural, optical and gas sensing properties on the nanoparticle size and composition by depositing In 2 O 3 and In 2 O 3 : Ag nanoparticle layers having well-defined sizes. 2. Experimental A two-step synthesis method has been used to prepare In 2 O 3 : Ag composite nanoparticle layers. In the first step, In 2 O 3 and Ag nanoparticles are synthesized by chemical capping method. In the chemical capping technique, capping molecules terminate the nanoparticle growth by blocking the active sites at the growing surfaces. For the synthesis of In 2 O 3 nanoparticles, alanine was used as the capping molecule. Two master solutions were prepared: solution A, consisting of 0.1 M solution of InCl 3 in ethanol and solution B consisting of M solution of alanine in ammonia. Solution A was added drop-wise to solution B with continuous stirring and the mixture was maintained at a temperature of 80 C at atmospheric pressure for 20 h. Due to the insolubility of synthesized nanoparticles in ethanol and ammonia, the gray white particles settle down. The nanoparticles were collected, washed several times in high purity water to remove the excess ions (NH + 4 and Cl ) and subsequently vacuum dried [8]. The In(OH) 3 nanoparticle samples synthesized using capping-to-reagent ratio of 25 : 4, 10 : 4 and 1 : 4 will be referred to as IHA, IHB and IHC, respectively. These samples were dried at 60 C in air to remove any moisture content. In 2 O 3 nanoparticle samples were prepared by heating In(OH) 3 (samples IHA, IHB and IHC) nanoparticles at 350 C and will be referred to as IOA, IOB, and IOC samples, respectively. The as-synthesized nanoparticle sample IHB was annealed at 350, 500, 700 and 900 C for 90 min in air. These nanoparticles samples will be referred to as IO3, IO5, IO7 and IO9, respectively. Unless stated, the capping ratio used in the present study is 10 : 4. For the synthesis of Ag nanoparticles, poly(vinyl pyrrolidone) (PVP) (0.5 m mol in monomer unit) and silver perchlorate (0.1 m mol) have been heated for 2 h in 1 : 1 solution of ethanol and water at 85 C. In 2 O 3 and In 2 O 3 : Ag composite nanoparticle layers were prepared from the precursor nanoparticles using a dip-coating method. The required quantities of nanoparticles (In 2 O 3 and Ag) were dispersed in a solution containing equal amounts of glacial acetic acid and ethanol. The layers were prepared by dipping the glass or quartz substrate in the solution containing nanoparticles using a vibration free dip- 950 Pramana J. Phys., Vol. 65, No. 5, November 2005

3 Structural, electrical and gas-sensing properties of In 2 O 3 ping system. After each dip, the layers were dried at a temperature of C for a few minutes to evaporate out the solvents. The dipping process was repeated till the required thickness was achieved. The substrates were pulled with a constant speed of 10 cm/min. The nanoparticle layers were annealed at 400 C for 90 min. For gas sensing measurements, nanoparticles dispersed in ethanol and ethylene glycol solution were spread on substrates with interdigitated electrodes. Speciallydesigned buried electrodes have been used for maintaining the homogeneity and uniformity of the nanoparticle layers. The resistance of the structure without any layer deposition is more than Ω. The structures are bonded to a DIL8 chip carrier and a poly-si layer is embedded in the structure and serves as the heating element, allowing a maximum surface temperature of 300 C under these conditions. An external heating arrangement has been made in order to heat the substrate at a higher temperature. A thermo-element close to the interdigitated electrodes allows the temperature measurement. The gas-sensing properties in terms of sensitivity and dynamic behavior were determined by measuring the time-dependent changes in resistance on changing the gas environment in the measurement cell in the temperature range of C. 3. Results and discussion The effect of nanoparticle size on the temperature of conversion from In(OH) 3 to In 2 O 3 has been carried out. TGA results in terms of the first derivative of weight loss (%) as a function of temperature have been shown in figure 1. The weight loss is related to the transformation of In(OH) 3 to In 2 O 3. According to the reaction 2In(OH) 3 2In 2 O 3 + 3H 2 O, the removal of water molecules results in a weight loss of 16.3%. A weight loss of 16.81% in the temperature range of C is observed for the sample IHB. It is important to note that the endothermic peak corresponding to the In(OH) 3 to In 2 O 3 transformation shifts towards lower temperature with the reduction in nanoparticle size from 15 nm to 8 nm. In the case of nanoparticle sample IHC (having initial size of 15 nm), the In(OH) 3 to In 2 O 3 transformation takes place at 285 C. In the case of samples with particle sizes 11 and 8 nm the transformation temperature is found to be 272 and 255 C, respectively. It was reported earlier that to obtain In 2 O 3 in bulk form, In(OH) 3 is ignited at 850 C followed by annealing at 1000 C for 30 min [9]. The observation of a reduced phase transition temperature in case of In(OH) 3 nanoparticles is of great importance both in terms of basic properties of materials at nano-dimensions and from application point of view. Figure 2 shows the results of X-ray diffraction analysis carried out on In(OH) 3 and In 2 O 3 nanoparticle samples annealed at different temperatures. XRD pattern shows a significant amount of line broadening which is a characteristic of nanoparticles. The intense peaks in the XRD spectrum of sample IHB at 2θ = 22.4, d = Å, I/I 0 = 100 and at 28.2, Å, 46.4 correspond to (2 0 0) and (2 1 1) h k l planes of cubic In(OH) 3. In the XRD spectrum for sample IO3, peaks corresponding to In(OH) 3 completely disappear and thus all the peaks in the XRD spectrum are due to cubic In 2 O 3 phase. The TEM micrographs of the nanoparticle samples IHA, IHC, IOA and IOC are shown in figure 3. The size of the nanoparticles is the same Pramana J. Phys., Vol. 65, No. 5, November

