- Email: [email protected]
In2O3 films deposited by spray pyrolysis: gas response to reducing (CO, H2) gases
Sensors and Actuators B 98 (2004) 122–129
In2 O3 films deposited by spray pyrolysis: gas response to reducing (CO, H2 ) gases G. Korotcenkov a,∗ , V. Brinzari a , A. Cerneavschi a , M. Ivanov a , A. Cornet b , J. Morante b , A. Cabot b , J. Arbiol b a
Laboratory of Microelectonics and Optoelectronics, Technical University of Moldova, Bld. Stefan cel Mare, 168 Chisinau 2004, Moldova b Dept. d’Electronica, Universitat de Barcelona, Barcelona, Spain Received 29 October 2002; received in revised form 28 January 2003; accepted 11 September 2003
Abstract This article discusses the influence of both technological modes and structural parameters of In2 O3 films deposited by spray pyrolysis from InCl3 water solution on response to reducing gases (CO and H2 ). Conclusions were drawn on the main factors, which influence both gas response of In2 O3 films and time constants of response and recovery processes. They are film thickness and porosity. It was established that films deposited at low pyrolysis temperatures (350–400 ◦ C) from low precursor concentration solutions (<0.2 M) had maximal gas response and minimal constant times of both response, and recovery processes. © 2003 Elsevier B.V. All rights reserved. Keywords: In2 O3 ; Spray pyrolysis; Gas response; Reducing gases
1. Introduction Recent research has shown that In2 O3 films have good response to oxidizing gases [1–8]. Therefore, highly sensitive detectors of O3 , NOx , Cl2, etc., can be manufactured on the basis of this material. In2 O3 sensors also detect reducing gases, such as CH4 and CO [9–13]. At that it was found that In2 O3 -based sensors may have a good selectivity to CO in the presence of H2 in surrounding atmosphere. According to Yamaura et al. [12,13] and our results the ratio of gas response (S) to CO and H2 (S(CO)/S(H2 )) may exceed 10 at the same concentrations of detected gases. It is important for design CO selective sensors, because for SnO2 the gas response to H2 is substantially higher than to CO [16], that creates certain difficulties in design of selective CO SnO2 -based gas sensors. This indicates that in the near future In2 O3 may become one of the base materials in the design of various gas sensors for several applications, along with such metal oxides as SnO2 and ZnO [14,15]. In this context the search for optimal technology for the deposition of In2 O3 films is particularly relevant. ∗ Corresponding author. Tel.: +34-373-2-235437; fax: +34-373-2-210432. E-mail addresses: [email protected], [email protected] (G. Korotcenkov).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.09.009
Earlier we showed that the spray technology we designed can be used successfully for deposition of SnO2 films with good gas-sensing parameters [17,18]. Moreover, the ability of this technology to affect various structural parameters of deposited films enabled us to analyze the influence of structural factors on the gas-sensing effects observed on SnO2 [19]. From the standpoint of the problem under discussion, determining the applicability of spray pyrolysis technology to In2 O3 deposition seems very important.
2. Experimental For deposition of In2 O3 films we used the technique of active spray pyrolysis. The principal scheme of apparatus used for this purpose was described in [17]. The aerosol phase was formed by pneumatic method. Purified air with pressure equaled 2 atm was used as gas carrier. Technological parameters, such as pyrolysis temperature, precursor concentration, and volume of sprayed solution, were used to control structural and electrophysical parameters of deposited films. Earlier we showed that these factors determine grain size (t), thickness (d), and porosity of deposited films [17,18]. InCl3 was used as precursor, and water as a solvent. Precursor concentration varied in the range 0.05–1.0 M. Spray pyrolysis was carried out at the temperature range 300–550 ◦ C.
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
123
Fig. 1. AFM images of In2 O3 films deposited by spray pyrolysis using different technological parameters: (a) Tpyr = 375 ◦ C; d ∼ 40–60 nm; (b) Tpyr = 510 ◦ C; d ∼ 40 nm; (c) Tpyr = 510 ◦ C; d ∼ 150 nm.
Structural parameters of deposited films were controlled using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques. The thickness of In2 O3 -deposited films was determined by laser ellipsometry. The main results of this research are presented in [3,20]. It was determined that In2 O3 films deposited by spray pyrolysis are nanocrystalline with grain sizes ranging from 10 to 80 nm. Typical AFM images of In2 O3 films deposited in different technological modes are shown in Fig. 1. Minimal grain sizes were observed in thin films (d < 40 nm) deposited at Tpyr ∼ 450–500 ◦ C. Moreover, In2 O3 films with a thickness of more than 40 nm, which were deposited at Tpyr > 400 ◦ C, are highly textured with predominant crystallite orientation in the (0 0 1) direction perpendicular to substrate. It is clearly shown in Fig. 2, where the XRD patterns of In2 O3 films deposited at low and high pyrolysis temperatures are presented. Gas-sensing properties were measured in a flow-type reactor with a cell of <0.2 cm3 . Film conductivity (G) and gas response (S) were measured in steady state and transient modes. Measurements were conducted in dry (∼1–2% RH) and wet (30–50% RH) atmospheres of two mixtures of air and detected gas ((0.5% CO + air) and (0.5% H2 + air)). Gas response was calculated as a ratio R(air)/R(gas), where R(air) and R(gas) are film resistance measured in air and in the atmosphere containing detected gas respectively. Studied samples had width equaled 2–3 mm, and distance between
Fig. 2. XRD patterns of In2 O3 films deposited at low (a) and high (b) pyrolysis temperatures (d ∼ 150–200 nm): (a) Tpyr = 375 ◦ C; (b) Tpyr = 510 ◦ C.
contacts equaled 5–7 mm. Time constants of response (τ res ) and recovery processes (τ rec ) were determined at a level of 0.9 from steady state magnitude of film conductivity, measured after changing the atmosphere in measurement cell.
3. Results and discussions 3.1. Electrical and gas-sensing properties of In2 O3 films The typical temperature dependencies of sheet resistance R(T) and gas response of In2 O3 films are shown in Figs. 3 and 4. Even without special doping, In2 O3 films have good gas response to CO, H2 , which is no worse than the gas response of undoped SnO2 films. Moreover, both the general shapes of R(T) and S(T) dependencies and the position of “extreme” points of these curves are very close to similar dependencies for SnO2 films [12,17,21]. This situation could give us some clue about the identity of the processes responsible for changing the electro-physical and gas-sensing properties of SnO2 and In2 O3 films. In particular, it could be assumed that in sympathy with the effects on SnO2 , gas chemisorption processes determine sensing characteristics of In2 O3 films. However, it was found that
Fig. 3. Influence of In2 O3 film thickness on temperature dependencies of film resistance: films deposited from high concentration InCl3 –water solutions (1.0 M); (1) d ∼ 400 nm; (2) d ∼ 200 nm; (3) d ∼ 30–50 nm; (4) d ∼ 10–15 nm. (RH = 35–45%).
124
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
Fig. 4. Temperature dependencies of In2 O3 gas response to 0.5% CO (3) and 0.5%H2 (1,2): (1) d ∼ 400 nm; (2, 3) d ∼ 30–50 nm (RH = 35–45%).
one part of the regularities established for SnO2 have another tendency of changing for In2 O3 films, and another one does not carry out at all. For example, we established that In2 O3 films with the same thickness as SnO2 film have a resistance 1–3 order of magnitude lower than SnO2 films. The change of In2 O3 film resistance, measured in both heating and cooling modes, is also smaller (Fig. 3). Moreover, normalized In2 O3 film resistance in air, presented in scale Rd = f (t) (Fig. 5, curve 2), does not depend on grain size (t). In polycrystalline materials the fact that resistance does not depend on grain size is typical for films, where conductivity is determined by bulk properties of the material. The combination of the above-mentioned parameters can be a reflection of small resistance of intergrain contacts. The latter can be caused by the low height of potential barriers on intergrain interfaces, or by the high surface concentration of ionized donors. For SnO2 films, where intergrain barriers play an important role in film conductivity limitation, sheet resistance sharply increases when grain size decreases [21,22]. For In2 O3 films, such behavior we observed only in an ozone atmosphere (Fig. 5, curves 3 and 4). Ozone is very strong oxidizing agent, and after interaction with this
Fig. 5. Sheet resistance of In2 O3 films, deposited from 1.0 M InCl3 –water solution versus on grain sizes, measured in the direction parallel to substrate: (1, 3) R = f (t), (2, 4) Rd = f (t); (1, 2)—atmosphere of usual air; (3, 4)—atmosphere containing ozone (∼1 ppm).
