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Influence of Oxygen in the Sensing Properties of Cadmium and Germanium Oxynitride J. J. Benı´tez,*,† M. A. Centeno,† C. Louis dit Picard,‡ J. Guyader,‡ Y. Laurent,‡ and J. A. Odriozola† Departamento de Quı´mica Inorga´ nica e Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla, CSIC Apdo. 874, 41071 Sevilla, Spain, and Laboratoire de Chimie des Mate´ riaux. URA 1496 CNRS “Verres et Ce´ ramiques”, Universite´ de Rennes I, Campus de Beaulieu, 35042 Rennes Cedex, France Received March 16, 1995X A novel cadmium and germanium oxynitride (CdGeON) sensor, originally designed for NH3 and SH2 detection, has been studied by in situ Fourier transform diffuse reflectance infrared spectroscopy (DRIFTS), XPS, and UV-vis in order to elucidate the influence of an oxygen-containing environment on the sensor’s electrical response. Under oxygen, the conductivity of this sensor is directly related with DRIFTS bands at 810, 780, and 580 cm-1 assigned to oxygenated species bonded to single Ge atoms. These species are reversibly eliminated or restored in the solid framework upon heating in N2 or synthetic air, respectively. XPS and UV-vis spectroscopies have confirmed the oxygen uptake and the increase in the coordination number of Ge atoms upon exposure of the CdGeON sensor to synthetic air. A similar conclusion is presumed for Cd atoms although no conclusive results can be inferred from the UV-vis spectra. From the analysis of the resistance-temperature Arrhenius-type relationships, a model involving the filling of anionic vacancies is proposed to explain such a differenciated behavior under synthetic air.
Introduction Environmental control continously requests better and more reliable low-cost sensing devices. In this line, zinc and germanium oxynitrides (ZnGeON) have been characterized as specific sensors for gaseous pollutants such as NH3, H2S, etc.1 Despite the efficiency of this material, its high electrical resistance (tipically 109 Ω cm-1 in air at 250 °C) prevents any low-cost industrial application. Due to its good performances and to a much lower electrical resistance (106 Ω cm-1 at 250 °C in air), cadmium and germanium oxynitride (CdGeON) has been proposed to replace ZnGeON as a sensing material. The electrical response of CdGeON sensors was found to depend on the presence of air in the probe stream.1 In order to find out the origin of such a differentiated behavior we have employed Fourier transform diffuse reflectance infrared (DRIFTS), XPS, and UV-vis spectroscopies in combination with in situ conductivity measurements under controlled atmosphere and temperature. Infrared spectroscopy is a very powerful method for both adsorbed species and solid characterization. However, traditional transmission IR requires radiation transparent samples. Unfortunately this condition is not fulfilled by most real sensor designs; they usually contain an opaque nonconductive substratum, electrode tracks, etc. This handicap can be overcome using DRIFTS. This technique analyzes not the transmitted information but the information contained in the radiation reflected by the sample. In a previous paper2 we have reported the potential of DRIFTS for gas sensor analysis. We also described a homemade DRIFTS chamber useful for in situ and simultaneous obtaining both the DRIFTS spectra and the conductivity response of sensors under the desired temperature and atmosphere composition. Preliminary results have shown that this technique is capable of detecting †
Universidad de Sevilla. Universite´ de Rennes I. Abstract published in Advance ACS Abstracts, February 15, 1996. ‡
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(1) Jamois, D. Thesis, Universite´ de Rennes, 1992. (2) Benı´tez, J. J.; Centeno, M. A.; Merdrignac, O.; Guyader, J.; Laurent, Y.; Odriozola, J. A. Appl. Spectrosc. 1995, 49, 1094.
