Effect of Silver Addition on the Ethanol-Sensing Properties of IndiumOxide Nanoparticle Layers : Optical Absorption Study

In2O3 and In2O3:Ag nanoparticle layers have been deposited using a two-step method consisting of chemical capping and dip coating techniques. The result of optical absorption analysis of In2O3:Ag samples shows the presence of Ag2O and Ag in air-annealed and vacuum-annealed samples, respectively. These results have been correlated with the gas sensing properties of these layers towards ethanol and support the proposed mechanism that increase in sensor response on Ag addition is due to the conversion of Ag2O to Ag in the presence of ethanol.


INTRODUCTION
Recently, gas-sensing properties of oxide semiconductors have been improved by preparing these materials in nanocrystalline or nanoparticle forms [1].The sensing mechanism is based on the fact that the adsorption of oxygen on the semiconductor surface causes a significant change in the electrical resistance of the material.The formation of oxygen adsorbate (O −2 or O − ) results in an electrondepletion surface layer due to electron transfer from the oxide surface to oxygen.The sensing properties of oxide semiconductor materials also have been modified by adding metals or metal oxides as catalytic activator [2].The effect of metal/metal oxide addition on gas-sensing phenomenon of oxide semiconductor has been explained in terms of doping of oxide matrix by metal atoms or formation of nano, micro, or macro-agglomerated metal clusters [3].Various possible mechanisms have been suggested for the enhanced gas-sensing properties in presence of metal/metal oxide.In almost all studies, the structure of the metal nanoparticles in the composite nanoparticles is not well defined.In the present study, In 2 O 3 :Ag composite nanoparticle layers have been synthesized by a two-step synthesis process using chemically grown In 2 O 3 and Ag nanoparticles.Based on the detailed optical absorption studies, a possible mechanism for the effects of metal addition on the gas sensing properties has been suggested.Formation of nanoparticles and layers in two separate steps provides the advantage of controlling the individual nanoparticle size, composition, and film thickness independently.

Synthesis of In 2 O 3 and Ag nanoparticles has been carried
out by chemical capping method as discussed earlier [4][5][6][7].The In(OH) 3 nanoparticles were synthesized by heating 0.01 M InCl 3 • 3H 2 O in ethanol and 0.01 M alanine in ammonia solutions (in a ratio of 1 : 2.5) at 80 • C for 20 hours with continuous stirring.The nanoparticles were collected and washed several times using deionised water to remove the excess ions.This is followed by vacuum drying.In 2 O 3 nanoparticles were prepared by heating In(OH) 3 nanoparticles at 350 • C. For the synthesis of Ag nanoparticles, PVP, poly (N-vinyl-2-pyrrolidone) and silver perchlorate were heated in ethanol/water mixture at 90 • C with continuous stirring.Silver nanoparticles were obtained after centrifugation.In 2 O 3 :Ag (IAG15) composite nanoparticle layers with 15 weight% Ag were prepared by a dip-coating method.The required amount of precursor nanoparticles, In 2 O 3 and Ag were dispersed in ethanol.The deposited layers were dried at a temperature of 100-120 • C for few minutes to evaporate out the solvents after each dip.
Glancing angle X-ray diffraction (GAXRD) analysis of nanoparticle layers was obtained using Rigaku diffractometer (Giegerflex-D/max-RB-Ru 200).A Philips CM12 electron microscope was used for transmission electron microscopy (TEM) studies.Nanoparticles were dispersed ultrasonically in water and spread over formvar coated copper grids for TEM analysis.For gas-sensing measurements, the dispersed nanoparticles were spread on substrates with interdigitated electrodes by carefully placing drops onto the substrate followed by drying in an oven at 100 • C, as described elsewhere [5,6].Specially designed buried electrodes have been used for maintaining the homogeneity and uniformity of the nanoparticle layers.The structures are bonded to a DIL16 chip carrier through gold bond wires.A poly-Si layer, which is embedded in the structure to serve as the heating element, allows a maximum surface temperature of 300 • C.An external heating arrangement has been used in order to heat the substrate to higher temperatures.A temperaturedependent resistor close to the interdigitated electrodes allows the temperature measurement.The gas-sensing properties in terms of sensor response and response time were determined by measuring the time-dependent changes in resistance on changing the gas environment in the measurement cell in the temperature range of 100-400 • C.

