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Sensor Effect in Oxide Films with Large Concentration of Conduction Electrons Mortko A. Kozhushner, Valeria L. Bodneva, Tatyana V. Belysheva, Mikhail I. Ikim, Ivan I. Oleynik, and Leonid I. Trakhtenberg J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10956 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 20, 2017

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The Journal of Physical Chemistry

Sensor Effect in Oxide Films with Large Concentration of Conduction Electrons M. A. Kozhushner,1,2 V. L. Bodneva,1 I. I. Oleynik,3 T. V. Belysheva,1 M. I. Ikim,1 L. I. Trakhtenberg1,2 1

Semenov Institute of Chemical Physics of RAS, 4 Kosygin Street, Moscow 119991, Russia

2

Moscow Institute of Physics and Technology (State University), 9 Institutskii Lane, Dolgoprudny, Moscow Region 141700, Russia 3

University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620-5700, United States

Keywords: nanoparticle, sensor response, semiconductor, conductivity, reduced gas, sensing layer ABSTRACT. This paper presents joint experimental and theoretical investigation of sensor response of nanostructured In2O3 semiconductor thin films containing a large concentration of conduction electrons. The capture of the conduction electrons by oxygen adsorbates from air causes redistribution of the electrons inside the nanoparticles, resulting in reduction of the subsurface electron density, and the drop of the conductivity of nanoparticle thin films. When СО and Н2 reduced gas analytes are introduced to the system, their reaction with previously adsorbed negative atomic oxygen ions O! releases electrons back to the nanoparticles, producing a noticeable increase of thin film conductivity, which constitutes the sensor effect. This work presents kinetic model of such processes, which allows to a quantitative description of the sensor effect including dependence of sensor sensitivity on temperature. Concurrently, experiments are performed to quantify the sensor response by nanostructured In2O3 thin film as a function of temperature and hydrogen concentration upon addition of hydrogen gas to the gas medium. The measured response is described well by the theoretical model developed in this work.

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INTRODUCTION

Sensing of reduced gas analytes such as carbon monoxide (CO) and hydrogen (Н2) is one the important environmental and technological problems requiring development of novel sensor materials.1-5 Most of them employ nanostructured metal oxide thin films of SnO2, In2O3, CeO2 to mention a few.2,6-10 In spite of a substantial effort in the field, the understanding of fundamentals of sensing response is lacking which negatively affects the development of optimal sensing materials. The scientific basis of sensor technology should include an adequate description of important physico-chemical and electro-physical processes contributing to the sensor response. The basic mechanisms of the sensor effect are well-known.5,11-14 The metal-oxide sensor materials are semiconductors containing oxygen vacancies, which are donors of electrons. The electrons supplied to the conduction band upon donor ionization are the major charge carriers in nanostructured semiconductor films. At their surface the oxygen molecules from air dissociate and effectively capture a large fraction of the electrons from the conduction band. As a result, the thin film conductivity is substantially reduced due to reduction of conduction electron concentration as well as the appearance of negatively-charged layer preventing transfer of electrons between neighboring nanoparticles. When reduced gas analytes are added to the environment, their molecules react with adsorbed oxygen at the nanoparticle surface, e.g. Н2 + О- → Н2О + е-, СО + О- → СО2 + е-, resulting in desorption of the reaction products and release of electrons back to the volume of the nanoparticles. The semiconductor nanoparticle sensor materials can be divided into two broad classes based on concentration of the conduction of electrons. The first class includes materials with low density of the conduction electrons, containing a few electrons (~1 e) per particle. Under such conditions, fixed surface charges do not influence the flow of electrons between the nanoparticles and the conductivity variation follow the variation of the conduction electron density. A specific example of first class sensor material, SnO2 nanoparticle thin film, has been considered in Refs.14,15, where theory of sensor response has been developed and applied to obtain the dependence of sensor sensitivity on temperature, concentration of hydrogen analyte, and average nanoparticle diameter in a good agreement with experiment.