4 B R Mehta and V N Singh Figure 1. Derivative of weight loss vs. temperature curves for nanoparticle samples. (a) IHA, (b) IHB and (c) IHC. Figure 2. X-ray diffraction spectra of In 2 O 3 nanoparticle samples. (a) IHB, (b) IO3, (c) IO5 and (d) IO9. Peaks corresponding to the cubic In 2 O 3 and cubic In(OH) 3 phases are indexed. as calculated using the XRD peaks. These results clearly show that the initial nanoparticle size is preserved and In(OH) 3 to In 2 O 3 transformation takes place without any coarsening or alteration of nanoparticle size. The Gibbs Thomson expression (eq. (1)) for the down-shifting of the melting point has been extended to explain the decrease in the phase transition temperature [10]. T D = T 0 + T 0 4V σ H D, (1) where T 0 is the transition temperature for bulk, D is the diameter of the nanoparticle sample, V is the molar volume of the initial crystalline phase, In(OH) 3 = cm 3, σ is the interface energy and H is the enthalpy or heat of transformation = 265 J/g. A plot (figure 4) of T D vs. 1/D approaches the value of 322 C (595 K) for bulk In(OH) 3. The calculated value of interfacial energy from the slope of the curve is 0.21 J/m 2, which is similar to the values calculated for WO 3 (0.24 J/m 2 ) nanoparticles in the study of Boulova and Lucazeau [11]. The TEM analysis was carried out to study the effect of annealing on the size of the nanoparticles and the micrographs for the samples IHB, IO3 and IO9 are shown in figure 5. In 2 O 3 nanoparticle samples, IO3 and IO9 have average particle size of 11 and 29 nm, respectively. It may be mentioned here that the as-synthesized nanoparticles have irregular shapes. On annealing, the nanoparticles acquire well-defined cubical shape. It is observed that up to a temperature of 400 C, only a marginal increase 952 Pramana J. Phys., Vol. 65, No. 5, November 2005

5 Structural, electrical and gas-sensing properties of In 2 O 3 Figure 3. TEM micrographs of nanoparticle samples. (a) IHA, (b) IHC, (c) IOA and (d) IOC. in size takes place and a sharp increase in size is observed at higher temperatures. The increase in size due to coarsening involves surface melting and diffusion which explains a larger increase in size at a temperature higher than 400 C. In 2 O 3 and In 2 O 3 : Ag nanoparticle composite layers have been deposited using the two-step method described elsewhere [3]. A detailed structural and composition analysis shows that particle size of precursor nanoparticle is preserved in the resulting nanoparticle layers. The electrical conductivity (σ) of In 2 O 3 nanoparticle layer sample was measured in vacuum as a function of temperature. The conductivity of In 2 O 3 nanoparticle layer annealed in air at 400 C is Ω 1 cm 1 and increases to Ω 1 cm 1 on vacuum annealing. The change in the electrical conductivity and a low value of activation energy of the particles indicate that the particles are well connected in the nanoparticle layers. These well-connected particulate features are important for forming a current conducting network. These results show that layers prepared by the present method are suitable for electronic and gas sensing applications. GAXRD results of the In 2 O 3 : Ag nanoparticle layer annealed at 400 C given in figure 6 show well-formed XRD peaks. The peak at 2θ = 30.5, d = Å, I/I 0 = 85 corresponds to the (2 2 2) h k l plane of the cubic In 2 O 3. Other peaks at 45.6, 51.1 and 56.1 also correspond to (4 3 1), (4 4 0) and (6 1 1) planes of cubic In 2 O 3. The peaks at 2θ = 31.85, d = Å correspond to the (1 1 1) h k l plane of Ag 2 O. The large FWHM of the XRD peaks is indicative of the nanoparticle character of these layers. The particle size in the In 2 O 3 nanoparticle layers is estimated to be 10 nm which is similar to the size of the precursor nanoparticles. For measuring gas sensing response, In 2 O 3 and In 2 O 3 : Ag nanoparticle layers having different sizes and compositions were deposited on buried interdigitated structures. Figure 7 shows a typical gas sensing response measured in In 2 O 3 layers Pramana J. Phys., Vol. 65, No. 5, November