RESULT AND DISCUSSION
GAXRD spectrum of composite nanoparticle layer sample IAG15 annealed at 400 • C for 90 minutes is shown in Figure 1 Figure 6 shows a comparison of the sensor response and the response time for the In 2 O 3 and In 2 O 3 :Ag nanoparticle layers at 400 • C for 1000 ppm ethanol.Figure 7 shows a typical sensor response for composite nanoparticle layer.Sensor response is the ratio of the conductance of the nanoparticle layer in ethanol in synthetic air to the conductance in      [9].This explains the high sensor response to ethanol in presence of Ag.In case of an In 2 O 3 layer without Ag addition, gas-sensing response depends on the degree of depletion of electrons from In 2 O 3 by adsorbed oxygen and the effectiveness of removal of oxygen species by ethanol.In case of In 2 O 3 having appropriate amount of Ag, gas-sensing response depends upon the effectiveness of reduction of Ag 2 O to Ag in the presence of reducing ambient (ethanol).The resistance in air and in 1000 ppm ethanol for the In 2 O 3 and for IAG15 nanoparticle layers is 550 K Ohms, 350 K Ohms, 1.21 K Ohms, and 0.820 K Ohms, respectively.As the gas sensing mechanism is based on the interaction of sensing gas with Ag, use of Ag nanoparticles is advantageous as it provides a large surface area for interaction with the sensing gas.Size of In 2 O 3 nanoparticles (13 nm) used in the present study falls in the range of Debye length values estimated for oxide semiconductors [10,11].The nanoparticle size used is thus quite optimum for high sensor response.Use of Ag and In 2 O 3 nanoparticles of approximately equal sizes is expected to result in homogenous mixing of two phases in composite nanoparticle samples.Our earlier studies have shown that particle size, structure, and the properties of the nanoparticles do not change at the operating temperature used in the present study [4,5] [13].The high value of sensor response and faster response observed in the present study in comparison to the reported values in various oxide semiconductors (especially, sensing response towards reducing gases) can thus be directly correlated with the electronic interaction, larger surface area due to nanoparticle nature of both the phases used in the present study.

Figure 1 :Figure 2 :Figure 3 :
Figure 1: (a) GAXRD spectrum of sample IAG15, XRD spectra of (b) Ag nanoparticles sample and (c) In 2 O 3 nanoparticles sample.All samples are annealed at 400 • C for 90 minutes.( * ) shows the peaks corresponding to the cubic Ag 2 O phase and (X) shows the peaks corresponding to cubic Ag phase.

Figure 7 :
Figure 7: Comparison of the ethanol-sensing behavior of In 2 O 3 and IAG15 nanoparticle layers for 1000 ppm ethanol at 400 • C.

Figure 8 :
Figure 8: A schematic diagram showing electronic interaction at (a) In 2 O 3 :Ag and (b) In 2 O 3 :Ag 2 O nanoparticle interfaces.In presence of ethanol, Ag 2 O converts to Ag, the electron-depletion surface layer of In 2 O 3 becomes smaller due to release of the electrons to In 2 O 3 , and thus the conductivity gets enhanced.In air, electron-depletion surface layer gets enhanced due to electron transfer to oxygen as well as to Ag 2 O from In 2 O 3 .

In 2 O
3 and In 2 O 3 :Ag nanoparticle layers having well-defined particle size and composition have been prepared using a two-step synthesis method.Optical absorption studies carried out on In 2 O 3 :Ag samples show the presence of In 2 O 3 and Ag 2 O phases in air-annealed samples.In vacuumannealed samples the presence of surface plasmon resonance peak shows the conversion of Ag 2 O to Ag phase.These results support the proposed mechanism based on the conversion of In 2 O 3 :Ag 2 O interface to In 2 O 3 :Ag interface on exposure to reducing gases and explain the enhance gas sensing properties of composite nanoparticle layers.
. Papadopoulos et al. have reported a sensor response of 33 in In 2 O 3 films for 26 ppm ethanol at 450 • C [12].Shukla et al. have reported a sensor response value of 16 for 1000 ppm H 2 at 50 • C [1].Chung et al. have reported sensor response of 88 for 2000 ppm ethanol at 350 • C