the generation of extra nonbridging (Ge-O) bonds in the CdGeON framework upon expossure to synthetic air. However, the lack of active (Cd-O) infrared modes prevents any hypothesis involving the modification of the (Cd-O) coordination polyhedron. X-ray photoelectron spectroscopy (XPS), as a surface and near-surface characterization technique, and UVvis, as a structure sensitive tool, are now employed to support and to complement the information obtained from DRIFTS. Conductivity results after sample exposure to synthetic air are also interpreted in this study. Experimental Section CdGeON is synthesized by nitridation of an amorphous cadmium germanate (Cd-Ge-O) phase under NH3 stream at 580 °C for 6 h. The Cd-Ge-O precursor was obtained by coprecipitation of GeO2 and CdO at 90 °C from an alkaline solution and further oven-dried at 120 °C.3 The CdGeON structure is tetrahedrical and derives from the wurtzite type. It can be described as diamond-like with Cd and Ge in equivalent positions filling half of the tetrahedral sites of the anionic arrangement. The chemical formula is Cd0.98GeO1.16N1.21. A more detailed description about preparation and structure characterization of samples and precursors is given in ref 3. DRIFTS spectra are recorded in a Fourier transform Nicolet 510P spectrometer with a DTGS detector operated at 4 cm-1 resolution and equipped with a commercial Spectra-Tech diffuse reflectance optics. One hundred scans are accumulated in each run, and spectra are smoothed and presented in the KubelkaMunk mode. The sensor is prepared by screen printing three layers of CdGeON powder on an Al2O3 support (8.5 mm × 6.4 mm and 0.635 mm thick), resulting in a 48 µm thick film. Two sets of Au tracks placed in between the CdGeON powder and the alumina support allow the measurement of its electrical response (Figure 1). A Pt coil, also screen-printed on the botton face of the Al2O3 support, will be used to both heat the sample and determine the temperature from a previously calibrated temperature-resistance relationship. Four Au wires (two for the Au electrodes and two for the Pt coil) are connected to a commercial eight pin electronic baseplate. The wires provide enough rigidity to keep the sample steady and horizontally inside the DRIFTS chamber. The heating (3) Louis dit Picard, C.; Merdrignac, O.; Guyader, J.; Laurent, Y. J. Solid State Chem. 1995, 119, 304.
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Figure 1. Schematics of CdGeON sensor as attached to the DRIFTS chamber. Temperature is controlled with a power supply (I) connected to a Pt filament. Pt coil is connected to a constant intensity power supply (I) and its resistance is simultaneously measured by monitoring the voltage decay across (V). Sensor resistance (R) is obtained from a Philips PM-2517X ohmmeter. Our DRIFTS chamber is a homemade design based on the commercial Spectra-Tech 300-101 model, and it has been fully described in a previous paper.2 The main modification is the redesign of the sample holder to allow both the supply to the Pt coil and the measurement of the electrical response of the sensor. Gases are flowed through a pair of 1/16 in. stainless steel tubes while a 1/8 in. exhaust ensures an effective removal. The dome of the DRIFTS chamber is equipped with a pair of 13 mm × 3 mm circular ZnSe windows. Before the DRIFTS analysis, the sensor was kept under N2 overnight at room temperature and, later, flash-heated up to 475 K in a 2 min ramp to eliminate impurities coming from the exposition to the atmosphere and from the screen-printing deposition. XPS spectra are obtained with a VG Escalab 210 spectrometer using a hemispherical analyser operated at 20 eV constant pass energy. Mg KR radiation (1253.6 eV, 400 W) was employed as the excitation source. O1s spectra are collected at 0.1 eV step in the (526-536 eV) range, and 10 scans were accumulated in each run. Samples (pure CdGeON powder) were either heated under vacuum in a preparation chamber (base pressure 5 × 10-7 Torr) for 1 h or in an external furnace under synthetic air flow (1 h) and later transferred to the preparation chamber. Equipment supplied software was used for data analysis. UV-vis spectra are collected in a Shimadzu 2101PC UV-vis scanning spectrophotometer using an integrating sphere accessory for powdered samples. In every case the 200-850 nm region was scanned every 0.5 nm. When necessary, samples were diluted in BaSO4 to prevent detector saturation, and in these cases, the same compound was used as background. Sample treatment (synthetic air or N2 at 535 K) was performed in a controlled environment furnace over the pure solids. N2 and synthetic air employed had a purity above 99.9990%, and they were used without further purification.