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The materials of the second class are those with high concentration of conduction electrons, typical example being In2O3. In this case, a substantial concentration of negatively charged oxygen anions is present at the surface of nanoparticles. For example, a nanoparticle with the diameter d = 100 nm contains (π/6)d3nc ~ 104 electrons, a substantial fraction of them is captured by the oxygen atoms at its surface. It is worth mentioning that because the sensor sensitivity is depended on the ratio of conductivities rather than absolute values of conductivities, the sensitivity of sensors of the first class is usually higher than that of second class. In the case of sensors of second class, sensor sensitivity is critically dependent on the distribution of conduction electrons inside nanoparticles, which by itself is a complex problem. Previous attempts were inconsistent3-6 as they used several ad hoc assumptions. For example, several previous publications consider idealized planar boundary between nanoparticles with surface electrons decoupled from their bulk counterparts.3-6 Such assumption is contradictory to experimental observations: the concentration of the electrons at the surface traps does depend on the total concentration of the bulk electrons. In addition, it was also assumed that the density of positively charged ionized donors is constant. This condition is valid only if the donors are fully ionized, which is not the case for most of the oxides as their ionization energy εd > kT. Moreover, the electron distribution in spherical semiconductor nanoparticles was obtained by using arbitrary concentration of the surface charges and assuming that the positive charges are fixed and distributed uniformly over the volume of nanoparticles.19,20 As was shown,21 such unphysical constraints produce substantial errors in concentration of the subsurface electrons. More importantly, the uniformly distributed surface charge of substantial concentration does not influence the distribution of the conduction electrons inside the nanoparticles as their electric field is zero according to Gauss law. To overcome the limitations of previous investigations, we have developed a selfconsistent theory of inhomogeneous spatial distribution of charge carriers within semiconductor oxide nanoparticles (using In2O3 as an example), which is based on minimization of free energy of system consisting of conduction electrons, oxygen donor vacancies in the bulk and oxygen adsorbates at the surface21. Once the problem of the charge distribution is solved, the comprehensive theory of sensor response of sensor materials of the second class can be established, which is the main goal of this work. We perform detailed comprehensive experimental and theoretical investigation of In2O3 sensor sensitivity at various temperatures and concentrations of H2 analyte to validate the fundamental mechanisms responsible for sensor properties by comparing theoretically predicted temperature and 3 ACS Paragon Plus Environment


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concentration dependence of sensor sensitivity with that obtained in experiment. MEASUREMENTS OF SENSOR RESPONSE IN NANOSTRUCTURED In2O3 THIN FILMS

The nanostructured thin films of In2O3 are produced by using nanopowder (AnalaR grade, 99.5%, BDH/Merck Ltd., Lutterworth, Leicestershire, UK) with average nanoparticle diameter of ~70 nm. The water suspension containing 50% by weight of In2O3 nanoparticles is deposited on aluminum oxide dielectric substrate with dimensions 1.5×1.5×0.3 mm3 containing platinum electrodes and platinum heater attached to the back surface of the substrate. The system is dried for 3 hours at 120 °С followed by annealing at 550 °С. The sensor is kept at this temperature until film electrical resistance stopped changing. The structural characteristics of the films were determined using X-ray diffraction characterization, as well as scanning electron, atomic force, and scanning tunneling microscopies. The SEM imaging of the resulting film shown in Fig. 1 confirms that it contains the nanoparticles with the same dimensions as those in the original nanopowder. The thickness of the film measured by SEM is ~1-1.5 𝜇𝑚.

Figure 1. The SEM image of nanostructured In2O3thin film.

Our samples consists of polycrystalline nanoparticles with their surface contain different crystalline orientations of the bulk crystal. Therefore, it is reasonable to assume that 4 ACS Paragon Plus Environment

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the distribution of adsorbed molecules and atoms (О2, О, Н2, Н) is uniform. We also focus on stationary sensor response. In this case, the average pore size between the nanoparticles can be neglected. Nonzero pore size affects only the diffusion of the gas molecules from the surface to the interior of nanoparticles. The latter contributes to the kinetics of the response rather than the stationary values of the charge distribution, which is used to determine the sensor response. The stationary sensor response of In2O3 thin film deposited on dielectric substrate is measured by placing the sample in a special chamber with a constant 200 ml/min flow of cleaned air or air containing a specific concentration of H2 gas. The temperature of the sensor is controlled by the heater on the back of the substrate with accuracy ±1 °С. Sensor response is defined as the ratio of the resistance of the sensor film in air to that with added H2 analyte gas. Five thin film samples were used to insure reproducibility of the results, the maximum relative error in measured resistance being 10%. The dependence of measured sensor response on H2 concentration and temperature is shown in Fig. 2. The increase of H2 concentration is accompanied naturally by the increase of