gas, resistance of In2 O3 films increases at Toper > 200 ◦ C more than 103 times [3]. At that if these samples to cool in atmosphere containing ozone the high resistance state of the films can remain long time even in atmosphere of usual air. Such behavior of In2 O3 film resistance confirms our assumption that low resistance of intergrain contacts is connected with partial reduction of the surface of In2 O3 grains. Besides, the maximum for gas response of undoped In2 O3 films to reducing gases (CO, H2 ) shows a shift in the region of higher temperatures in comparison with SnO2 films. For SnO2 film gas-sensing characteristics, measured in the heating mode, gas response to CO and H2 appears at Toper ∼ 200 ◦ C, which correspond to resistance minimum. For In2 O3 films gas sensitivity to CO and H2 appears only at Toper > 300–350 ◦ C (Fig. 4). Moreover, for In2 O3 films S(T) maximum lies at temperatures, which equaled 400–450 ◦ C independently from film structure. As we showed earlier in [18,19] for undoped SnO2 films, the position of S(T) maximum depends on film structure and this maximum can be observed in the range from 250 to 400 ◦ C. In comparison with SnO2 films, In2 O3 films present the more clearly shown acceptor-like behavior of reducing gases. At Toper ∼ 150–250 ◦ C during In2 O3 interaction with CO and H2 (0.5% + air) film resistance can increase more than 10–20 times (Fig. 4). For SnO2 films, such a clearly shown effect we observed only for SnO2 :Pd samples when interacting with H2 taken place. Acceptor-like behavior of reducing gases during interaction with n-type metal oxides is a very interesting effect. However, the temperature range of the demonstration of this effect lies below the operating temperature of In2 O3 sensor used for reducing gas detection. Therefore, in this article we will not discuss this problem. This effect will be analyzed in special article. Air humidity influence on In2 O3 gas-sensing characteristics is not so brightly shown as for SnO2 films as well [21]. As it is shown in Fig. 6, gas response of In2 O3 films deposited at high pyrolysis temperatures have very weak dependence on the air humidity. Moreover, the change of air humidity does not shift the position of the gas sensitivity
Fig. 6. Dependencies of gas response (1,2,3) and recovery time (4) on pyrolysis temperature: Toper = 370 ◦ C; (1) (0.5% CO + air); (2, 3) (0.5% H2 +air); (1, 2) wet gas atmosphere (35–45% RH); (3) dry gas atmosphere (1–3% RH).
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
maximum in S(T) dependence. Only in the region of low pyrolysis temperatures, In2 O3 films have appreciable dependence of gas-sensing properties on the humidity of surrounding atmosphere. The above indicates that gas-sensing effects on In2 O3 films are described by laws, which are different from those describing gas response in SnO2 films. At present two approaches can be used to explain gas-sensing properties of In2 O3 films. The surface-trap mechanism was used to describe In2 O3 interaction with oxidizing gases (NO2 , O3 , Cl2 ) (Toper < 300 ◦ C) [8], and the “redox” mechanism for interaction with reducing gases (CO, H2 ) (Toper > 350 ◦ C) [23,24]. According to the last model, the change in the valence state of lattice In (In3+ → In2+ ) takes place during In2 O3 interaction with reducing gases. Results of ESR measurements indicate that the concentration of F-centers and In2+ ions in In2 O3 lattice increases following treatment with CO and H2 [23]. This chemical interaction takes place with the participation of lattice oxygen. In this model, the role of adsorbed oxygen is both In2+ oxidation and annealing of structural defects, i.e. oxygen vacancies, formed during In2 O3 interaction with reducing gases [23]. In other words two different processes with their own energetic parameters are responsible for response and recovery during gas detection by In2 O3 . They are reduction and reoxidation. For example these reactions for CO detection can be written in the form: Olat 2− + COads → CO2 ↑ +VO + 2e− In3+ + e− → In2+ VO + e− → [F - center]
reduction (1)
O2ads + 2e− → 2Oads − In2+ − e− → In3+ reoxidation (2) [F - center] + Oads − → Olat 2− In2 O3 is known to be a partially reduced highly defective oxide. In accordance with [23,25], the electrical conductivity of In2 O3 at low temperatures is controlled by the effectiveness of electron exchange due to the overlapping of In3+ and In2+ . ESR measurements show that at T = 196 ◦ C uncoupled electrons in In2 O3 lattice are delocalized [23]. Therefore, the reduction process results in an increase in film conductivity, and the reoxidation process gives a decrease in this parameter. So, in such an approach, film conductivity is determined by the concentration of cations, which changed their valence state, and chemisorption is only an intermediate step in these chemical reactions. As we know, according to the chemisorption model, the change of film conductivity is connected with the change of concentration of free electrons captured by chemisorbed oxygen. Of course, this scheme is very simplified because it does not take into account the presence of water in different forms on the In2 O3 surface. It is well known that water can have a big surface concentration and can exert noticeable influence on both absolute value and kinetic parameters of gas sensitivity [21]. However, we agree that at high temperatures (Toper > 400 ◦ C) this “redox” mechanism can be dominant in the re-
125
action of reducing gas detection. For this temperature range we observe the following regularities, which are not characteristic of the chemisorption model: 1. In2 O3 has high enough gas response to reducing gases even for films with both low film resistance and big crystallite sizes; 2. Structural parameters of films, such as grain size and texturing, exert weak influence on absolute gas response to reducing gases. This conclusion follows from the results, which are shown in Fig. 6. Depending on Tpyr crystallite sizes in In2 O3 films can change more than three times. At that minimal crystallite sizes are observed in films deposited at Tpyr ∼ 450–475 ◦ C [3,20]. However, only for these Tpyr , when In2 O3 films have minimal crystallite sizes, we observed both the maximal values of τ rec and minimal values of gas response (Fig. 6). 3. Surface doping by Pd exerts weak influences on absolute gas response to CO and strong influences on τ rec . The appearance of Pd on In2 O3 surface makes τ rec decrease more than 10 times. However, we need to remember that the reduction and reoxidation processes described in (1) and (2) have specific energy parameters. As a result both time constants and Eact for these processes must differ. In real conditions this difference is not observed. As for CO–In2 O3 and for H2 –In2 O3 interactions in temperature range of donor-like reactions (Toper > 300 ◦ C) τrec ∼ τres , and Eact of τ = f (1/kT) dependencies have close values for both response and recovery processes (Fig. 7). It indicates that one and the same process is responsible for transient characteristics during response and recovery. At that this process cannot be connected with detected gas, because the magnitudes of indicated parameters Eact and τ are the same for CO and H2 detection. As we discussed earlier, the “redox” mechanism of gas response assumes that the reaction of gas detection is accompanied by generation and annealing of point structural defects in In2 O3 lattice. Therefore, for this gas-detection mechanism, the role of bulk diffusion processes must in-
Fig. 7. The influence of In2 O3 film structure on the temperature dependencies of response and recovery times (RH ∼ 35–45%): (1) 1.0 M InCl3 solution, d ∼ 400 nm; (2, 3) 0.2 M InCl3 solution, d ∼ 40 nm.
126
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
Fig. 8. The influence of film thickness on the response and recovery times of In2 O3 films deposited from low concentration (1, 2) and high concentration (3, 4) InCl3 –water solutions (RH = 35–45%, Toper = 370 ◦ C): (1, 3) recovery; (2, 4) response.
Fig. 9. Time constants of In2 O3 gas response to 0.5%CO versus on film refraction index: (A, B)—In2 O3 films deposited from 0.2 M and 1.0 M InCl3 solutions correspondingly. (1, 2) Toper = 370 ◦ C; (3) Toper = 270 ◦ C; (1) d ∼ 40–60 nm; (2, 3) d ∼ 400 nm.
