Cr2−xTixO3 (x ≤ 0.5) as CH3COCH3 sensitive resistors

Sensors and Actuators B 125 (2007) 550–555

Cr2−xTixO3 (x ≤ 0.5) as CH3COCH3 sensitive resistors Suman Pokhrel 1 , Yang Ming, Lihua Huo ∗ , Hui Zhao, Shan Gao Laboratory of Functional Materials, School of Chemistry and Materials Science, Heilongjiang University, 150080 Harbin, PR China Received 16 October 2006; received in revised form 28 February 2007; accepted 28 February 2007 Available online 12 March 2007

Abstract Fine powders of Cr2−x Tix O3 (x = 0–0.5, CTO) were prepared by combustion technique, followed by heat treatment at 1000 ◦ C in ambient air. The XRD results showed the presence of a solid solution of TiO2 in Cr2 O3 as the major phase along with CrTiO3 as the minor phase for x ≥ 0.4. EDAX studies on Cr2−x Tix O3 (x = 0.1 and 0.3) suggested the composition of the oxides close to the theoretical values. The XPS studies showed the presence of Cr3+ and Ti4+ species on the surface. Ti4+ inclusion in the eskolait Cr2 O3 had significant influence on the activation energy for conduction (Ea ) at 100–700 ◦ C in air. The commercial Taguchi sensors coated with CTO, when subjected to acetone in ambient air, showed an abrupt change in resistance within 10 s and took the same time for recovery at 400 ◦ C. Different humidity levels were achieved by various water vapor buffers thermostated at room temperature where sensor response decreased nearly by 40% at 80% RH. The stability test indicated the first order exponential decay to a constant sensor potential value of 4.7. The origin of the gas response was attributed to the surface reaction of (CH3 )2 CO to form an intermediate geminal diol, followed by formation of an unstable ethanoyl radical and then final product CH3 COOH, which was desorbed as a gas at 400 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Cr2−x Tix O3 ; Acetone sensing; Sensing mechanism

1. Introduction The sensor technology of semiconducting oxides has been progressing abruptly in the last few years. Advances in the fabrication methodology and materials technology have given stable, simple, robust and reliable devices [1]. Theoretical understanding of the surface processes and a detailed, atomistic understanding of the elementary processes of the sensor behavior have brought a total revolution in the sensor industries [2]. New ways of using the devices, which turn their general lack of selectivity into an advantage rather than a disadvantage, are being developed and are benefiting from the improvements in stability and reliability delivered by the new technology [1,3]. After the introduction of SnO2 as a gas sensing materials in 1960s, it has been used extensively in different forms to achieve better stability, short- and long-term performance, less influence of humidity, etc. [4,5]. To overcome these problems, earlier reports on gas sensors based on Cr2−x Tix O3 (CTO)

∗ 1

Corresponding author. Tel.: +86 451 86608737; fax: +86 451 86682541. E-mail address: [email protected] (L. Huo). He is a Humboldt Research Scholar in Tuebingen University, Germany.

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.02.055

[6,7] generated a new insight in the sensor technology. Following this, several other reports are available on the CTO sensors for H2 S, CO, alcohols, NH3 , LPG, and hydrocarbon [8–17]. Recently, a few more papers in which better performance of this material was exploited for large-volume commercialization [18–20]. The substitution of Ti in the eskolait Cr2 O3 strongly decreases the proportion of surface high-valency chromium ions of the porous sintered bodies [2]. This effect controls the gas sensing behavior in combination with the surface segregation of Ti4+ ions [9]. Defect models of the surfaces, assessed by computational modeling with and without Ti, show stable defect pairs which are surface segregated and contribute to the relatively high p-type conductivity at elevated temperatures [21]. Distortion of the arrangement of surface oxygen above the Cr vacancy after introduction of Ti4+ creates a possible binding site, while the high-valency surface cation creates another site. This results in creation of two binding sites to promote the dissociation of oxygen and the surface reaction needed for gas sensing [8]. In recent years, VOC’s emitting materials such as building and interior materials, paints, electric appliances and furniture are in large consumption. Sick house syndrome is a kind of

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disorder caused by long exposure to these gases [22] especially for a diabetes people. In the present investigation we report Ti4+ doped Cr2 O3 as a highly sensitive acetone sensor and its sensing mechanism for indoor application. 2. Experimental 2.1. Powder preparation [Cr(NO3 )3 ·9H2 O] (0.1 M), titanium(IV) n-butoxide (0.1 M) and conc. HNO3 (0.5 mL) were mixed and stirred for 10 min at 80 ◦ C to obtain a clear green solution. Ethylene glycol (0.9 mL) was added to a citric acid solution (99.5%, 4.0 g dissolved in 2 mL hot water) followed by heating at 100 ◦ C to obtain a viscous gel. The green solution mixture was then added to the gel and heated at 100 ◦ C for 0.5 h [23,24]. A resin-like solid product was formed 1 h after the temperature was raised to 110 ◦ C. The solid product was calcined in two steps: first at 250 ◦ C for 1 h to obtain a greenish glassy powder and then at 1000 ◦ C at a heating rate of 2 ◦ C/min for 12 h to obtain a dark green powder.