Results In order to investigate the influence of synthetic air over the samples, they were first heated up to 535 K under N2 and analyzed; later N2 was replaced by synthetic air and the procedure repeated. For reversibility purposes, a final treatment under N2, after synthetic air exposure, was performed. For clarity, results are presented in separated sections.
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1. DRIFTS Spectroscopy. 1.1. Evolution under N2. After the initial cleaning treatment, the sensor was heated at several temperatures under N2 flow (20 mL/min), and resistance measurements were obtained after a 15 min stabilization period. Relative values (R/Ro, Ro being the value obtained at 535 K in N2) are plotted versus temperature in Figure 2A (triangles). Once the maximum temperature (535 K) was reached, the sensor was cooled down to room temperature via the temperature sequence indicated in Figure 2A by circles. As observed, the heating and the cooling sequences are not coincident, which apparently means some degree of hysteresis in the electrical response of CdGeON versus temperature. If the sensor is again heated and cooled, the cooling sequence (circles in Figure 2A) is exactly reproduced in both senses. It is no longer possible to recover the initial behavior (triangles) by simply heating or cooling under N2 flow. DRIFTS spectrum in the 475-1350 cm-1 range after heating in N2 at 535 K (trace a) is presented in Figure 3. For traces a and b, the spectrum at room temperature corresponding to the sample after the initial cleaning process is used as background. In general, the DRIFTS spectrum obtained before any treatment will be used as background. This way small changes are better characterized and negative peaks mean eliminated species while positive ones indicate generated species. With respect to room temperature, broad peaks at 1130 and 1070 cm-1, a doublet at 810 and 780 cm-1, and a strong band at 580 cm-1 are removed by heating at 535 K under N2. However, the same spectrum remains (trace b) when the sensor is cooled in N2 to room temperature afterward. The irreversible elimination of these bands is parallel to the conductivity jump observed at room temperature in Figure 2A (triangles vs circles). Conductivity values (1/R) for the N2 treated sensor follow an Arrhenius-type relationship T/R ) A exp(-Ea/kT) versus temperature (T). This behavior is typical for ionic conductivity mechanisms, as already described for GeO2 solid solutions.4 The Arrhenius plot for the heating sequence (triangles) is also included in Figure 2A. The slope of both the heating and cooling (not shown) sequences are the same, resulting in Ea ) 0.205 eV. For the initial heating process, resistance at 535 K is clearly out of the linear trend and at this temperature is when the DRIFTS spectrum is modified by the elimination of bands at 870 and 590 cm-1. However, in the cooling sequence the whole set of data fits the Arrhenius plot and, accordingly, no substantial change in the DRIFTS spectrum versus temperature is observed (Figure 3, trace b). Preexponential factor (A) for the cooling series (Ac ) 881 K/Ω) is higher than the one for the initial heating sequence (Ah ) 373 K/Ω). If we consider this factor to be proportional to the number of charge carriers, then their concentration after heating in N2 is increased. However no change in the nature of charge carriers can be expected since the activation energy is the same for both sequences. 1.2. Evolution under Synthetic Air. After the sample was cooled to room temperature, N2 atmosphere was replaced by a synthetic air flow (20 mL/min) in the DRIFTS chamber. After a 1 h stabilization period at room temperature, the evolution of resistance versus temperature is plotted in Figure 2B by filled squares. As observed, after a soft decay below 350 K, the resistance of CdGeON sensors strongly increases with temperature under synthetic air flow. This behavior is clearly different from the results obtained under N2 and presented in Figure 2A. DRIFTS spectrum after synthetic air exposure at 535 K is included in Figure 3 (trace c). In this case, the (4) Rulmont, A.; Tarte, P.; Winand, J. M.; Almou, M. J. Solid State Chem. 1992, 97, 156.
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Figure 2. CdGeON conductivity dependence with temperature and atmosphere composition: (A) initial heating and following cooling ramps and (B) exposure to synthetic air (sa) and restoration under N2 (in this latter case, values are reduced by a 10-3 factor to fit the scale). Resistance values (R) are referred to the one obtained under N2 at 535 K (Ro). Conductivity-temperature Arrhenius relationships are also included for the two heating sequences under N2.