crease considerably, especially when point structural defects have high enough mobility. The reduction and reoxidation processes give the same diffusion flows, which move in opposite directions i.e. bulk diffusion of oxygen or indium in In2 O3 grains could explain the coincidence of Eact and time constants for response and recovery processes. However, as we know bulk diffusion is a grain size dependent process. Therefore for processes, controlled by bulk diffusion, we must have the time constants with specific dependencies on crystallite sizes (τdif ∼ t 2 /D, where D is the coefficient of diffusion). However, for reactions of CO and H2 detection we did not find such inter-correlation between response time and grain size. For example, for In2 O3 samples deposited from solutions with low precursor concentration grain size increases from 10 to 80 nm, when film thickness increases from 20 to 400 nm. Time constants of gas response measured for these samples do not change yet (Fig. 8). So, we can assume that, in general, for In2 O3 reaction with reducing gases bulk diffusion does not determine response and recovery times. At the same time the absence of τ = f (t) dependence is not the evidence that diffusion processes do not exert influence on gas-sensing characteristics of In2 O3 films at all. As we have shown earlier for porous In2 O3 films deposited at low pyrolysis temperatures with no τ = f (d, t) dependence we observed both some difference in τ res and τ rec (Figs. 7 and 8), and a noticeable influence of air humidity on both S and τ (Fig. 6). These are really typical for processes controlled by surface reactions. For “dense” films with big thickness (d > 100 nm) we have other situation. In this case τres ∼ τrec , and neither S nor τ depend on air humidity. Moreover, τ has a strong dependence on film thickness (Fig. 8). Such combination of dependencies corresponds better to diffusion type processes [26,27]. For the above-mentioned films, it was also established that film density is one of the main factors, which controls the time constants of gas response. During our experiments this parameter was checked by determining refraction index (n). For this purpose we used laser ellipsomentry. It is known
the greater the film density, the bigger the refraction index. Interrelations between τ and n, which were determined for both film thickness, are shown in Fig. 9. The influence of refraction index on response and recovery times can be understood in the framework of the “diffusion” approach, which was used to explain gas response characteristics of thick ceramic sensors [26,27]. This approach assumes that if the film is denser, the intergrain distance is smaller. In this case the diffusion of both detected gas and products of gas detection reaction into the film is slower. As a result the time constant of gas response of thick dense film is bigger in comparison with porous film. As it was shown in [3,20], In2 O3 are highly textured films with cubic structure of the crystallites. These films look like columns. In certain conditions, in such highly textured columnar films, the crystallites can be compactly packed together without pores between them. Therefore, in thick In2 O3 films, deposited from a high concentration InCl3 solution the response time really can be controlled by diffusion processes. However, under the “diffusion” process, which limit τ res and τ rec , we probably need to understand intergrain diffusion, but not bulk diffusion. The change of the nature of the processes controlling the kinetics of gas response in porous and dense In2 O3 films that we have discussed may also account for the observed change of activation energy of τ = f (1/kT) dependencies, calculated for In2 O3 films with a different film structure (Fig. 7). If for porous In2 O3 films we have Eact = 0.45–0.60 eV (curves 2 and 3), then for thick dense In2 O3 films (d > 70–100 nm) Eact can be equaled to more than 0.80–0.9 eV (curve 1). It is important to note that indicated values of activation energies are considerably smaller than activation energy of oxygen bulk diffusion in In2 O3 . According to [30] for this process Eact = ∼1.5 eV. Such difference is additional confirmation that oxygen bulk diffusion does not control the kinetics of In2 O3 gas response to reducing gases in the range of used operating temperatures (Toper < 450 ◦ C). For In2 O3 films deposited from high concentration solutions simultaneously with an increase in response time, when film thickness increases we observed the shift of tem-
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
Fig. 10. The influence of In2 O3 film thickness on the temperature position of R(min) on R = f (T) dependencies for films deposited from 1.0 M and 0.2 M InCl3 -solution (RH = 35–45%). Arrow indicates the direction of temperature change during measurements.
perature position of R(min) on R = f (T) dependencies in the region of higher temperatures (Figs. 3 and 10). As we noted earlier, In2 O3 films deposited from a solution with both precursor concentrations 1.0 and 0.2 M have very close main structural parameters such as grain size and film texturing [3,20]. Only in the values of refraction indexes, and then in film porosity, we observed noticeable difference for these films. These results allow us to assume that the shift of R(min) and the above-mentioned τ increase for thick films, deposited from high concentration solution, have the same nature. 3.2. Possibilities for optimization of In2 O3 gas sensor parameters It is necessary to note that the gas sensitivity of analyzed In2 O3 films is not worse than gas sensitivity in In2 O3 films prepared by the sol–gel method, and the rate of response is even better for our films [8,9]. Even at Toper ∼ 300–350 ◦ C response time did not exceed 5–6 s for undoped In2 O3 films with optimal structure. This situation confirms the possibility of using the selected method for deposition of In2 O3 films with high gas-sensing parameters. If we have to select the technological modes, which optimize gas-sensing characteristics of In2 O3 films, they will be the following:
127
Fig. 11. The influence of InCl3 precursor concentration in sprayed solution on the recovery times during CO and H2 detection (Toper = 370 ◦ C, RH = 35–45%).
size decreases and film sheet resistance increases, while Tpyr rises [3,20]. For standard SnO2 gas sensors, this change is accompanied by both gas response increase, and response time decrease [19,22,28]. We established such regularity earlier for In2 O3 ozone sensors [3]. In the case of reducing gas detection we observed opposite dependencies between gas response, response time, grain size of In2 O3 films, and pyrolysis temperature. On the basis of these results, it can be assumed that the grain size factor is not crucial to the influence of pyrolysis temperature on In2 O3 gas response to reducing gases. Taking into account our results, it may be suggested that surface properties, and film structure (morphology) are key factors causing the dependence of In2 O3 film gas response on Tpyr . Perhaps In2 O3 films, deposited at low pyrolysis temperatures, have more optimal surface structure or surface chemical composition for detection of reducing gases than films, deposited at high pyrolysis temperature. For example In2 O3 films deposited at low pyrolysis temperatures have smaller degree of texturing. It is clearly shown in Fig. 2. XRD patterns of such films are close to standard XRD patterns of In2 O3 powders, i.e. at Tpyr < 350 ◦ C In2 O3 films consist from randomly oriented crystallites. So, In2 O3 films deposited at these pyrolysis temperatures are not always textured and therefore, they are more porous. This assertion is coincided with conclusion which was made earlier, that the
• to use low temperature mode of pyrolysis deposition ∼350–400 ◦ C (Fig. 6); • to decrease precursor concentration in sprayed solution (<0.2 M) (Fig. 11); • to increase the In2 O3 film thickness (d > 80 nm) (Fig. 12). These technological modes guarantee maximal gas response to CO and H2 and minimal constant times of both response and recovery processes. The obtained results demonstrate that gas response of nanocrystalline In2 O3 films depends significantly on pyrolysis temperature (Fig. 6). This dependence can be attributed to a change in a number of factors such as crystallite size, porosity, film resistance, surface characteristics, etc. Grain
Fig. 12. Dependencies of gas response to 0.5%CO (1, 2) and 0.5%H2 (3) on film thickness for In2 O3 deposited from high concentration (1.0 M) (1) and low concentration (0.2 M) sprayed InCl3 solutions (2, 3) (RH ∼ 1–3%; Toper = 370 ◦ C).
128
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
increase of film porosity is an important factor of In2 O3 gas response improvement. However, as we indicated earlier, at low pyrolysis temperatures, as gas response increases, we observed an increase in the sensitivity of gas-sensing characteristics to air humidity (Fig. 6), that is not always acceptable for practical applications. Therefore, during selection of optimal pyrolysis temperatures for In2 O3 film deposition, we have to search the compromise between maximum gas response and minimum sensitivity to air humidity. As shown in Fig. 11, the decrease in precursor concentration in sprayed solution from 1.0 to 0.05–0.1 M results in a 5–6 times decrease in τ res for In2 O3 films with a thickness of ∼60 nm. Moreover, for films deposited from low concentration solutions no film thickness influence on τ res was observed (Fig. 8, curves 1 and 2). At the same time, for In2 O3 films deposited from solution with high precursor concentration (∼1.0 M), when d increases from 10 to 400 nm, τ res and τ rec increased nearly 10 times (Fig. 8, curves 3 and 4). As we discussed earlier, indicated film thickness influence on response time can be a reflection of the influence of such factors as film porosity. As we mentioned above, In2 O3 films deposited from high concentration solutions are dense. Therefore, in these films the role of intergrain gas diffusion in the process of gas detection considerably increases. For porous films (films deposited from low concentration solutions) we do not find this limitation. For these films, in all ranges of film thickness (20–400 nm), τ res and τ rec do not exceed 6–10 s even at Toper ∼ 300 ◦ C. Regarding film thickness, we can confirm that films, deposited from low concentration solutions, which had a bigger thickness, showed better gas response to CO and H2 (Fig. 12, curves 2 and 3). Therefore, they are more preferable for the design of In2 O3 -based gas sensors to reducing gases. It is necessary to note that such thickness influence on gas response of thin film gas sensors to reducing gas was an unexpected phenomena. However, the same effect was observed earlier in [29] for In2 O3 films prepared by spin-coating method. So, this conclusion regarding film thickness is not so unbelievable, because indicated film thickness influence on gas responser can be a reflection of the influence of other factors, such as structure and resistance of the films. Earlier we assumed that In2 O3 grains in atmosphere of reducing gas contain conductive surface layer. Apparently when film thickness increases the increase of shunting influence of surface high conductive layer takes place. It is very likely that In2 O3 grains with bigger size have better bulk stoichiometry, and as a result, they have higher bulk resistance than small grains. As it is shown in Fig. 13, we have clearly demonstrated the relationship between film sheet resistance and gas response. In general, where conductivity is less, gas response to reducing gases is higher. In this situation decreasing film conductivity can become the most important way to optimize gas-sensing properties. Usually sheet resistance of In2 O3 films deposited by spray pyrolysis does not exceed 105 –106 , even at a thickness of 10–15 nm. Sheet resis-
Fig. 13. Dependence of In2 O3 gas response to 0.5%H2 on sheet resistance of deposited films: Toper = 450 ◦ C.
tance increases for a little for films deposited at Tpyr ∼ 450–475 ◦ C. However, as it was shown earlier this pyrolysis temperature is not optimal for improving other gas-sensing parameters. Therefore, we think that the search for optimal components for bulk doping of In2 O3 films is one of the promising ways to optimize In2 O3 film parameters for In2 O3 -based gas sensor design.