6 B R Mehta and V N Singh Figure 1/D. 4. A plot of transformation temperature, T D (K) as a function of Figure 5. TEM micrographs of nanoparticle samples. (a) IHB, (b) IO3 and (c) IO9. having nanoparticle sizes of 5 and 11 nm. Figure 8 shows the sensitivity and response time of nanoparticles having 5 nm, 11 nm, 15 nm, 21 nm and 29 nm sizes at 200 C temperature and 1000 ppm ethanol. Sensitivity is the ratio of the resistance of the nanoparticle layer in air ambient to the resistance in sensing gas (ethanol+air). Response time has been taken as the time required to reach 90% of the saturated value of resistance in gas sensing. The sensitivity increases from 1.2 to 10 with decrease in size from 21 to 5 nm. The response time decreases from 420 to 140 s on reduction in nanoparticle size. Thus there is a large improvement in sensitivity and the response time of the In 2 O 3 nanoparticle layers with decrease in size. Variation of the sensitivity and response time was also studied as a function of measurement temperatures and ethanol concentrations. For a In 2 O 3 nanoparticle sample having nanoparticle size of 11 nm, a sensitivity of 325 and the response time of 8 s at 400 C for 1000 ppm ethanol has been observed. Figure 9 shows a 954 Pramana J. Phys., Vol. 65, No. 5, November 2005

7 Structural, electrical and gas-sensing properties of In 2 O 3 Figure 6. GAXRD spectrum of In 2 O 3 : Ag composite nanoparticle layer annealed at 400 C. Figure 7. Gas sensing response of In 2O 3 nanoparticle layers having sizes 5 and 11 nm. comparison of the sensitivity and the response time for the In 2 O 3 and In 2 O 3 : Ag nanoparticle layers at 400 C temperature for 1000 ppm ethanol. The sensitivity in the case of In 2 O 3 : Ag increases to 436 and the response time is about 6 s. As already mentioned, the adsorption of O on the surface of In 2 O 3 extracts electrons from the donor level. This extraction of electrons creates an electron depleted space charge region near the surface resulting in an increase in the resistance. The interaction of the reducing gas with pre-adsorbed oxygen ions removes the oxygen leaving the electrons in the conduction band resulting in decrease in the resistance. Thus, the mechanism responsible for the large conductivity changes in oxide semiconductor in the presence of oxidizing and reducing gas is because of the formation and annihilation of oxygen vacancies. The depletion of carriers takes Pramana J. Phys., Vol. 65, No. 5, November

