Effect of Water vapour on Gallium doped Zinc Oxide nanoparticle sensor gas response Ruth Pearce, Fredrik Söderlind, Alexander Hagelin, PerOlov Käll, Rositza Yakimova, and Anita Lloyd Spetz
Elin Becker and Magnus Skoglundh Competence Centre for Catalysis Chalmers University of Technology SE-412 96 Goteborg, Sweden
Department of Biology Chemistry and Physics Linköping University SE-581 83 Linköping, Sweden
[email protected] Abstract—Zinc oxide is a wide band gap (~3.4ev) semiconductor material, making it a promising material for high temperature applications, such as exhaust and flue environments where NO and NO2 monitoring is increasingly required due to stricter emission controls[1]. In these environments water vapour and background levels of oxygen are present and, as such, the effect of humidity on the sensing characteristics of these materials requires further study. The reaction mechanisms in the presence of water vapour are poorly understood and there is a need for deeper understanding of the principles and mechanisms of gas response of these materials. An investigation of the influence of changing water vapour (H2O) and oxygen (O2) backgrounds on the response of nanoparticulate Ga-doped ZnO resistive sensors is presented.
I.
INTRODUCTION
Earlier work[2][3][4]demonstrated promising results for ZnO films and particles as high temperature sensors with water vapour showing interesting effects on the sensing capabilities of ZnO particle sensors[5]. Dopants such as Ga have shown promise in providing further enhancement of sensor stability[6]. The Ga:ZnO response to gases including O2, NO, NO2, H2, CO, and NH3 was investigated. Different sensor responses in the presence of water vapour were observed for all gases, with water vapour affecting the sensitivity, rate and, for some gases, direction of response. The sensing mechanisms in humid conditions are not clear at present and further study utilizing in situ FTIR spectroscopy in the diffuse reflectance mode (DRIFT) will be undertaken to elucidate surface species. II.
sputter deposited on a silicon substrate. The silicon substrates were then glued onto a ceramic heater with a Pt100 temperature sensor and gold wire bonded to a 16 pin gold coated header (see Fig.1). In-house built electronics using Labview controlled sensor temperature and recorded sensor output signal. Gas mixtures were flowed over the sensor from an in-house built gas mixer board comprising of Brönkhurst mass flow controllers. Water vapour was added by flowing the dry carrier gas (N2 or N2 with 10% O2) through a water bubbler at 20oC. The temperature of sensor operation was maintained at 500oC as this was found to be the optimum temperature for O2 sensitivity of ZnO nanoparticle sensors[4] and earlier work[5][6] showed that sensing of NO and NO2 was unoptimal below this temperature for ZnO nanoparticle sensors.
Figure 1. Sensor device showing gold wire bound interdigitated electrodes on silicon substrate glued to a ceramic heater with Pt100 temperature sensor.
EXPERIMENTAL
Nanoparticulate Ga-doped ZnO was produced via an electrochemical deposition under oxidising conditions (EDOC) method[7], as reported earlier[3][8]. The particle size after annealing at 500oC was 50-100nm[6]. The Ga:ZnO particles were dispersed in EtOH and drop cast over interdigitated finger electrode structures to form resistive sensors. The interdigitated finger electrode structures had finger spacing of ~40μm which were defined by photolithography and composed of a Cr/Au ~100/2500Å
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III.
RESPONSE TOWARDS WATER VAPOUR AND OXYGEN
A.
Response to water vapour At 500oC a clear, reversible decrease in the baseline resistance is observed upon exposure to water vapour. This shift is thought to be due to the formation of OH groups on the surface of the ZnO particles. At lower temperature (300 and 400oC) a response towards water vapour was seen, however upon returning the sensor to a dry environment
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recovery to a stable baseline was slow (in the order of hours), with the Ga doped sensor showing a larger response (Rg/RN) to water vapour and a slower recovery upon re-exposure to dry carrier gas than the undoped ZnO. B. Oxygen response in dry and humid environments As was found with the undoped ZnO[5] the Ga:ZnO response and rate of response to O2 increased with temperatures up to 500oC. The response time toward O2 increases in humid conditions, (see Fig. 2). Although the absolute response is lower in a humid environment the relative response (R-R0/R0) increases toward O2. The response is stable and reproducible in both dry and humid conditions. In dry environments the O2 response decreases slightly with continued exposure time.
Figure 2. Response of Ga:ZnO sensor to O2 in dry and humid conditions.
IV.