4. Conclusions On the basis of the conducted experiments we can make the following conclusions: • In2 O3 films deposited by spray pyrolysis from InCl3 –water solutions possess good sensitivity to reducing gases such as CO and H2 . They have a high enough gas response, and short response and recovery times; • The proposed deposition technology allows us both to control gas-sensing properties and to search for optimal In2 O3 film structure for gas sensor applications; • Even for In2 O3 films with less thickness 200–400 nm high film porosity is a mandatory requirement for achievement both the high gas response to reducing gases (CO, and H2 ), and small response time of In2 O3 -based gas sensors. Film deposition at low pyrolysis temperatures (350–400 ◦ C) from solutions with low precursor concentration (<0.2 M) guarantee the accomplishment of this request; • To explain In2 O3 gas sensor characteristics we have to use specific approaches, which take into account “redox”, diffusion and chemisorbed effects.
Acknowledgements This work was supported by EU in the framework of INCO-Copernicus Program (Grant CT2-CA-2000-10017).
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
References [1] T. Takada, K. Suzuki, M. Nakane, Highly sensitive ozone sensor, Sens. Actuators B 13–14 (1993) 404–407. [2] A. Galdikas, Z. Marturas, A. Setkus, SnInO-based chlorine gas sensor, Sens. Actuators B 7 (1992) 633–636. [3] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Ozone sensing properties of In2 O3 films deposited by spray pyrolysis, Moldavian J. Phys. Sci. 1 (3) (2002) 24–27. [4] G. Kiriakidsis, M. Bender, N. Katsarakis, E. Gogaoudakis, E. Hourdakis, E. Douloufakis, V. Cimalla, Ozone sensing properties of polycrystalline indium oxide films at room temperature, Phys. Stat. Sol. (a) 185 (1) (2001) 27–32. [5] H. Seffes, C. Imawan, F. Solzbacher, E. Obermeir, Enhancement of NO2 sensing properties of In2 O3 -based thin films using Au or Ti surface modification, Sens. Actuators B 78 (2001) 106–112. [6] K. Scharnagl, K. Anothainart, A. Fuchs, R. Winter, M. Zimmer, W. Wlodarski, T. Doll, I. Eisele, Room temperature ozone detection: comparison between indium oxide and potassium iodide, in: Proceedings of the 13th European Conference on Solid State Transducers “Eurosensors XIII”, 12–15 September 1999, The Hague, The Netherlands pp. 255–258. [7] G. Faglia, B. Allieri, E. Comini, L.E. Depera, L. Sangaletti, G. Sberveglieri, Electrical and structural properties of RGTO-In2 O3 sensors for ozone detection, Sens. Actuators B 57 (1999) 188– 191. [8] A. Gurlo, M. Ivanovskaya, N. Barsan, U. Weimar, W. Gopel, In2 O3 and In2 O3 –MoO3 thin film semiconductor sensors: interaction with NO2 and O3 , Sens. Actuators B 47 (1998) 92–99. [9] M. Ivanovskaya, P. Bogdanov, G. Faglia, S. Sberveglieri, The features of thin films and ceramic sensors at the detection of CO and NO2 , Sens. Actuators B 68 (2000) 344–350. [10] H. Yamaura, T. Jinkawa, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Indium oxide-based gas sensors for selective detection of CO, Sens. Actuators B 35–36 (1996) 325–332. [11] H.J. Lee, J.H. Song, Y.S. Yoon, T.S. Kim, K.J. Kim, W.K. Choi, Enhancement of CO sensitivity of indium oxide-based semiconductor gas sensor through ultra-thin cobalt adsorption, Sens. Actuators B 79 (2001) 200–205. [12] H. Yamaura, K. Moriya, N. Miura, N. Yamazoe, Mechanism of sensitivity promotion in CO sensor using indium oxide and cobalt oxide, Sens. Actuators B 65 (2000) 39–41. [13] H. Yamaura, T. Jinkawa, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Indium oxide based gas sensor for selective detection of CO, Sens. Actuators B 35–36 (1996) 325–332. [14] W. Goepel, J. Hense, H. Baltes (Eds.), Sensors Update, vol. 1, VCH, New York, 1996, 350 pp. [15] G. Sberveglieru, Recent development in semiconducting thin film gas sensors, Sens. Actuators B 23 (1995) 103–109.
129
[16] G.S. Coles, G. Williams, Nanocrystalline materials as potential gas-sensing elements, in: K.E. Gonsalves, M.-I. Baraton, R. Singh, H. Hofmann, J.X. Chen, J.A. Akkara (Eds.), Proceedings of the Materials Residue Society Symposium 501, Surface-controlled Nanoscale Materials for High-added-Value Applications, MRS, Warrendale, PA, 1998, pp. 33–40. [17] G. Korotcenkov, V. Brinzari, M. DiBattista, J. Schwank, A. Vasiliev, Peculiarities of SnO2 thin film deposition by spray pyrolysis for gas sensor application, Sens. Actuators B 77 (1–2) (2001) 244–252. [18] G. Korotcenkov, V. Brinzari, J. Schwank, A. Cerneavschi, Possibilities of aerosol technology for deposition of SnO2 -based films with improved gas-sensing characteristics, J. Mater. Sci. Eng. C 19 (1–2) (2001) 73–77. [19] V. Brinzari, G. Korotcenkov, V. Golovanov, Factors influencing the gas-sensing characteristics of tin dioxide films deposited by spray pyrolysis: understanding and possibilities for control, Thin Solid Films 391 (2) (2001) 167–175. [20] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Crystallographic characterization of In2 O3 films deposited by spray pyrolysis, Sens. Actuators B 84 (1) (2002) 37–42. [21] N. Barsan, M. Schweizer-Berberich, W. Gopel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: status report, Fresenius. J. Anal. Chem. 365 (1999) 287–304. [22] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B 5 (1991) 7–19. [23] A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, W. Gopel, Sol–gel prepared In2 O3 thin films, Thin Solid Films 307 (1997) 288–293. [24] M. Ivanovskaya, P. Bogdanov, N. Barsan, J. Kapler, The influence of humidity on a sensitive behavior of In2 O3 -based sensors, in: Proceedings of the 14th European Conference on Solid State Transducers “Eurosensors XIV”, 27–30 August 2000, Copenhagen, Denmark pp. 145–148. [25] T.M. Ratcheva, M.D. Nanova, L.V. Vassilev, M.G. Mikhailov, Properties of In2 O3 :Te films prepared by the spraying method, Thin Solid Films 139 (1986) 189–199. [26] D.E. Williams, G.S. Henshaw, K.F.E. Pratt, R. Peat, Reactiondiffusion effects and systematic design of gas-sensitive resistors based on semiconductor oxides, J. Chem. Soc., Faraday Trans. 91 (23) (1995) 4299–4307. [27] G. Sakai, N.S. Baik, N. Miura, N. Yamazoe, Gas-sensing properties of tin oxide thin films fabricated from hydrothermally treated nanoparticles. Dependence of CO and H2 response on film thickness, Sens. Actuators B 77 (2001) 116–121. [28] G. Korotcenkov, V. Brinzari, S. Dmitriev, SnO2 films for thin film gas sensor design, Mater. Sci. Eng. B 56 (1999) 195–204. [29] W.Y. Chung, G. Sakai, K. Shimanoe, N. Miura, D.D. Lee, N. Yamazoe, Preparation of indium oxide thin film by spin-coating method and its gas-sensing properties, Sens. Actuators B 46 (1998) 139–145. [30] J. Berger, I. Riess, D.S. Tannhauser, Dymanic measurement of oxygen diffusion in indium-tin oxide, Solid State Ionics 15 (1985) 225–231.