Figure 3. DRIFTS spectra corresponding to (a) CdGeON sensor initially heated to 535 K under N2, (b) cooled to room temperature under N2 flow, (c) heated in synthetic air at 535 K, and (d) again heated in N2 at 535 K.
spectrum corresponding to the sample previously heated and cooled under N2 flow is employed as background. Broad bands around 1000 cm-1 and well-defined peaks at 795 and 570 cm-1 are generated, meaning that species responsible for these peaks are now present in the solid. The appearance of these peaks at 535 K is accompanied by a sharp increase of the resistance of the sensor, Figure 2B. If the sample is cooled to room temperature still under synthetic air flow, the DRIFTS spectrum remains unchanged, even if N2 is readmitted to the chamber at room temperature (spectra not shown). However, compared
with the initial value recorded in Figure 2B (filled square), the resistance increases dramatically after such a temperature reduction under synthetic air (open triangle). Thus, resistance values obtained in the heating sequence under synthetic air are not reversibly reproduced when temperature is lowered. 1.3. Readmission of N2. If N2 is again flowed at room temperature over the sample resulting from the previous experiment and the temperature is increased, the resistance of the sensor drops until values corresponding to the primitive heating sequence in N2 are achieved (triangles in Figure 2B and 2A). Restoration of the sensor conductivity under N2 after synthetic air treatment occurs above 440 K, and it is parallel to the disappearance of both the doublet at 810 and 780 cm-1 and band at 580 cm-1 (trace d in Figure 3). When the CdGeON sensor is heated under N2 flow after exposure to synthetic air at 535 K, only resistance values obtained below 440 K fit the Arrhenius relationship but, in this case, with a higher activation energy (Ea ) 0.362 eV) (Figure 2B). Again deviation from linearity is observed when DRIFTS spectra are modified by elimination of bands at 810, 780, and 580 cm-1. In summary, this section shows the differentiated electrical response of CdGeON sensors under N2 and synthetic air, and also their irreversible behavior versus temperature and their dependence from the sample history. Besides, a very clear and direct relationship between such a differentiated pattern and the modification of the DRIFTS spectrum exists. These experimental observations will be later recalled and explained in the Discussion section. 2. XPS Spectroscopy. X-ray photoelectron spectroscopy (XPS) can provide both quantitative and qualitative information about changes induced in the CdGeON sensor after exposure to synthetic air. The sample has been studied after evacuation at room temperature, evacuation at 535 K for 1 h, exposure to synthetic air at 535 K (1 h), and subsequent evacuation at 535 K. In these cases evacuation is supposed to substitute the treatment under N2. Quantitative analysis is done over the areas of Cd3d, Ge3d, O1s, and N1s peaks and with sensitivity factors
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Figure 5. UV-vis spectra in the 200-850 nm region for CdGeON sensors treated under N2 and synthetic air at 535 K. CdO, GeO2 (cristobalite), and GeO2 (rutile) references are also included. Figure 4. O1s XPS region for CdGeON (a) after evacuation at 535 K for 1 h and (b) after exposure to synthetic air in the same conditions. Table 1. Parameters for the O1s XPS Peak-Fitting Algorithm component
BE (eV)
fwhm
G/L
OI OII
531.3 530.3
2.30 1.40
0.49-0.58 0.38-0.47
Table 2. OII/OI Ratio Obtained from XPS after Successive Treatments of the CdGeON Sensor treatment
OII/OI
evacuated room temperature evacuated 535 K synthetic air 535 K evacuation 535 K
0.19 0.15 0.20 0.17
reported in the literature.5 The amounts of Cd, Ge, and N were found to remain unchanged despite the successive treatments. However, the sample heated under synthetic air is slightly enriched in oxygen with respect to the evacuated one. The overall increment is about 1.7%. No binding energy shifts are observed along the whole series of experiments. Only the shape of the O1s peak is significatively altered as a consequence of synthetic air treatment. Figure 4 shows that two oxygen peaks, a more intense component at 531.3 eV (OI) and a weaker one at 530.3 eV (OII), are necessary to properly fit experimental data. O1s analysis was done very carefully, trying to achieve the best fit. Best results were obtained after a Shirley baseline correction and with a combination of symmetric Gauss and Lorentz functions. Fitting parameters for both components along the treatment series are compiled in Table 1. While OI component remains unchanged, the contribution of OII is modified by the sample treatment. In Table 2 the OII/OI ratio is presented as a function of the sample manipulation. As observed, the oxygen uptake after synthetic air exposure at 535 K is directly reflected into (5) Briggs, D., Seah, M. P., Eds. Practical Surface Analysis; John Willey & Sons Ltd.: New York, 1983.