In the presence of humidity the response of the Ga:ZnO sensor toward H2 and NH3 is inverted and the response becomes smaller with increasing H2 concentration (see Fig. 3 and 4a and b). The change in direction of response of these materials in dry and humid air demonstrates the amphoteric nature of this material allowing hydrogen to either reduce or oxidise the surface, and suggests a number of different binding sites for a gaseous molecule, this will be discussed in further detail in section VI. a)
b)
RESPONSE TOWARDS REDUCING GASES
A. Change in direction of response in dry and humid environments. The response towards hydrogen in dry environments becomes saturated at ~100ppm, the magnitude of maximum response being the same as that on exposure to humid environments (see Fig. 3). This may suggest a small number of available sites for H2 to bind.
Figure 4. a) Response of the Ga:ZnO sensor towards NH3 in dry and humid environments with 10% O2 in the carrier gas. b) Enlargement of Figure a showing change of direction of NH3 response in dry and humid carrier gas.
Figure 3. Response of Ga:ZnO sensor towards H2 in dry and humid conditions with 10% O2 in the carrier gas.
B. Effect of addition the carrier gas on NH3 sensing. Upon the addition of 10% O2 to the carrier gas NH3 response is observed in both dry and humid environments (see Fig. 4a and b) with a reversal in direction of response. However when O2 is not present little response towards NH3 is observed in humid environments (see Fig.5). This is contrary to the results for un-doped ZnO in humid carrier gas where NH3 response was observed [5], albeit with a reduced magnitude and
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reversed direction than when O2 was present. It is interesting that reversal of direction of response was not observed for NH3 when switching between dry and humid environments when O2 is not present in the carrier gas. As expected the NH3 response in dry air is slower when only N2 is present as a carrier gas (see Fig.4 and Fig.5).
An increase in resistance of the sensor is observed on switching back to dry air after water vapour and NO exposure which was not observed with the reducing gases. This response becomes larger with continued switching between dry and humid environments. Figure 7 shows an enlargement of the response toward NO. An interesting peak is seen on removal of NO from the system in dry environments. The peak at the start of the NO pulses in humid environments may be due to errors in the gas mixer, or may be due to a larger response when initial concentrations are very low.
Figure 5. Response of Ga:ZnO sensor to NH3 with N2 as the carrier gas.
V.
Figure 7. Enlargement of Figure 6 showing the response of Ga:ZnO towards NO in dry and humid environments with 10% O2 in carrier gas.
RESPONSE TOWARDS OXIDISING GASES.
A. Response towards NO As was seen with the reducing gases, the response toward NO is also observed to reduce with increasing concentration, however contrary to the reducing gases this effect is observed in both dry and humid environments (see Fig. 6 and Fig. 7).
Figure 6. Response of Ga:ZnO towards NO in dry and humid environments with 10% O2 in the carrier gas.
With no O2 present in the system a longer recovery upon re-exposure to dry carrier gas is observed, suggesting O2 aids in the recovery of the sensor from water vapour exposure. The response towards NO is very similar with and without the presence of O2, no changes in direction of response are observed under any of the investigated conditions with oxidising gases always causing an increase of sensor resistance.
Figure 8. Response of Ga:ZnO towards NO in dry and humid environments with N2 as a carrier gas.
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B. Response towards CO Under dry conditions the response toward CO varies in direction of response, at low concentrations an increase in resistance is observed whereas at higher concentrations a decrease in resistance is seen. Under humid conditions an increase in resistance is always observed under all CO concentrations attempted. As was observed with other gases under humid conditions the response becomes smaller for larger concentrations of CO.
sensor material; sensor sites could be blocked and H2/NH3 then binds at different sensor sites which evoke an opposite response, or, the electronic structure of the sensor material may be altered upon surface OH group formation which leads to the usually electron deficient ZnO surface becoming electron rich. The electron rich oxide surface may then donate electrons to the gas molecules in the environment instead of withdrawing electrons. B. Reduction in response with increasing concentration. Reduction in response with increasing concentration is seen for all gases in humid environments except oxygen. The reducing response with increasing concentration it is also seen in dry environments with NO. The reason for the observed decrease in response at higher gas concentration is open to conjecture, a number of possible theories are postulated here; •
One could speculate that there may be different absorption sites, low energy site/sites and high energy site/sites on the surface. There are thought to be many different defects in ZnO crystals[14]. At low gas concentrations the low energy site/sites is/are occupied whereas at higher concentration both the lower energy and the higher energy site/sites are occupied. At higher gas concentrations, when the surface is saturated with electron donating molecules the higher energy site/sites cause an opposite response (electron donation toward the gas molecules) whereby reducing the overall response.