In2 O3 films deposited by spray pyrolysis: gas response to reducing (CO, H2 ) gases G. Korotcenkov a,∗ , V. Brinzari a , A. Cerneavschi a , M. Ivanov a , A. Cornet b , J. Morante b , A. Cabot b , J. Arbiol b a
Laboratory of Microelectonics and Optoelectronics, Technical University of Moldova, Bld. Stefan cel Mare, 168 Chisinau 2004, Moldova b Dept. d’Electronica, Universitat de Barcelona, Barcelona, Spain Received 29 October 2002; received in revised form 28 January 2003; accepted 11 September 2003
Abstract This article discusses the influence of both technological modes and structural parameters of In2 O3 films deposited by spray pyrolysis from InCl3 water solution on response to reducing gases (CO and H2 ). Conclusions were drawn on the main factors, which influence both gas response of In2 O3 films and time constants of response and recovery processes. They are film thickness and porosity. It was established that films deposited at low pyrolysis temperatures (350–400 ◦ C) from low precursor concentration solutions (<0.2 M) had maximal gas response and minimal constant times of both response, and recovery processes. © 2003 Elsevier B.V. All rights reserved. Keywords: In2 O3 ; Spray pyrolysis; Gas response; Reducing gases
1. Introduction Recent research has shown that In2 O3 films have good response to oxidizing gases [1–8]. Therefore, highly sensitive detectors of O3 , NOx , Cl2, etc., can be manufactured on the basis of this material. In2 O3 sensors also detect reducing gases, such as CH4 and CO [9–13]. At that it was found that In2 O3 -based sensors may have a good selectivity to CO in the presence of H2 in surrounding atmosphere. According to Yamaura et al. [12,13] and our results the ratio of gas response (S) to CO and H2 (S(CO)/S(H2 )) may exceed 10 at the same concentrations of detected gases. It is important for design CO selective sensors, because for SnO2 the gas response to H2 is substantially higher than to CO [16], that creates certain difficulties in design of selective CO SnO2 -based gas sensors. This indicates that in the near future In2 O3 may become one of the base materials in the design of various gas sensors for several applications, along with such metal oxides as SnO2 and ZnO [14,15]. In this context the search for optimal technology for the deposition of In2 O3 films is particularly relevant. ∗ Corresponding author. Tel.: +34-373-2-235437; fax: +34-373-2-210432. E-mail addresses: [email protected], [email protected] (G. Korotcenkov).
0925-4005/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2003.09.009
Earlier we showed that the spray technology we designed can be used successfully for deposition of SnO2 films with good gas-sensing parameters [17,18]. Moreover, the ability of this technology to affect various structural parameters of deposited films enabled us to analyze the influence of structural factors on the gas-sensing effects observed on SnO2 [19]. From the standpoint of the problem under discussion, determining the applicability of spray pyrolysis technology to In2 O3 deposition seems very important.
2. Experimental For deposition of In2 O3 films we used the technique of active spray pyrolysis. The principal scheme of apparatus used for this purpose was described in [17]. The aerosol phase was formed by pneumatic method. Purified air with pressure equaled 2 atm was used as gas carrier. Technological parameters, such as pyrolysis temperature, precursor concentration, and volume of sprayed solution, were used to control structural and electrophysical parameters of deposited films. Earlier we showed that these factors determine grain size (t), thickness (d), and porosity of deposited films [17,18]. InCl3 was used as precursor, and water as a solvent. Precursor concentration varied in the range 0.05–1.0 M. Spray pyrolysis was carried out at the temperature range 300–550 ◦ C.
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
123
Fig. 1. AFM images of In2 O3 films deposited by spray pyrolysis using different technological parameters: (a) Tpyr = 375 ◦ C; d ∼ 40–60 nm; (b) Tpyr = 510 ◦ C; d ∼ 40 nm; (c) Tpyr = 510 ◦ C; d ∼ 150 nm.
Structural parameters of deposited films were controlled using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) techniques. The thickness of In2 O3 -deposited films was determined by laser ellipsometry. The main results of this research are presented in [3,20]. It was determined that In2 O3 films deposited by spray pyrolysis are nanocrystalline with grain sizes ranging from 10 to 80 nm. Typical AFM images of In2 O3 films deposited in different technological modes are shown in Fig. 1. Minimal grain sizes were observed in thin films (d < 40 nm) deposited at Tpyr ∼ 450–500 ◦ C. Moreover, In2 O3 films with a thickness of more than 40 nm, which were deposited at Tpyr > 400 ◦ C, are highly textured with predominant crystallite orientation in the (0 0 1) direction perpendicular to substrate. It is clearly shown in Fig. 2, where the XRD patterns of In2 O3 films deposited at low and high pyrolysis temperatures are presented. Gas-sensing properties were measured in a flow-type reactor with a cell of <0.2 cm3 . Film conductivity (G) and gas response (S) were measured in steady state and transient modes. Measurements were conducted in dry (∼1–2% RH) and wet (30–50% RH) atmospheres of two mixtures of air and detected gas ((0.5% CO + air) and (0.5% H2 + air)). Gas response was calculated as a ratio R(air)/R(gas), where R(air) and R(gas) are film resistance measured in air and in the atmosphere containing detected gas respectively. Studied samples had width equaled 2–3 mm, and distance between
Fig. 2. XRD patterns of In2 O3 films deposited at low (a) and high (b) pyrolysis temperatures (d ∼ 150–200 nm): (a) Tpyr = 375 ◦ C; (b) Tpyr = 510 ◦ C.
contacts equaled 5–7 mm. Time constants of response (τ res ) and recovery processes (τ rec ) were determined at a level of 0.9 from steady state magnitude of film conductivity, measured after changing the atmosphere in measurement cell.
3. Results and discussions 3.1. Electrical and gas-sensing properties of In2 O3 films The typical temperature dependencies of sheet resistance R(T) and gas response of In2 O3 films are shown in Figs. 3 and 4. Even without special doping, In2 O3 films have good gas response to CO, H2 , which is no worse than the gas response of undoped SnO2 films. Moreover, both the general shapes of R(T) and S(T) dependencies and the position of “extreme” points of these curves are very close to similar dependencies for SnO2 films [12,17,21]. This situation could give us some clue about the identity of the processes responsible for changing the electro-physical and gas-sensing properties of SnO2 and In2 O3 films. In particular, it could be assumed that in sympathy with the effects on SnO2 , gas chemisorption processes determine sensing characteristics of In2 O3 films. However, it was found that
Fig. 3. Influence of In2 O3 film thickness on temperature dependencies of film resistance: films deposited from high concentration InCl3 –water solutions (1.0 M); (1) d ∼ 400 nm; (2) d ∼ 200 nm; (3) d ∼ 30–50 nm; (4) d ∼ 10–15 nm. (RH = 35–45%).
124
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
Fig. 4. Temperature dependencies of In2 O3 gas response to 0.5% CO (3) and 0.5%H2 (1,2): (1) d ∼ 400 nm; (2, 3) d ∼ 30–50 nm (RH = 35–45%).
one part of the regularities established for SnO2 have another tendency of changing for In2 O3 films, and another one does not carry out at all. For example, we established that In2 O3 films with the same thickness as SnO2 film have a resistance 1–3 order of magnitude lower than SnO2 films. The change of In2 O3 film resistance, measured in both heating and cooling modes, is also smaller (Fig. 3). Moreover, normalized In2 O3 film resistance in air, presented in scale Rd = f (t) (Fig. 5, curve 2), does not depend on grain size (t). In polycrystalline materials the fact that resistance does not depend on grain size is typical for films, where conductivity is determined by bulk properties of the material. The combination of the above-mentioned parameters can be a reflection of small resistance of intergrain contacts. The latter can be caused by the low height of potential barriers on intergrain interfaces, or by the high surface concentration of ionized donors. For SnO2 films, where intergrain barriers play an important role in film conductivity limitation, sheet resistance sharply increases when grain size decreases [21,22]. For In2 O3 films, such behavior we observed only in an ozone atmosphere (Fig. 5, curves 3 and 4). Ozone is very strong oxidizing agent, and after interaction with this
Fig. 5. Sheet resistance of In2 O3 films, deposited from 1.0 M InCl3 –water solution versus on grain sizes, measured in the direction parallel to substrate: (1, 3) R = f (t), (2, 4) Rd = f (t); (1, 2)—atmosphere of usual air; (3, 4)—atmosphere containing ozone (∼1 ppm).
