Effect of Temperature and Nanoparticle Size on Sensor Properties of

Article pubs.acs.org/JPCC

Effect of Temperature and Nanoparticle Size on Sensor Properties of Nanostructured Tin Dioxide Films Mortko A. Kozhushner,†,§ Leonid I. Trakhtenberg,†,§ Valeria L. Bodneva,† Tatyana V. Belisheva,† Aaron C. Landerville,‡ and Ivan I. Oleynik*,‡ †

Semenov Institute of Chemical Physics RAS, 4 Kosygin St., Moscow 119991, Russia University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620-5700, United States § Institute of Physics and Technology (State University), Institutskii per. 9, Dolgoprudny, Moscow Region 141700, Russia ‡

ABSTRACT: The previously developed theory of sensor response is validated by detailed comparison with experiment. For this purpose, new experiments are performed to investigate the sensitivity of semiconductor tin dioxide (SnO2) nanostructured thin films with average nanoparticle diameter ≈120 nm as a function of temperature and concentration of analyte hydrogen gas. Concurrently, the sensor properties are calculated at experimental conditions by taking into account the increase of surface chemical reactions with temperature as well as subsequent dominance of desorption over adsorption processes at high temperatures. Comparative analysis of temperature and nanoparticle size dependence of sensor response is also performed using experimental data from other groups. Qualitative agreement between experiment and theory is achieved.

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INTRODUCTION Chemical sensors based on nanostructured semiconductor oxide films are widely used for detecting various gas-phase analytes in air.1−8 Although substantial efforts are undertaken to study the sensor response,9,2−17 a unifying description of major physicochemical processes taking place within the sensing layer is lacking. To address this challenge, a theory of sensor response of nanostructured semiconductor films to reducing analyte gases (H2, CO) has been recently developed based on the chemical kinetics of surface chemical reactions and statistical mechanics calculations of equilibrium concentrations of conduction electrons.18 By solving the stationary system of equations for chemical kinetics, the dependence of sensor response on average nanoparticle size, analyte concentration, and temperature has been predicted. The sensor response by semiconductor nanoparticles involves a change in the conductance of the film upon adsorption of a gas phase analyte on the surface of oxide nanoparticles. The flow of electrons between individual nanoparticles is determined by the concentration of conduction electrons nc, which in turn depends on the temperature and the concentration of dopant impurities. The dopants donate or capture electrons to/from the conduction band. In SnO2 films the oxygen vacancies created during the materials processing serve as donor impurities.19 The distance between the bottom of the conduction band and the average electron energy in the oxygen vacancy is relatively small, ∼0.7 eV; therefore, an appreciable fraction of the vacancies are thermally ionized at the sensor’s operating temperature ∼400 °C, which adds a substantial amount of the electrons to the conduction band. An additional factor, the adsorption of considerable amounts of oxygen molecules, O2, appears upon exposure of the nanostructured SnO2 film to air. Thanks to the extended © 2014 American Chemical Society

surface area of the nanostructured material, the equilibrium concentration of the oxygen can exceed the concentration of the oxygen vacancies in the bulk of the material. The SnO2 nanoparticles catalyze oxygen dissociation reactions, producing oxygen atoms, which serve as effective traps for the electrons.5,18 des Oad 2 ⇌ O2

(1)

ad Oad 2 → 2O

(2)

Oad + e− ⇌ O(−)ad

(3)

Equations 1, 2, and 3 depict the oxygen adsorption/desorption, dissociation, and capture/release of an electron by O atom/ion, respectively, and subscripts “ad” and “des” correspond to adsorbed and desorbed species, respectively. As a result, the electron concentration in the conduction band drops substantially upon exposure to air, causing a decrease in sensor conductivity. The addition of a reducing gas H2 to air causes adsorption and subsequent dissociation of the analyte molecules, followed by the reactions with atomic oxygen at the nanoparticle surface, as shown by (−)ad H ad → H 2Odes + e− 2 + O

(4)

the product H2O being released to the gas phase. The electrons previously trapped by oxygen atoms are transferred back to the conduction band, resulting in an increase in the sensor conductivity, which is measured as a sensor effect. Also, there Received: February 25, 2014 Revised: May 2, 2014 Published: May 4, 2014 11440