Figure 2. Experimental temperature dependence of nanostructured In2O3films sensitivity at various concentrations of Н2.

sensor response. The temperature dependence of sensor response to H2 displays a maximum, which is typical observation in case of conductometric sensors. Importantly, the maximum in sensor sensitivity shifts to larger temperatures upon increase of H2 concentration. THEORY/MODELING OF NANOSRUCTURED In2O3 SENSOR RESPONSE 5 ACS Paragon Plus Environment


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In this section, the method of sensor sensitivity calculation is proposed. The sensitivity depends on the oxygen atoms and ions density on the nanoparticles surface. The stationary surface density of O-is found based on the equilibrium adsorption-desorption processes and chemical reactions between the adsorbed molecules, atoms and ions. Subsurface charge density in semiconductor nanoparticles. The method for calculation of equilibrium charge distribution inside the semiconductor nanoparticle was developed in Ref 21. It requires solution of system of equations obtained by minimizing Gibbs free energy F of the system of charged nanoparticles. The contributions to F include the free energies of electron gas and ionized donor positive charges, the potential energy of the Coulomb interaction between all positive and negative charges in the system as well as the free energy of electrons captured by oxygen atoms at the surface of nanoparticles. The minimization of the free energy functional F over unknown functions – radial distribution of the conduction electrons nc(r), positive charges on ionized donors as well as unknown density of oxygen surface traps of electrons produces the set of equations which allows us to obtain nc(r),21 the key input for calculation of the sensor sensitivity. The equations are solved under assumption the donor centers, such as oxygen vacancies and electron traps are uniformly distributed over the volume and the surface of nanoparticle respectively. It turned out that surface concentration of O atoms is stationary during adsorption and desorption of molecules of analyte gas as well as other reactions at the surface of the nanoparticles. Its values for the cases with and without H2 analyte are determined by solving the system of kinetic equations and then subsequently used to obtain the radial distributions of the positive and negative charges inside the nanoparticle. The electric current in nanostructured thin film flows through the contacts between neighboring nanoparticles. Therefore, it's the contact conductivity that determines the conductance of the sensor. The two possible mechanisms of charge transfer from one nanoparticle to another are the regular conductivity and tunneling electron transfer between the oxygen cations and neutral atoms. Due to a large distance (~3 nm) between O- cations and O atoms, the tunneling electron transfer between O atoms is neglected in the calculations. The external electric field is much smaller than that inside the nanoparticles for typical voltages applied to the sensor film. Therefore, the electric current can be calculated by using equilibrium distribution of charges inside the neighboring nanoparticles, the chemical potential being shifted by the potential difference between them. The current flowing from one 6 ACS Paragon Plus Environment

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nanoparticle to another is proportional to the potential difference and the equilibrium subsurface concentration of the conduction electrons (𝑛! 𝑅; 𝑇, 0 𝑎𝑛𝑑 𝑛! (𝑅; 𝑇, 𝑃!! )) in the nanoparticle of radius R. Then, the sensor sensitivity Θ(𝑇) at temperature T is defined as the ratio of film conductivity in air with hydrogen gas at pressure 𝑃!! to the conductivity without it Θ(𝑇) =

!! (!; !,!!! ) !! (!; !,!)

.

(1)

It is worth noting that the surface concentration of the adsorbed oxygen atoms nO = NO/4πR2, where NO being the number of O atoms at the surface, is the major parameter determining the distribution of charges in the nanoparticle, as well as the nanoparticle conductance. In fact, the oxygen atoms capture an appreciable fraction of the conduction electrons resulting in redistribution of the electrons and ions inside the nanoparticle. As the conductivity is proportional to the concentration of subsurface electrons, there is a unique correspondence between the conductance and nO. Therefore, to determine the film sensitivityΘ(𝑇), it is necessary to obtain nO(0) and nO(𝑃!! ), surface concentrations of atomic oxygen with and without H2. Then, nc(R; T, 0) and nc(R; T, 𝑃!! ) are calculated using approach developed in Ref. 21, followed by the calculation of sensor sensitivity using expression (1) above. Adsorption and chemical reactions of oxygen and hydrogen molecules at the surface of nanoparticles. The steady-state equations for the equilibrium concentrations of adsorbed oxygen atoms nO(0) and molecular oxygen 𝑛!! (0) describing the dissociation and association upon adsorption and desrption of O2 molecules are 𝐾!"# 𝑛!! (0) − 𝐾!"# 𝑛! (0)

!