gas, resistance of In2 O3 films increases at Toper > 200 ◦ C more than 103 times [3]. At that if these samples to cool in atmosphere containing ozone the high resistance state of the films can remain long time even in atmosphere of usual air. Such behavior of In2 O3 film resistance confirms our assumption that low resistance of intergrain contacts is connected with partial reduction of the surface of In2 O3 grains. Besides, the maximum for gas response of undoped In2 O3 films to reducing gases (CO, H2 ) shows a shift in the region of higher temperatures in comparison with SnO2 films. For SnO2 film gas-sensing characteristics, measured in the heating mode, gas response to CO and H2 appears at Toper ∼ 200 ◦ C, which correspond to resistance minimum. For In2 O3 films gas sensitivity to CO and H2 appears only at Toper > 300–350 ◦ C (Fig. 4). Moreover, for In2 O3 films S(T) maximum lies at temperatures, which equaled 400–450 ◦ C independently from film structure. As we showed earlier in [18,19] for undoped SnO2 films, the position of S(T) maximum depends on film structure and this maximum can be observed in the range from 250 to 400 ◦ C. In comparison with SnO2 films, In2 O3 films present the more clearly shown acceptor-like behavior of reducing gases. At Toper ∼ 150–250 ◦ C during In2 O3 interaction with CO and H2 (0.5% + air) film resistance can increase more than 10–20 times (Fig. 4). For SnO2 films, such a clearly shown effect we observed only for SnO2 :Pd samples when interacting with H2 taken place. Acceptor-like behavior of reducing gases during interaction with n-type metal oxides is a very interesting effect. However, the temperature range of the demonstration of this effect lies below the operating temperature of In2 O3 sensor used for reducing gas detection. Therefore, in this article we will not discuss this problem. This effect will be analyzed in special article. Air humidity influence on In2 O3 gas-sensing characteristics is not so brightly shown as for SnO2 films as well [21]. As it is shown in Fig. 6, gas response of In2 O3 films deposited at high pyrolysis temperatures have very weak dependence on the air humidity. Moreover, the change of air humidity does not shift the position of the gas sensitivity
Fig. 6. Dependencies of gas response (1,2,3) and recovery time (4) on pyrolysis temperature: Toper = 370 ◦ C; (1) (0.5% CO + air); (2, 3) (0.5% H2 +air); (1, 2) wet gas atmosphere (35–45% RH); (3) dry gas atmosphere (1–3% RH).
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
maximum in S(T) dependence. Only in the region of low pyrolysis temperatures, In2 O3 films have appreciable dependence of gas-sensing properties on the humidity of surrounding atmosphere. The above indicates that gas-sensing effects on In2 O3 films are described by laws, which are different from those describing gas response in SnO2 films. At present two approaches can be used to explain gas-sensing properties of In2 O3 films. The surface-trap mechanism was used to describe In2 O3 interaction with oxidizing gases (NO2 , O3 , Cl2 ) (Toper < 300 ◦ C) [8], and the “redox” mechanism for interaction with reducing gases (CO, H2 ) (Toper > 350 ◦ C) [23,24]. According to the last model, the change in the valence state of lattice In (In3+ → In2+ ) takes place during In2 O3 interaction with reducing gases. Results of ESR measurements indicate that the concentration of F-centers and In2+ ions in In2 O3 lattice increases following treatment with CO and H2 [23]. This chemical interaction takes place with the participation of lattice oxygen. In this model, the role of adsorbed oxygen is both In2+ oxidation and annealing of structural defects, i.e. oxygen vacancies, formed during In2 O3 interaction with reducing gases [23]. In other words two different processes with their own energetic parameters are responsible for response and recovery during gas detection by In2 O3 . They are reduction and reoxidation. For example these reactions for CO detection can be written in the form: Olat 2− + COads → CO2 ↑ +VO + 2e− In3+ + e− → In2+ VO + e− → [F - center]
reduction (1)
O2ads + 2e− → 2Oads − In2+ − e− → In3+ reoxidation (2) [F - center] + Oads − → Olat 2− In2 O3 is known to be a partially reduced highly defective oxide. In accordance with [23,25], the electrical conductivity of In2 O3 at low temperatures is controlled by the effectiveness of electron exchange due to the overlapping of In3+ and In2+ . ESR measurements show that at T = 196 ◦ C uncoupled electrons in In2 O3 lattice are delocalized [23]. Therefore, the reduction process results in an increase in film conductivity, and the reoxidation process gives a decrease in this parameter. So, in such an approach, film conductivity is determined by the concentration of cations, which changed their valence state, and chemisorption is only an intermediate step in these chemical reactions. As we know, according to the chemisorption model, the change of film conductivity is connected with the change of concentration of free electrons captured by chemisorbed oxygen. Of course, this scheme is very simplified because it does not take into account the presence of water in different forms on the In2 O3 surface. It is well known that water can have a big surface concentration and can exert noticeable influence on both absolute value and kinetic parameters of gas sensitivity [21]. However, we agree that at high temperatures (Toper > 400 ◦ C) this “redox” mechanism can be dominant in the re-
125
action of reducing gas detection. For this temperature range we observe the following regularities, which are not characteristic of the chemisorption model: 1. In2 O3 has high enough gas response to reducing gases even for films with both low film resistance and big crystallite sizes; 2. Structural parameters of films, such as grain size and texturing, exert weak influence on absolute gas response to reducing gases. This conclusion follows from the results, which are shown in Fig. 6. Depending on Tpyr crystallite sizes in In2 O3 films can change more than three times. At that minimal crystallite sizes are observed in films deposited at Tpyr ∼ 450–475 ◦ C [3,20]. However, only for these Tpyr , when In2 O3 films have minimal crystallite sizes, we observed both the maximal values of τ rec and minimal values of gas response (Fig. 6). 3. Surface doping by Pd exerts weak influences on absolute gas response to CO and strong influences on τ rec . The appearance of Pd on In2 O3 surface makes τ rec decrease more than 10 times. However, we need to remember that the reduction and reoxidation processes described in (1) and (2) have specific energy parameters. As a result both time constants and Eact for these processes must differ. In real conditions this difference is not observed. As for CO–In2 O3 and for H2 –In2 O3 interactions in temperature range of donor-like reactions (Toper > 300 ◦ C) τrec ∼ τres , and Eact of τ = f (1/kT) dependencies have close values for both response and recovery processes (Fig. 7). It indicates that one and the same process is responsible for transient characteristics during response and recovery. At that this process cannot be connected with detected gas, because the magnitudes of indicated parameters Eact and τ are the same for CO and H2 detection. As we discussed earlier, the “redox” mechanism of gas response assumes that the reaction of gas detection is accompanied by generation and annealing of point structural defects in In2 O3 lattice. Therefore, for this gas-detection mechanism, the role of bulk diffusion processes must in-
Fig. 7. The influence of In2 O3 film structure on the temperature dependencies of response and recovery times (RH ∼ 35–45%): (1) 1.0 M InCl3 solution, d ∼ 400 nm; (2, 3) 0.2 M InCl3 solution, d ∼ 40 nm.
126
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
Fig. 8. The influence of film thickness on the response and recovery times of In2 O3 films deposited from low concentration (1, 2) and high concentration (3, 4) InCl3 –water solutions (RH = 35–45%, Toper = 370 ◦ C): (1, 3) recovery; (2, 4) response.
Fig. 9. Time constants of In2 O3 gas response to 0.5%CO versus on film refraction index: (A, B)—In2 O3 films deposited from 0.2 M and 1.0 M InCl3 solutions correspondingly. (1, 2) Toper = 370 ◦ C; (3) Toper = 270 ◦ C; (1) d ∼ 40–60 nm; (2, 3) d ∼ 400 nm.