the OII component. Accordingly, oxygen is removed by evacuation at 535 K from the OII contribution. 3. UV-Vis Spectroscopy. UV-vis spectrometry is a suitable method for detecting changes in the coordination polyhedron in systems absorbing in the visible-ultraviolet region. This is the case of the CdGeON sensor. GeO2 and CdO also have absorption peaks in UV-vis; this way GeO2 and CdO (already known structures) can be used as reference patterns to explain the changes observed in CdGeON after exposure to synthetic air. Figure 5 shows the UV-vis spectra corresponding to the sensor after N2 and synthetic air treatment at 535 K for 1 h. In this case, CdGeON powder before any manipulation is used as background to better characterize treatment-induced changes. CdO and GeO2, in two varieties, rutile and cristobalite spectra, referred to BaSO4, are also included. As observed in Figure 5, a very broad band in the 500600 nm range is generated in the CdGeON solid upon exposure to synthetic air at 535 K. However, the same band is eliminated when the original CdGeON powder is heated under N2 at 535 K. Both CdO and GeO2 (rutile) have strong absorption peaks in that range. So, from UVvis data, the band at 500-600 nm in the synthetic air treated CdGeON sensor can be explained by an increment in the proportion of Ge-O or/and Cd-O polyhedra with the GeO2 rutile and CdO arrangement, respectively. Discussion Main infrared bands corresponding to GeO2 and to germanates, i.e. asymmetric and symmetric stretching of germanium-oxygen-bridged bonds (Ge-O-Ge), are respectively located in the 880-890 and 520-580 cm-1 ranges.6-10 Also, weak features at 1050 and 1100 cm-1 have been described.11 The doublet around 800 cm-1 and the band at 580 cm-1 observed in our DRIFTS spectra (6) Farmer, V. C. The Infrared Spectra of Minerals; Mineralogical Society: London, 1974; p 484. (7) Murthy, M. K.; Kirby, E. M. Phys. Chem. Glasses 1964, 5, 144. (8) Cheng, J.; Xu, R. J. Chem. Soc., Chem. Commun. 1991, 483. (9) Cheng, J.; Xu, R.; Yang, G. J. Chem. Soc. Dalton Trans. 1991, 1537. (10) Reynoso, V. C. S.; Barbosa, L. C.; Alves, O. L.; Aranha, N.; Cesar, C. L. J. Mat. Chem. 1994, 4, 529. (11) Margaryan, A. A. J. Mat. Sci. Lett. 1993, 12, 230.