= 0,

𝐾!!" 1 − 𝑛!! /𝑛!!"# − 𝐾!!"# 𝑛!! − 𝐾!"# 𝑛!! + 𝐾!"# 𝑛! (0) ! ! !

(2) !

= 0,

(2а)

where 𝐾!"# is dissociation constant of adsorbed О2 molecules, 𝐾!"# is the recombination constant of adsorbed О atoms, 𝐾!!" и 𝐾!!"# are the adsorption and desorption rate constants of ! ! О2 molecules, 𝑛!!"# is the limiting concentration of the adsorbed oxygen molecules. The ! constants introduced above depend on temperature as follows 𝐾!"# = 𝜈!!! 𝑒𝑥𝑝 −

!!"#

,

(3)

𝐾!"# = 𝑎! 𝜈! exp − !"! ,

(4)

!" !!



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!

𝐾!!" = ! 𝑛!! !

!!" ! !!

𝛼!! ,

𝐾!!"# = 𝜈!! 𝑒𝑥𝑝 − !

!!"# !"

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(5) ,

(6)

where 𝜈!!! is the vibrational frequency, 𝜀!"# is the dissociation energy of O2 adsorbates, 𝑎 is an average length of О atom jumps at the surface, 𝜈! is the frequency of O atom vibrations in the local potential well at the surface of nanoparticle, 𝜀!! is the binding energy of O atom in this well, 𝑛!! is the concentration of O2 molecules in air, 𝑚!! is their mass, 𝛼!! is the adsorption coefficient of O2 molecules to the surface of the nanoparticles. To calculate 𝑛! 𝑃!! , is it necessary to add to eq. (2) the equations describing adsorption of H2 molecules and their chemical reactions with oxygen: O2 + 2H2 → 2H2O, O + H2 → H2O,

(7)

O- + H2 → H2O + e-. The net reactions in (7) take into account the reactions of oxygen with hydrogen atoms as H2 molecules quickly and irreversibly dissociate. These reactions are very exothermic, and H2O is quickly detached from the surface of the nanoparticles. Therefore, only direct reactions are taken into account in (7). The stationary concentrations for the case of H2 present in the air are obtained from solving equations similar to (2) and (2a), which take into account the chemical reactions with hydrogen. In addition, an equation for the stationary concentration of adsorbed hydrogen is added to the system of equations 𝐾!"# 𝑛!! − 𝐾!"# 𝑛! (𝑃! )

!

– 𝐾!" 𝑛! (𝑃! )𝑛!! = 0,

𝐾!!" 1 − 𝑛!! /𝑛!!"# − 𝐾!!"# 𝑛!! − 𝐾!"# 𝑛!! + 𝐾!"# 𝑛! (𝑃! ) ! ! !

!

− 2𝐾!" 𝑛!! 𝑛!! = 0,

!"# 𝐾!!"! 1 − 𝑛!! /𝑛! − 𝐾!" 𝑛! 𝑃! 𝑛!! − 2𝐾!" 𝑛!! 𝑛!! = 0. !

(8) (8а) (8b)

The equations (8), (8а) are similar to (2), (2а), with an additional term taking into account the disappearance of oxygen atoms and molecules in reaction with hydrogen, where 𝐾!" is the net constant of reactions (7). The constants of these reactions are limited by surface


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diffusion of H atoms described by the diffusion coefficient DH; therefore, the net constant KHO≈ DH. In equation (8b) 𝐾!!"! is the constant of hydrogen adsorption, the hydrogen desorption being neglected due to large binding energy of H atoms to the surface of the nanoparticles. The additional constants depend on temperature as follows !

𝐾!!"! = ! 𝑛!!

!!" ! !!

𝛼!! ,

(9)

where 𝑛!! is the density of H2 molecules in air, 𝑚!! is their mass, 𝛼!! is the adsorption coefficient of H2 molecules upon collision with the surface 𝐾!" =

! !! !