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are other mechanisms of sensor response operational, for example, in metal−polymer composites.20−22 However, as in the case of oxide semiconductor oxide nanocomposites, the response is related to the increase in electron concentration. The goal of this work is to validate the theory of sensor response, developed recently by the authors,18 by comparing the theoretical predictions with experimental measurements. For this purpose, new experiments have been performed to measure the sensitivity of semiconductor SnO2 nanostructured films, with average nanoparticle diameter ≈120 nm, as a function of temperature and concentration of analyte hydrogen gas. These dependences are also calculated using the theory presented in ref 18 under conditions matching experiment and compared with experimental data obtained in this work and by other groups.9,23

Figure 1. Experimental temperature dependence of SnO2 sensor sensitivity to H2 analyte gas at concentrations 106, 212, 550, and 1100 ppm. The average size of SnO2 nanoparticles is d = 120 nm.

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COMPARISON OF THEORETICAL AND EXPERIMENTAL DATA The theory of sensor response developed in ref 18 is validated by calculating sensor sensitivity at conditions of specific experiments and comparing with experimental data presented in this paper as well as with data obtained by other groups.9,23 The theoretical sensor sensitivity is related to the equilibrium concentration of electrons in the conduction band nc(PH2,T) at temperature T and concentration of H2 analyte, PH2:

EXPERIMENTAL RESULTS The tin dioxide nanoparticle films are prepared using a commercial nanopowder of tin dioxide (99.9% of SnO2, Sigma-Aldrich Chemical Co., Gillingham, UK) with average particle size ∼70−80 nm. A water suspension of nanopowder is deposited on an insulating Al2O3 substrate with dimensions 1.5 mm × 1.5 mm × 0.3 mm, containing specially fabricated platinum contacts for electrical measurement of the sensor effect, as well as a platinum heater on the back of the substrate. After template printing of the water suspension containing 50% of SnO2 nanopowder, the sample is heated at 120 °C for 3 h followed by annealing at 500 °C while monitoring the resistance of the sample. During the annealing, the resistance of the sample is increased from 300 kΩ to 1000 kΩ, the annealing being stopped when resistance ceases to change. Scanning electron microscopy is employed for structural characterization of the film. The film thickness is measured to be ∼3 μm, while the size of SnO2 nanoparticles is within the broad range 70−200 nm, the average diameter being ∼120 nm. The sensor effect, measured as the change in conductivity of the film, is investigated in the interval of temperatures 330−470 °C and for four concentrations of H2 analyte, 106, 212, 550, and 1100 ppm. The sensor is placed in a chamber with constant 200 cm3/min flow of pure air or air−analyte mixture. The sensor temperature is maintained with an accuracy of ±1 °C. The sensor sensitivity Θ is determined as the ratio of sensor resistance in pure air R0 and that with analyte H2 gas added, R(PH2): Θ(PH2,T) = R0(T)/R(PH2,T). The results of measurements are averaged using five different samples, the maximum relative error being less than 5%. The temperature dependence of the sensor sensitivity Θ(PH2,T) of the nanostructured SnO2 film is shown in Figure 1 at several concentrations of H2 analyte in air. The results display a characteristic feature of sensor response  the maximum of sensitivity Θmax achieved at temperature Tmax. The maximum sensitivity is due to the competition of absorption and desorption of analyte gas molecules.23,24 Although the rate of every chemical reaction involved in sensor response depends exponentially with temperature, desorption becomes dominant at high temperatures, resulting in the decrease of the equilibrium concentration of surface hydrogen. For the specific concentration 1100 ppm of H2 analyte gas, the maximum of sensor response Θmax ≈ 90 is achieved at temperature Tmax = 703 K.