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are, despite the frequency difference, tentatively assigned to these stretching modes. A similar decrease (about 100 cm-1) in the frequency of the asymmetric (Ge-O-Ge) stretching with respect to the GeO2 (880 cm-1) was early described by Murthy and Kirby in K2O-GeO2 systems.7 The frequency shift is accompanied by a split up of the band into a doublet at 740 and 810 cm-1. They explained this phenomenon on the basis of the generation of extra nonbridged (Ge-O) bonds in the GeO2 network induced by the presence of K2O. The possibility of ascribing the observed DRIFTS features to nitrogenated species removal is discarded since Ge-N stretching frequency has been reported at 720 cm-1 12 and no nitrogen loss is detected by XPS. The density of synthesized CdGeON is d ) 5.63 g/cm3; however, the theoretical value obtained from the Cd0.98GeO1.16N1.21 formula distributed in an orthorombic network with lattice parameters a ) 5.646 Å, b ) 6.844 Å, and c ) 5.392 Å should be d ) 6.58 g/cm3.1 The presence of both cationic and anionic vacancies is proposed to account for such a density difference. Thus, the stoichiometry of the CdGeON compound can be described by the formula Cd0.71Ge0.7290.57O0.84N0.8700.29 (9 cationic vacancy, 0 anionic vacancy). The explanation of our results is as follows: initial exposure of the sample to air, screen-printing deposition and the specific treatment under synthetic air partially fill anionic vacancies with oxygenated species. Such oxygenated species should be directly coordinated to one Ge atom in a nonbridging position, as indicated by the apparition of the doublet at 780 and 810 cm-1 in the DRIFTS spectra and in concordance with the model proposed by Murthy and Kirby.7 Germanium coordination number is then increased from formally four up to six. Nonbridged (Ge-O) bonds are eliminated at 535 K in N2 as showed by the disappearance of the doublet, Figure 3 (spectrum a), thus restoring the original oxygen-bridged (Ge-O-Ge) structure in the solid. The presence of an inert environment (N2) prevents vacancy refill by residual oxygen, this way explaining the similarities between spectra a and b in Figure 3 and the hysteresis observed in the conductivity-temperature curves (Figure 2A). Incorporation of oxygen to the solid framework is proven from XPS quantification. Also the coordination in nonbridging position can be inferred from the selective modification of the OII (530.3 eV) contribution along the successive sample treatments (Figure 4 and Table 2). The increment in the germanium coordination number induced by exposure of CdGeON to synthetic air is also derived from UV-vis spectra. Germanium is tetrahe(12) Vicalrromero, J.; Marques, F. C. J. Appl. Phys. 1994, 76, 615.
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drically coordinated in GeO2 (cristobalite) while the arrangement is octahedrical in GeO2 (rutile), showing an absorption band at 510 nm in the UV-vis spectrum (Figure 5). The broad band at 500-600 nm observed in CdGeON samples exposed to synthetic air can then be ascribed to the increase in the Ge-O coordination number. The appearance of the 500-600 nm UV-vis band can also account for anionic vacancy filling around Cd ions. Cd is also tetrahedrically coordinated in the CdGeON framework. Incorporation of oxygen in octahedrical positions would resemble CdO structure (NaCl type), which has a strong absorption in the 500-550 nm range (Figure 5). Unfortunately, in this case a more detailed structural discussion is impeded by the absence of Cd-O peaks in the DRIFTS spectra. Regarding the electrical behavior, the conductivity of the CdGeON sensor is improved after oxygenated species removal from anionic vacancies at high temperature in N2 (Figure 2). The increase in the preexponential factor (A), obtained from the Arrhenius plots in these conditions, points to a higher number of charge carriers available. The conduction mechanism can then be described as anionic vacancy migration through the CdGeON framework as an activated process with Ea ) 0.205 eV. This value is indeed in the order of magnitude of previously reported activation energies for good ionic conductors having an ordered channel structure.13 Admission of synthetic air fills vacancies in the CdGeON framework and causes a dramatic reduction of the sensor conductivity. Subsequent heating under N2 reversibly eliminates oxygenated species from vacancies leading to the recovery of conductivity values observed upon initial exposure to N2. With respect to an empty vacancy state, a vacancy filled situation causes both a smaller preexponential factor (A ) 69 vs 881 K/Ω) and a higher activation energy (Ea ) 0.362 vs 0.205 eV). So, a decrease in the number of charge carriers (smaller preexponential factor) as well as a modification of carrier migration pathway (higher activation energy) can be argued for synthetic air treated CdGeON sensors. However, the presence of extra oxygen ions within the sensor framework can also modify the electronic structure of the solid accounting for the observed differences in these parameters. To confirm the above hypothesis, studies about the conduction mechanism under different oxygen partial pressures using ac techniques are in progress. LA950207J (13) Whittingham, M. S.; Huggins, R. A. In Solid State Chemistry; NBS Special Publication; 1972; Vol. 364.