!

!

! = 𝑎! 𝜔𝑒𝑥𝑝 − !"! . !

(10)

Here 𝑎 is the average length of H atom jumps upon escape from local potential well at the surface, ω and εH are the its frequency and binding energy of H atoms in the potential well. The temperature-dependent concentrations of oxygen atoms in the absence (𝑛! (0; 𝑇)) and the presence (𝑛! (𝑃!! ; 𝑇)) of hydrogen can be found by solving system of equations (2)(2а) and (8)-(8b) respectively. Then, these concentrations are used as input to obtain the concentrations of the conduction electrons 𝑛! (𝑅; 𝑇, 0) and 𝑛! (𝑅; 𝑇, 𝑃!! ) by solving the system of equations obtained in Ref. 21 by minimizing total free energy F of interacting charges in the nanoparticle, followed by the calculation of the sensor sensitivity Θ(𝑇) using formula (1). Fig. 3 displays the radial distribution of the concentration 𝑛! (𝑟; 𝑇, 𝑃!! ) inside the nanoparticle of D = 70 nm at temperature Т = 590 K at three concentrations of H2 added to air: 0 ppm, 1100 ppm and 104 ppm. The curves are obtained using a unique set of parameters of the kinetic scheme, which is that same as the one used to calculate the sensor effect, see below. Solid curve displays a sharp inhomogeneity in the conduction electrons density, as it varies from



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Figure 3. Radial distribution of the conduction electron concentration 𝑛! (𝑟; 𝑇, 𝑃!! ) (in atomic units) at D = 70 nm, Т = 590 K at three concentrations of H2 in air.

nonzero value at the center to a value 3×10-10 a.u. close to zero at the surface of the nanoparticles. As follows from Fig. 3, the concentration of conduction electrons inside the nanoparticle increases with hydrogen concentration. It happens because H atoms react with adsorbed O- ions at the nanoparticle surface releasing the electrons to the bulk of nanoparticle and producing water molecules released to the gas phase. The higher the hydrogen concentration, the more electrons are released to the bulk of nanoparticles. SENSOR EFFECT To uniquely determine the distribution of conduction electrons inside In2O3, several parameters were obtained from experiment. In particular, the activation energies 𝜀! = 7.4×10-3; 𝜀! = 2×102

(here and further we use the atomic units), was determined by measuring the sensor

conductivity in vacuum and in air.22,23 However, several important parameters, controlling absorption and desorption of oxygen and hydrogen are unknown. They are obtained by comparing experimental and theoretical temperature dependencies of sensitivity Θ 𝑃!! ; 𝑇 and choosing the values that give the best agreement between experimental and theoretical sensitivities and corresponding temperatures at maxima of Θ 𝑇

curves for the case of

concentration of hydrogen 1100 ppm. The best agreement is achieved using the following parameters: 𝜈! = 4×10-3; 𝜈!!! = 7.5×10-3; 𝜈!! = 2×10-4; 𝑎 = 13; 𝛼!! = 1.3×10-4; 𝜀!"# = 5.2×10-2; !"# 𝜀!! = 3.08×10-2; 𝜀!"# = 3.27×10-2; 𝑛!!"# = 2.66×10-3; 𝑛! = 7×10-2; 𝑎`!!! 𝜔 = 1.69×10-7; 𝜀! = ! !


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1.56×10-2. The adsorption coefficient 𝛼!! was found to be temperature dependent within the range of experimental temperatures 250-350 °C: 𝛼!! = α + βT, where α = – 5.58×10-2, β = 1.3×10-4. This dependence is valid only in the above interval of temperatures, at smaller temperatures 𝛼!! becomes negative, which is unphysical. All the kinetic parameters determined in this work have values within physically reasonable bounds. The sensor response is most sensitive to the values of the adsorption coefficients, 𝛼!! и 𝛼!! , which are the two main parameters varied to find the best agreement between the theory and experiment. The calculated temperature dependence of sensor sensitivity for the system with average diameter of nanoparticles 𝐷 = 70 nm as in experimental samples shown in Fig. 1 at various concentrations of H2 are presented in Fig. 4.