Figure 9. Response towards CO with 10% O2 in the carrier gas.
CO may be converted to H2 under humid conditions by the water gas shift reaction[9] however, high temperatures do not promote this reaction[10]. Although ZnO is commonly used along with Cu, in water gas shift catalysts it is the Cu which is thought to be the active component[11]. However small amount of both CO and H2O may be formed on the surface under humid conditions due to the OH groups on the surface which are thought to play a role in the catalysis of the reaction[12]. VI.
Theory one:
•
Theory two:
The binding of analytes at a higher energy or less frequently occurring site/sites on the surface causes a change in the electronic structure in the local area, making it less favourable for gas molecules to bind to the sensor surface, so causing the desorption of surrounding binding analytes, and a reduction in response. At higher analyte partial pressures more of the surface sites will be covered and higher energy binding sites will be covered.
DISCUSSION
•
Theory three:
Ga:ZnO is an interesting wide band gap semiconductor material, and this study has shown that at an operating temperature of 500oC response are seen towards a wide range of different gases which may be relevant for environmental monitoring. There are many interesting and surprising responses presented in this abstract, some of which will be discussed here;
All of the tested gases may react with surface oxygen at the operation temperature of 500oC resulting in the consumption of surface oxygen. If the consumption of surface oxygen is greater than the oxidation and replacement of surface oxygen then the response will diminish with increasing concentration.
A. Change of direction of response of H2 and NH3 in dry and humid environments. Hydrogen and Ammonia were observed to evoke a response with an opposite direction when switching from a dry to a humid environment. Under humid conditions both hydrogen and ammonia, which are usually electron donating molecules, acted as oxidising gases. This may be due to their solvation in water vapour changing the species, or may be due to the sensor surface being saturated with OH groups, it is thought that the ZnO surface develops different OH surface groups upon exposure to water vapour[13]. This saturation with surface OH groups could have a number of effects on the
C. Increase in resistance upon re-exposure to dry air. After cycling between wet and dry environments in the presence of CO or NO a large resistance shift was observed upon re-exposure to dry carrier gas, this response recovered more rapidly in the presence of oxygen. This increase in resistance may be due to strongly bound CO or NO molecules remaining on the surface which react slowly with oxygen on the removal of water vapour from the system. D. Observed peaks at the end or beginning of gas pulses. Spikes or peaks were observed at the end of the NO and NO2 pulses in dry conditions and the beginning of the peaks in
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humid environment. These peaks could be due to flow problems with the gas mixer program however this is unlikely. The peaks could be caused by transient species on the surface which give a larger response, or very low concentrations which have been shown to give greater responses than higher gas concentrations. It is interesting that in dry conditions the peak is seen at the end of the pulse and in humid conditions the peak is seen at the beginning of the pulse. E. DRIFT spectroscopy Many of the interesting results discussed above require knowledge of the species on the Ga:ZnO surface to interpret them and decide which of the postulated theories are most likely. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy detects surface species from their characteristic vibrations. DRIFT spectroscopy has been used to evaluate surface species on ZnO and also to aid in the elucidation of response mechanisms of sensor surfaces[15] [16].
IX. [1] [2]
[3]
[4]
[5]
[6]
[7]
VII. SUMMARY AND CONCLUSIONS Ga-doped ZnO nanoparticles produced via an electrochemical deposition under oxidising conditions (EDOC) method was investigated for its response as a resistive sensor toward O2, NO, NO2, H2, CO, and NH3 with and without a 10% background of oxygen and in humid and dry environments. Water vapour was found to actually increase the rate of response and recovery see Fig. 3,4,7,8 and 9 the baseline stability is also improved. In the presence of humidity the sensor response is reproducible even after many cycles between humid and dry carrier gases. Whilst the magnitude of response decreases the relative response (R-R0/R0) increases toward O2 and NO in the presence of humidity. In humid environments the response was found to decrease with increasing gas concentration, this unusual result is thought to be due to competing reaction mechanisms. The competing sensing mechanisms leading to the observed inverse response scaling with concentration require deeper study and DRIFT spectroscopy will be employed to detect surface species. VIII. ACKNOWLEDGEMENTS
[8]
[9] [10] [11] [12]
[13]
[14]
[15]
Our research is supported by grants from the Swedish Agency for Innovation Systems, and Swedish Industry through the VINN Excellence Centre FunMat, and the Swedish Research Council. The authors would also like to acknowledge useful discussion with Mike Andersson.
[16]
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