crease considerably, especially when point structural defects have high enough mobility. The reduction and reoxidation processes give the same diffusion flows, which move in opposite directions i.e. bulk diffusion of oxygen or indium in In2 O3 grains could explain the coincidence of Eact and time constants for response and recovery processes. However, as we know bulk diffusion is a grain size dependent process. Therefore for processes, controlled by bulk diffusion, we must have the time constants with specific dependencies on crystallite sizes (τdif ∼ t 2 /D, where D is the coefficient of diffusion). However, for reactions of CO and H2 detection we did not find such inter-correlation between response time and grain size. For example, for In2 O3 samples deposited from solutions with low precursor concentration grain size increases from 10 to 80 nm, when film thickness increases from 20 to 400 nm. Time constants of gas response measured for these samples do not change yet (Fig. 8). So, we can assume that, in general, for In2 O3 reaction with reducing gases bulk diffusion does not determine response and recovery times. At the same time the absence of τ = f (t) dependence is not the evidence that diffusion processes do not exert influence on gas-sensing characteristics of In2 O3 films at all. As we have shown earlier for porous In2 O3 films deposited at low pyrolysis temperatures with no τ = f (d, t) dependence we observed both some difference in τ res and τ rec (Figs. 7 and 8), and a noticeable influence of air humidity on both S and τ (Fig. 6). These are really typical for processes controlled by surface reactions. For “dense” films with big thickness (d > 100 nm) we have other situation. In this case τres ∼ τrec , and neither S nor τ depend on air humidity. Moreover, τ has a strong dependence on film thickness (Fig. 8). Such combination of dependencies corresponds better to diffusion type processes [26,27]. For the above-mentioned films, it was also established that film density is one of the main factors, which controls the time constants of gas response. During our experiments this parameter was checked by determining refraction index (n). For this purpose we used laser ellipsomentry. It is known
the greater the film density, the bigger the refraction index. Interrelations between τ and n, which were determined for both film thickness, are shown in Fig. 9. The influence of refraction index on response and recovery times can be understood in the framework of the “diffusion” approach, which was used to explain gas response characteristics of thick ceramic sensors [26,27]. This approach assumes that if the film is denser, the intergrain distance is smaller. In this case the diffusion of both detected gas and products of gas detection reaction into the film is slower. As a result the time constant of gas response of thick dense film is bigger in comparison with porous film. As it was shown in [3,20], In2 O3 are highly textured films with cubic structure of the crystallites. These films look like columns. In certain conditions, in such highly textured columnar films, the crystallites can be compactly packed together without pores between them. Therefore, in thick In2 O3 films, deposited from a high concentration InCl3 solution the response time really can be controlled by diffusion processes. However, under the “diffusion” process, which limit τ res and τ rec , we probably need to understand intergrain diffusion, but not bulk diffusion. The change of the nature of the processes controlling the kinetics of gas response in porous and dense In2 O3 films that we have discussed may also account for the observed change of activation energy of τ = f (1/kT) dependencies, calculated for In2 O3 films with a different film structure (Fig. 7). If for porous In2 O3 films we have Eact = 0.45–0.60 eV (curves 2 and 3), then for thick dense In2 O3 films (d > 70–100 nm) Eact can be equaled to more than 0.80–0.9 eV (curve 1). It is important to note that indicated values of activation energies are considerably smaller than activation energy of oxygen bulk diffusion in In2 O3 . According to [30] for this process Eact = ∼1.5 eV. Such difference is additional confirmation that oxygen bulk diffusion does not control the kinetics of In2 O3 gas response to reducing gases in the range of used operating temperatures (Toper < 450 ◦ C). For In2 O3 films deposited from high concentration solutions simultaneously with an increase in response time, when film thickness increases we observed the shift of tem-
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
Fig. 10. The influence of In2 O3 film thickness on the temperature position of R(min) on R = f (T) dependencies for films deposited from 1.0 M and 0.2 M InCl3 -solution (RH = 35–45%). Arrow indicates the direction of temperature change during measurements.
perature position of R(min) on R = f (T) dependencies in the region of higher temperatures (Figs. 3 and 10). As we noted earlier, In2 O3 films deposited from a solution with both precursor concentrations 1.0 and 0.2 M have very close main structural parameters such as grain size and film texturing [3,20]. Only in the values of refraction indexes, and then in film porosity, we observed noticeable difference for these films. These results allow us to assume that the shift of R(min) and the above-mentioned τ increase for thick films, deposited from high concentration solution, have the same nature. 3.2. Possibilities for optimization of In2 O3 gas sensor parameters It is necessary to note that the gas sensitivity of analyzed In2 O3 films is not worse than gas sensitivity in In2 O3 films prepared by the sol–gel method, and the rate of response is even better for our films [8,9]. Even at Toper ∼ 300–350 ◦ C response time did not exceed 5–6 s for undoped In2 O3 films with optimal structure. This situation confirms the possibility of using the selected method for deposition of In2 O3 films with high gas-sensing parameters. If we have to select the technological modes, which optimize gas-sensing characteristics of In2 O3 films, they will be the following:
127
Fig. 11. The influence of InCl3 precursor concentration in sprayed solution on the recovery times during CO and H2 detection (Toper = 370 ◦ C, RH = 35–45%).
size decreases and film sheet resistance increases, while Tpyr rises [3,20]. For standard SnO2 gas sensors, this change is accompanied by both gas response increase, and response time decrease [19,22,28]. We established such regularity earlier for In2 O3 ozone sensors [3]. In the case of reducing gas detection we observed opposite dependencies between gas response, response time, grain size of In2 O3 films, and pyrolysis temperature. On the basis of these results, it can be assumed that the grain size factor is not crucial to the influence of pyrolysis temperature on In2 O3 gas response to reducing gases. Taking into account our results, it may be suggested that surface properties, and film structure (morphology) are key factors causing the dependence of In2 O3 film gas response on Tpyr . Perhaps In2 O3 films, deposited at low pyrolysis temperatures, have more optimal surface structure or surface chemical composition for detection of reducing gases than films, deposited at high pyrolysis temperature. For example In2 O3 films deposited at low pyrolysis temperatures have smaller degree of texturing. It is clearly shown in Fig. 2. XRD patterns of such films are close to standard XRD patterns of In2 O3 powders, i.e. at Tpyr < 350 ◦ C In2 O3 films consist from randomly oriented crystallites. So, In2 O3 films deposited at these pyrolysis temperatures are not always textured and therefore, they are more porous. This assertion is coincided with conclusion which was made earlier, that the
• to use low temperature mode of pyrolysis deposition ∼350–400 ◦ C (Fig. 6); • to decrease precursor concentration in sprayed solution (<0.2 M) (Fig. 11); • to increase the In2 O3 film thickness (d > 80 nm) (Fig. 12). These technological modes guarantee maximal gas response to CO and H2 and minimal constant times of both response and recovery processes. The obtained results demonstrate that gas response of nanocrystalline In2 O3 films depends significantly on pyrolysis temperature (Fig. 6). This dependence can be attributed to a change in a number of factors such as crystallite size, porosity, film resistance, surface characteristics, etc. Grain
Fig. 12. Dependencies of gas response to 0.5%CO (1, 2) and 0.5%H2 (3) on film thickness for In2 O3 deposited from high concentration (1.0 M) (1) and low concentration (0.2 M) sprayed InCl3 solutions (2, 3) (RH ∼ 1–3%; Toper = 370 ◦ C).
128
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
increase of film porosity is an important factor of In2 O3 gas response improvement. However, as we indicated earlier, at low pyrolysis temperatures, as gas response increases, we observed an increase in the sensitivity of gas-sensing characteristics to air humidity (Fig. 6), that is not always acceptable for practical applications. Therefore, during selection of optimal pyrolysis temperatures for In2 O3 film deposition, we have to search the compromise between maximum gas response and minimum sensitivity to air humidity. As shown in Fig. 11, the decrease in precursor concentration in sprayed solution from 1.0 to 0.05–0.1 M results in a 5–6 times decrease in τ res for In2 O3 films with a thickness of ∼60 nm. Moreover, for films deposited from low concentration solutions no film thickness influence on τ res was observed (Fig. 8, curves 1 and 2). At the same time, for In2 O3 films deposited from solution with high precursor concentration (∼1.0 M), when d increases from 10 to 400 nm, τ res and τ rec increased nearly 10 times (Fig. 8, curves 3 and 4). As we discussed earlier, indicated film thickness influence on response time can be a reflection of the influence of such factors as film porosity. As we mentioned above, In2 O3 films deposited from high concentration solutions are dense. Therefore, in these films the role of intergrain gas diffusion in the process of gas detection considerably increases. For porous films (films deposited from low concentration solutions) we do not find this limitation. For these films, in all ranges of film thickness (20–400 nm), τ res and τ rec do not exceed 6–10 s even at Toper ∼ 300 ◦ C. Regarding film thickness, we can confirm that films, deposited from low concentration solutions, which had a bigger thickness, showed better gas response to CO and H2 (Fig. 12, curves 2 and 3). Therefore, they are more preferable for the design of In2 O3 -based gas sensors to reducing gases. It is necessary to note that such thickness influence on gas response of thin film gas sensors to reducing gas was an unexpected phenomena. However, the same effect was observed earlier in [29] for In2 O3 films prepared by spin-coating method. So, this conclusion regarding film thickness is not so unbelievable, because indicated film thickness influence on gas responser can be a reflection of the influence of other factors, such as structure and resistance of the films. Earlier we assumed that In2 O3 grains in atmosphere of reducing gas contain conductive surface layer. Apparently when film thickness increases the increase of shunting influence of surface high conductive layer takes place. It is very likely that In2 O3 grains with bigger size have better bulk stoichiometry, and as a result, they have higher bulk resistance than small grains. As it is shown in Fig. 13, we have clearly demonstrated the relationship between film sheet resistance and gas response. In general, where conductivity is less, gas response to reducing gases is higher. In this situation decreasing film conductivity can become the most important way to optimize gas-sensing properties. Usually sheet resistance of In2 O3 films deposited by spray pyrolysis does not exceed 105 –106 , even at a thickness of 10–15 nm. Sheet resis-
Fig. 13. Dependence of In2 O3 gas response to 0.5%H2 on sheet resistance of deposited films: Toper = 450 ◦ C.
tance increases for a little for films deposited at Tpyr ∼ 450–475 ◦ C. However, as it was shown earlier this pyrolysis temperature is not optimal for improving other gas-sensing parameters. Therefore, we think that the search for optimal components for bulk doping of In2 O3 films is one of the promising ways to optimize In2 O3 film parameters for In2 O3 -based gas sensor design.