Θ(PH2 , T ) =

nc(PH2 , T ) nc(0, T )

(5)

where nc(0,T) is the electron concentration in the absence of the analyte. The electron concentration nc is obtained by solving equations that are derived by minimizing the free energy of the system. The resulting expression contains the concentration of adsorbed oxygen atoms and atomic ions O− as parameters, which in turn depend on PH2 and T. These concentrations of O and O− are obtained by solving the system of kinetic equations for reactions 1−4. Finally, the sensor sensitivity Θ, eq 5, is obtained as a function of concentration of analyte, temperature, and the effective diameter of the nanoparticles d,18 the latter being related to the specific area of the film, which is inversely proportional to d. Using the approach outlined above, the sensor sensitivity of the SnO2 nanoparticle film is calculated as a function of temperature at experimental concentrations of H2 analyte gas  106, 212, 550, and 1100 ppm  as well as the experimental average size of SnO2 nanoparticles, d = 120 nm (Figure 2). As in experiment, such a wide range of analyte concentrations allows the investigation of both the low concentration regime, at which the sensor sensitivity is relatively small (∼1), and large concentrations close to saturation, i.e., when the sensitivity is almost constant upon further increase of concentration of the analyte. As in ref 18, the parameters of the theoretical model, which specify the rate constants of the chemical processes contributing to the sensor effect, are uniquely determined by using only one experimental point: the maximum sensitivity at the concentration 1100 ppm: (Θmax, Tmax) = (90, 703 K). The following parameters are obtained: binding energies of an electron to a donor vacancy εd = 0.75 eV and to a surface oxygen atom εO = 0.82 eV, dissociation energy of the O2 molecular adsorbate Δ = 1.32 eV, the O2 binding energy to the SnO2 surface εO2 = 0.86 eV, and the activation energy of O surface diffusion ε aO = 0.2 eV. The limiting surface 11441

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Figure 2. Theoretical temperature dependence of SnO2 sensor sensitivity to H2 analyte gas at concentrations 106, 212, 550, and 1100 ppm. The average size of SnO2 nanoparticles is d = 120 nm. Two lim −13 cm−2 and Nlim sets of Nlim O2 parameter are used: NO2 = 1.0 × 10 O2 = 4.0 −13 −2 × 10 cm (see text).

Figure 3. Experimental temperature dependence of SnO2 sensor sensitivity to H2 analyte gas at concentrations 100, 500, 2500, and 10000 ppm; data taken from ref 23. The average size of SnO2 nanoparticles is 500 nm.

13 concentrations of O2 and H2 are found to be Nlim O2 = 4.0 × 10 −2 lim 14 −2 cm and NH2 = 1.5 × 10 cm , respectively, with corresponding sticking probabilities αO2 = 0.001 and αH2 = 0.4. All other curves in Figure 2 are obtained with the same set of parameters within the model. By comparing Figures 1 and 2, it is clearly seen that good quantitative agreement between theory and experiment is achieved at high concentrations of H2; however, for smaller concentrations, 212 and 106 ppm, only a qualitative agreement is obtained. The general feature of both experimental and theoretical sensitivity curves Θ(PH2,T) is that the maximum of Θ(T) is shifted to the left (smaller Tmax) as the concentration of analyte H2 is reduced. A better agreement of theory with experiment at low concentrations of H2 is achieved if parameter Nlim O2 is reduced to 1.0 × 1013 cm−2. Such apparent dependence of Nlim O2 on analyte concentration can be qualitatively explained by a substantial dispersion in the size of the SnO2 nanoparticles: as nanoparticle size increases, its surface becomes smoother and less defective, resulting in an effective reduction of O2 adsorption from air and, correspondingly, in a reduction in the surface concentration of O2. At smaller concentrations of analyte gas, H2 molecules are adsorbed primarily at nanoparticles possessing larger surface area, which are less effective in the absorption of O 2 molecules. Therefore, the sensor effect at low H 2 concentrations is better described by system kinetic equations18 with a smaller value of Nlim O2 . We also analyzed available experimental data obtained by other groups. For example, ref 23 reports similar data on the temperature dependence of sensor response with a substantially larger average size of SnO2 nanoparticles, 500 nm, and at different concentrations of H2 analyte gas, 100, 500, 2500 and 10 000 ppm (see Figure 3). Again, a qualitative agreement between experiment (Figure 3) and theory (Figure 4) is apparent. The parameters for this case are εd = 0.7 eV, εO = 0.85 eV, Δ = 1.27 eV, εO2 = 0.77 eV, εOa = 0.15 eV, Nlim O2 = 8.0 × 13 −2 lim 14 −2 10 cm , NH2 = 2.0 × 10 cm , αO2 = 0.0052, and αH2 = 0.22. As reported in ref 18, the temperature dependence of sensor response is highly sensitive to the values of the parameters for the model, which allows the determination of an almost unique parameter set through fitting of the experimental Θmax and Tmax values at a single value of analyte concentration. However, the energy and kinetic parameters of the model vary from sample