8 B R Mehta and V N Singh Figure 8. Effect of particle size on the sensitivity and response time of In 2O 3 nanoparticle layers. place in a thickness equal to the Debye length of electrons in In 2 O 3. Debye length for electrons, which depends upon the carrier concentration, temperature and semiconductor dielectric constant, has been estimated to be about 3 nm in the case of metal oxide semiconductors [12]. This explains the maximum response observed in In 2 O 3 nanoparticle layer having a size of about 5 nm. It may be mentioned here that sensitivity and response time depend upon the thickness, porosity and electrode configuration. A detailed XPS analysis of In 2 O 3 : Ag nanoparticle layer was carried out. Ag 3d and O 1s spectra for In 2 O 3 : Ag (10%) layer is shown in figure 10. Peak intensity of Ag 3d peak at ev shows that Ag is present in the form of Ag 2 O. Ag 2 O and In 2 O 3 are p- and n-type semiconductors, respectively. Ag 2 O and In 2 O 3 form p n junction at In 2 O 3 surface and the conduction electrons of In 2 O 3 are drawn toward Ag 2 O producing an electron-depleted layer in the In 2 O 3 nanoparticle resulting in an increased electrical resistance. Upon exposure to ethanol, Ag 2 O is reduced to Ag and the conduction electrons are given back to In 2 O 3 accompanied by a decrease or disappearance of the space charge and appearance of an accumulation layer due to conversion of Ag 2 O In 2 O 3 interface to Ag In 2 O 3 interface, leading to larger sensor response. To eliminate the changes in gas sensing response due to variation in porosity or thickness in In 2 O 3 layers having different nanoparticle sizes, a fixed thickness of In 2 O 3 layer (150 nm) has been used. This thickness is also found to give a stable gas sensing response. Thus the variation in gas sensing response in different nanoparticle samples is due to change in nanoparticle size. In the case of In 2 O 3 : Ag composite layer, both In 2 O 3 and Ag nanoparticles have been synthesized separately and the composite layers have been annealed at 400 C before studying gas sensing response. The structural and compositional studies carried out on In 2 O 3 : Ag composite layer show that, there is no change in the size of the nanoparticles up to 400 C. Thus, the change in sensitivity and response time can be directly linked to change in silver composition and surface modification at In 2 O 3 : Ag interfaces. 956 Pramana J. Phys., Vol. 65, No. 5, November 2005

9 Structural, electrical and gas-sensing properties of In 2 O 3 Figure 9. Comparison of gas sensing response of the In 2 O 3 : Ag composite nanoparticle for 1000 ppm ethanol at 400 C. Figure 10. XPS spectra showing (a) Ag 3d and (b) O 1s peaks for In 2 O 3 : Ag (10%) layers. 4. Conclusions In(OH) 3 nanoparticles having well-defined particle sizes of 15, 11 and 8 nm have been grown using the chemical capping method. The lowering of transformation temperature from In(OH) 3 to In 2 O 3 observed in the present study is a direct measure of lowering of surface cohesive energy and increase in surface-to-volume ratio. It has been successfully demonstrated that In 2 O 3 and In 2 O 3 : Ag nanoparticle layers having well-defined particle size and composition can be prepared using a two-step synthesis method. Gas sensing properties have been measured by depositing nanoparticle layers on specially designed buried interdigitated structure. The Pramana J. Phys., Vol. 65, No. 5, November

10 B R Mehta and V N Singh addition of silver nanoparticles having 13 nm size in the In 2 O 3 nanoparticle layer leads to increase in the sensitivity from 325 to 436. The reduction of silver oxide in the presence of the reducing gas is observed to be responsible for the increased sensitivity. Acknowledgement The present study was carried out with the financial support from Nano Science and Technology Initiative of the Department of Science and Technology, Government of India. One of the authors (VNS) acknowledges Council for Scientific and Industrial Research, New Delhi, India for the grant of Senior Research Fellowship and Research Associateship for carrying out this work. References [1] N Yamazoe, Sensors and Actuators B5, 7 (1991) [2] S Shukla, S Seal, L Ludwig and C Parish, Sensors and Actuators B97, 256 (2004) [3] V N Singh and B R Mehta, Jpn. J. Appl. Phys. 42, 4226 (2003) [4] Y Shimizu, T Maekawa, Y Nakamura and M Egashira, Sensors and Actuators B46, 163 (1998) [5] V A Chaudhary, I S Mulla and K Vijayamohanan, Sensors and Actuators B55, 154 (1999) [6] A Diéguez, A Vilà, A Cabot, A Romano-Rodríguez, J R Morante, J Kappler, N Bârsan, U Weimar and W Göpel, Sensors and Actuators B68, 94 (2000) [7] J A Cobos, Metal additive distribution in TiO 2 and SnO 2 semiconductor gas sensor nanostructured materials, PhD Thesis (Department of Physics, Universitat de Barcelona, Spain, 2001) p. 34 [8] V N Singh and B R Mehta, J. Nanosci. Nanotechnol. 5, 437 (2005) [9] A Thiel and H Lukmann, Z. Anorg Alloy Chem. 172, 353 (1928) [10] P J Desre, Nanostruct. Mater. 8, 687 (1997) [11] M Boulova and G Lucazeau, J. Solid State Chem. 167, 425 (2002) [12] H Ogawa, A Abe, M Nishikawa and S Hayakawa, J. Electrochem. Soc. 28, 2020 (1981) 958 Pramana J. Phys., Vol. 65, No. 5, November 2005