Figure 4. Theoretical temperature dependence of nanostructured In2O3 film sensitivity at various concentrations of Н2. The agreement between theory and experiment, seen in Figs. 2 and 4, demonstrates the adequate description of major physico-chemical processes responsible for sensors properties of type two sensors, i.e. with large number of conduction electrons per nanoparticle as in In2O3 nanostructured thin films. It is worth noting that the theoretical sensor sensitivity Θ 𝑃!! ; 𝑇 is !"# the most sensitive to the following parameters: 𝑛!!"#! , 𝑛! , 𝛼!! . The dashed curve in the Fig. 4 !

corresponds to the highest concentration of H2 close to saturation. Such sensitivity saturation is reached at smaller hydrogen pressures in SnO2 sensor.14,15 In addition, a modest shift of maximum of the theoretical temperature dependence Θ 𝑃!! ; 𝑇 to the left, i.e. to the lower 𝑇! , upon decrease of H2 concentration is observed in agreement with experiment. Some deviation



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from experiment might be due to the dispersion of the diameters of nanoparticles around its average value, which is neglected in this model. CONCLUSSIONS The consistent theory of sensor effect is developed in this work by taking into account the fundamental physico-chemical processes taking place in semiconductor sensor nanomaterial of second type with large concentration of conduction electrons using specific samples of In2O3. The distinct feature of this work is uncovering the inhomogeneous distribution of the electrical charges inside the nanoparticles developing in response to addition of analyte gas H2 to air.The sensor response depends on concentration of H2 and temperature and it is measured using In2O3 nanostructured thin film with average diameter of nanoparticles is approximately 70 nm. As in sensors of the first type, such as SnO2,14,15 the sensor effect is due to the increase of the conduction electrons concentration upon removal of surface atomic oxygen electron traps upon their reactions with adsorbed hydrogen. The theoretical dependence of sensor sensitivity on the concentration of H2 and temperature is in good agreement with experiment, which validates the description of basic processes under physically reasonable choice of unknown parameters of the model. As the mechanism of sensor response to other reduced analytes (CO, ethanol) is the same, the approach developed in this paper can also be applied to other specific cases as well. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (I.I.O.). Notes The authors declare no competing ﬁnancial interest. ACKNOLEGMENTS This work was supported by the National Science Foundation (grant No. CMMI-1030715), the Russian Foundation of Basic Research (grant No 16-29-05138) and Defense Threat Reduction Agency (grant No. HDTRA1-12-1-0023).


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REFERENCES

(1) Gopel, W.; Schierbaum, K. SnO2 Sensors: Current and Future Prospects. Sens. Actuators B 1995, 26, 1-12. (2) Shimizu, Y.; Egashira, M. Basic Aspects and Challenges of Semiconductor Gas Sensors. MRS Bull. 1999, 24, 18-24. (3) Yamazoe, N. Towards Innovations of Gas Sensor Technology. Sens. Actuators B 2005, 108, 2-14. (4) Korotcenkov, G. Metal Oxides for Solid-State Gas Sensors: What Determines Our Choice? Mat. Sci. Eng. B 2007, 139, 1-23. (5) Yamazoe, N.; Shimanoe, K. New Perspectives in Gas Sensor Technology. Sens. Actuators B 2009, 138, 100-107. (6) Xu, C.; Tamaki, J.; Miura, N.; Yamazoe, N. Grain Size Effects on Gas Sensitivity of Porous SnO2-Based Elements. Sens. Actuators B 1991, 3, 147-155. (7) Williams, D. E. Semiconducting Oxides as Gas-Sensitive Resistors. Sens. Actuators B 1999, 57, 1-16. (8) Lu, F.; Liu, Y.; Dong, M.; Wang, X.P. Nanosized Tin Oxide as the Novel Material with Simultaneous Detection Towards CO, H2 and CH. Sens. Actuators B 2000, 66, 225-227. (9) Rothschild, A.; Komem, Y. The Effect of Grain Size on the Sensitivity of Nanocrystalline Metal-Oxide Gas Sensors. J. Appl. Phys. 2004, 95, 6374-6380. (10) Korotcenkov, G. The Role of Morphology and Crystallographic Structure of Metal Oxides in Response of Conductometric-Type Gas Sensors. Mat. Sci. Eng. R. 2008, 61, 1-39.



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Table of Contents: Graphic


Sensor Effect in Oxide Films with a Large ... - ACS Publications

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