Figure 4. Theoretical temperature dependence of SnO2 sensor sensitivity to H2 analyte gas at concentrations 100, 500, 2500, and 10000 ppm. The average size of SnO2 nanoparticles is d = 500 nm.

to sample. For example, the energy parameters εd and εO, which are the properties of bulk and surface SnO2 nanoparticles, change insignificantly; whereas the kinetic parameters Nlim O2 , lim NH2 , αO2, and αH2 are strongly dependent on the film’s synthesis and processing conditions. This observation, i.e., sample dependence of sensor parameters, must always be taken into account while comparing results obtained by different groups. In addition to temperature and concentration dependencies, there are experimental data9 concerning the dependence of the sensor response on the size of SnO2 nanoparticles (see Figure 5). According to ref 18, the specific area of the sensor is inversely proportional to the effective (average) diameter of

Figure 5. Experimental maximum sensor sensitivity to H2 analyte gas as a function of average size of SnO2 nanoparticles; data taken from ref 9. 11442

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well as reactions of the CO molecules with O− ions, should be used in the system of kinetic equations.

SnO2 nanoparticles, The larger the area, the more atomic oxygen is available for effective capture of the electrons and, consequently, the larger the sensor response. Thus, the theory predicts the increase in sensor sensitivity with decreasing nanoparticle size in excellent agreement with experiment. This observation is confirmed by comparing the experimental (Figure 5) and theoretical (Figure 6) dependencies of the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.I.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grant CMMI-1030715) and the Russian Foundation for Basic Research (Grants 13-03-00447 and 13-07-00141).

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REFERENCES

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Figure 6. Theoretical maximum sensor sensitivity to H2 analyte gas as a function of average size of SnO2 nanoparticles calculated under conditions of experiment.9

maximum sensor sensitivity Θmax on the average (effective) size of SnO2 nanoparticles. The theoretical sensitivity is calculated by determining the parameters of the model for each nanoparticle diameter. It is found that the O2 sticking coefficient αO2 decreases monotonically from 0.0052 to 0.0005 upon increase of nanoparticle diameter from 5 to 27.5 nm, whereas the H2 sticking coefficient αH2 displays nonmonotonic change from 0.44 to 0.2 while passing the minimum 0.09 at nanoparticle diameter 10 nm. Such behavior of kinetic parameters is not surprising as the morphology of the nanoparticle surface is size dependent. However, the energy parameters used to calculate sensor response displayed in Figure 6 barely change, thus reflecting the fact that the measurements displayed in Figure 5 were performed using the same batch of oxide films.

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CONCLUSIONS The validation of the theory of sensor response18 is performed by detailed comparison of experimental and calculated sensor characteristics of SnO2 nanoparticle film. For this purpose, new experiments are conducted to obtain temperature and concentration dependence of sensor sensitivity to analyte H2 by films with average SnO2 nanoparticle size ∼120 nm. The sensor response is also simulated under experimental conditions and compared with experiment. Both theory and experiment display a sharp maximum in the temperature dependence of sensor sensitivity, which is due to the temperature-dependent increase of adsorption and desorption processes as well as their competition at different temperatures. In addition to experiments presented in the paper, detailed comparison with experimental data from other groups9,23 is performed, which demonstrates good agreement between experiment and the theory across the samples obtained at different synthesis and processing conditions. The developed theory of the sensor effect is applicable to other reducing analytes such as CO. To do so, corresponding constants for the chemical reactions, such as CO adsorption and desorption, as 11443

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