4. Conclusions On the basis of the conducted experiments we can make the following conclusions: • In2 O3 films deposited by spray pyrolysis from InCl3 –water solutions possess good sensitivity to reducing gases such as CO and H2 . They have a high enough gas response, and short response and recovery times; • The proposed deposition technology allows us both to control gas-sensing properties and to search for optimal In2 O3 film structure for gas sensor applications; • Even for In2 O3 films with less thickness 200–400 nm high film porosity is a mandatory requirement for achievement both the high gas response to reducing gases (CO, and H2 ), and small response time of In2 O3 -based gas sensors. Film deposition at low pyrolysis temperatures (350–400 ◦ C) from solutions with low precursor concentration (<0.2 M) guarantee the accomplishment of this request; • To explain In2 O3 gas sensor characteristics we have to use specific approaches, which take into account “redox”, diffusion and chemisorbed effects.
Acknowledgements This work was supported by EU in the framework of INCO-Copernicus Program (Grant CT2-CA-2000-10017).
G. Korotcenkov et al. / Sensors and Actuators B 98 (2004) 122–129
References [1] T. Takada, K. Suzuki, M. Nakane, Highly sensitive ozone sensor, Sens. Actuators B 13–14 (1993) 404–407. [2] A. Galdikas, Z. Marturas, A. Setkus, SnInO-based chlorine gas sensor, Sens. Actuators B 7 (1992) 633–636. [3] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Ozone sensing properties of In2 O3 films deposited by spray pyrolysis, Moldavian J. Phys. Sci. 1 (3) (2002) 24–27. [4] G. Kiriakidsis, M. Bender, N. Katsarakis, E. Gogaoudakis, E. Hourdakis, E. Douloufakis, V. Cimalla, Ozone sensing properties of polycrystalline indium oxide films at room temperature, Phys. Stat. Sol. (a) 185 (1) (2001) 27–32. [5] H. Seffes, C. Imawan, F. Solzbacher, E. Obermeir, Enhancement of NO2 sensing properties of In2 O3 -based thin films using Au or Ti surface modification, Sens. Actuators B 78 (2001) 106–112. [6] K. Scharnagl, K. Anothainart, A. Fuchs, R. Winter, M. Zimmer, W. Wlodarski, T. Doll, I. Eisele, Room temperature ozone detection: comparison between indium oxide and potassium iodide, in: Proceedings of the 13th European Conference on Solid State Transducers “Eurosensors XIII”, 12–15 September 1999, The Hague, The Netherlands pp. 255–258. [7] G. Faglia, B. Allieri, E. Comini, L.E. Depera, L. Sangaletti, G. Sberveglieri, Electrical and structural properties of RGTO-In2 O3 sensors for ozone detection, Sens. Actuators B 57 (1999) 188– 191. [8] A. Gurlo, M. Ivanovskaya, N. Barsan, U. Weimar, W. Gopel, In2 O3 and In2 O3 –MoO3 thin film semiconductor sensors: interaction with NO2 and O3 , Sens. Actuators B 47 (1998) 92–99. [9] M. Ivanovskaya, P. Bogdanov, G. Faglia, S. Sberveglieri, The features of thin films and ceramic sensors at the detection of CO and NO2 , Sens. Actuators B 68 (2000) 344–350. [10] H. Yamaura, T. Jinkawa, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Indium oxide-based gas sensors for selective detection of CO, Sens. Actuators B 35–36 (1996) 325–332. [11] H.J. Lee, J.H. Song, Y.S. Yoon, T.S. Kim, K.J. Kim, W.K. Choi, Enhancement of CO sensitivity of indium oxide-based semiconductor gas sensor through ultra-thin cobalt adsorption, Sens. Actuators B 79 (2001) 200–205. [12] H. Yamaura, K. Moriya, N. Miura, N. Yamazoe, Mechanism of sensitivity promotion in CO sensor using indium oxide and cobalt oxide, Sens. Actuators B 65 (2000) 39–41. [13] H. Yamaura, T. Jinkawa, J. Tamaki, K. Moriya, N. Miura, N. Yamazoe, Indium oxide based gas sensor for selective detection of CO, Sens. Actuators B 35–36 (1996) 325–332. [14] W. Goepel, J. Hense, H. Baltes (Eds.), Sensors Update, vol. 1, VCH, New York, 1996, 350 pp. [15] G. Sberveglieru, Recent development in semiconducting thin film gas sensors, Sens. Actuators B 23 (1995) 103–109.
129
[16] G.S. Coles, G. Williams, Nanocrystalline materials as potential gas-sensing elements, in: K.E. Gonsalves, M.-I. Baraton, R. Singh, H. Hofmann, J.X. Chen, J.A. Akkara (Eds.), Proceedings of the Materials Residue Society Symposium 501, Surface-controlled Nanoscale Materials for High-added-Value Applications, MRS, Warrendale, PA, 1998, pp. 33–40. [17] G. Korotcenkov, V. Brinzari, M. DiBattista, J. Schwank, A. Vasiliev, Peculiarities of SnO2 thin film deposition by spray pyrolysis for gas sensor application, Sens. Actuators B 77 (1–2) (2001) 244–252. [18] G. Korotcenkov, V. Brinzari, J. Schwank, A. Cerneavschi, Possibilities of aerosol technology for deposition of SnO2 -based films with improved gas-sensing characteristics, J. Mater. Sci. Eng. C 19 (1–2) (2001) 73–77. [19] V. Brinzari, G. Korotcenkov, V. Golovanov, Factors influencing the gas-sensing characteristics of tin dioxide films deposited by spray pyrolysis: understanding and possibilities for control, Thin Solid Films 391 (2) (2001) 167–175. [20] G. Korotcenkov, A. Cerneavschi, V. Brinzari, A. Cornet, J. Morante, A. Cabot, J. Arbiol, Crystallographic characterization of In2 O3 films deposited by spray pyrolysis, Sens. Actuators B 84 (1) (2002) 37–42. [21] N. Barsan, M. Schweizer-Berberich, W. Gopel, Fundamental and practical aspects in the design of nanoscaled SnO2 gas sensors: status report, Fresenius. J. Anal. Chem. 365 (1999) 287–304. [22] N. Yamazoe, New approaches for improving semiconductor gas sensors, Sens. Actuators B 5 (1991) 7–19. [23] A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, W. Gopel, Sol–gel prepared In2 O3 thin films, Thin Solid Films 307 (1997) 288–293. [24] M. Ivanovskaya, P. Bogdanov, N. Barsan, J. Kapler, The influence of humidity on a sensitive behavior of In2 O3 -based sensors, in: Proceedings of the 14th European Conference on Solid State Transducers “Eurosensors XIV”, 27–30 August 2000, Copenhagen, Denmark pp. 145–148. [25] T.M. Ratcheva, M.D. Nanova, L.V. Vassilev, M.G. Mikhailov, Properties of In2 O3 :Te films prepared by the spraying method, Thin Solid Films 139 (1986) 189–199. [26] D.E. Williams, G.S. Henshaw, K.F.E. Pratt, R. Peat, Reactiondiffusion effects and systematic design of gas-sensitive resistors based on semiconductor oxides, J. Chem. Soc., Faraday Trans. 91 (23) (1995) 4299–4307. [27] G. Sakai, N.S. Baik, N. Miura, N. Yamazoe, Gas-sensing properties of tin oxide thin films fabricated from hydrothermally treated nanoparticles. Dependence of CO and H2 response on film thickness, Sens. Actuators B 77 (2001) 116–121. [28] G. Korotcenkov, V. Brinzari, S. Dmitriev, SnO2 films for thin film gas sensor design, Mater. Sci. Eng. B 56 (1999) 195–204. [29] W.Y. Chung, G. Sakai, K. Shimanoe, N. Miura, D.D. Lee, N. Yamazoe, Preparation of indium oxide thin film by spin-coating method and its gas-sensing properties, Sens. Actuators B 46 (1998) 139–145. [30] J. Berger, I. Riess, D.S. Tannhauser, Dymanic measurement of oxygen diffusion in indium-tin oxide, Solid State Ionics 15 (1985) 225–231.