Fig. 1. Powder XRD patterns of Cr2−x Tix O3 (x = 0–0.5). (*) indicates the low intensity CrTiO3 peaks (JCPDS card 33-408).

3. Results and discussion 2.2. Powder characterization and sensor testing 3.1. Powder XRD and electrical properties X-ray powder diffraction (XRD) pattern of the sample annealed at 1000 ◦ C was recorded in the region 2θ = 10–80◦ with a scanning speed of 4◦ min−1 on a Rigaku diffractometer (Model ˚ radiation within D/MAX-3B, Japan) using Cu K␣ (1.5046 A) mass% detection threshold of impurity phases. EDAX analyses of the powders were performed using cobalt as a reference standard. The samples after heat treatment were analyzed by Xray photoelectron spectroscopy (XPS, VG Scientific ESCALAB MK II, Thermo Avantage V 3.20 analyzer). The radiation source used was Al K␣ (1486.6 eV) calibrated at C 1s binding energy (284.6 eV) with standard deviation of 0.2 eV. The vacuum level during XPS measurement was 2.6 × 10−9 mbar. Survey scans were performed up to 1000 eV at a rate of 1 eV/s and detailed scans for Cr 2p, Ti 2p and O 1s were obtained with a 0.1 eV/s step. The spectra were smoothened using Origin graphical software. The DC resistance measurements were performed with a Keithley instrument interfaced with a PC and analyzed using test point software. The gas response of the material was determined using a digital electrometer (Model RQ-2, China). The sensor used for response measurements comprised of a commercially available Taguchi Gas Sensors (TGS) fabricated on a cylindrical Al2 O3 tube of 4 mm length with two permanent gold coatings spaced 1 mm. The sensor fabrication and testing was performed similarly as described in our previous report [18]. The controlled humidity environments to measure the sensor response at different humidity levels were achieved using saturated solutions of Ca(NO3 )2 ·4H2 O (50% RH) and NH4 Cl (80% RH) in a closed 5 L test chamber thermostated at room temperature [25]. The relative gas response (S) of the CTO-sensors was determined using the relation S = (Rg − Ra )/Ra , where Rg and Ra are the resistance in acetone-containing atmospheres balanced with air and in clean air, respectively.

Typical XRD patterns of samples Cr2−x Tix O3 (x = 0.1–0.5) after heat treatment at 1000 ◦ C are shown in Fig. 1. The patterns of the CTO (x = 0.1–0.5) were identical to that of Cr2 O3 (x = 0), indicating formation of a solid solution of TiO2 in Cr2 O3 . This behavior is expected as both Cr3+ and Ti4+ ions have the same ionic radius (0.061 nm). Chabanis et al. [14] reported the presence of a second minor phase corresponding to CrTiO3 (JCPDS card 33–408) for CTO with x ≥ 0.2 at 2θ = 54.35◦ . Similarly Niemeyer et al. [2] reported that this second CrTiO3 phase could just be discernable for x = 0.3 and was prominent at x = 0.5. However, in the present investigation this phase was not found to be prominent even up to x = 0.5 but a very low intense peak at 2θ = 28.30◦ corresponding to the CrTiO3 was observed for x ≥ 0.4. The chromia-rich TiO2 –Cr2 O3 system reported by Somiya et al. [26] corresponded to a nominal composition of Cr1.8 Ti0.2 O3−δ with CrTiO3 and Cr2 O3 . Subsequently, Nagai and Ohbayashi [27], in their sintering studies on Cr2 O3 using TiO2 as an additive, suggested the formation of solid solutions with very low titania concentration. The present study shows that the limiting solubility of the solid solution is more than 20 mol% of TiO2 (Cr1.6 Ti0.4 O3 ) at 1000 ◦ C. The mixing of the Cr3+ and Ti4+ ions from the combustion route provides a more uniform Ti distribution than the conventional grinding mixing and sintering process. The EDAX study of Cr1.9 Ti0.1 O3 and Cr1.7 Ti0.3 O3 (Fig. 2) showed the Ti/Cr ratios to be 0.045 and 0.154, respectively. These values closely agree with the theoretical data [9]. XPS data of the Cr 2p, Ti 2p and O 1s core levels of the CTO (x = 0.2) powder are presented in Fig. 3. The variation of the impurity distribution of the surface was not detected. The Cr 2p3/2 and Ti 2p3/2 peaks of all the CTO powders at around 577.0 and 458.0 eV are attributed to Cr3+ and Ti4+ , respectively.

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al., in their electrical conductivity study [12] on these same compositions as ours, suggested the same activation energy value of 0.40 eV irrespective of the Ti4+ addition. Moreover, the report stated that Cr2 O3 eskolait also exhibited the same activation energy. Thus, in the present investigation we obtained different results from theirs. 3.2. Sensing characteristics and humidity dependence

Fig. 2. EDAX patterns of (a) Cr1.7 Ti0.3 O3 and (b) Cr1.9 Ti0.1 O3 .

The DC conductivity of Cr2 O3 and Cr2−x Tix O3 (x = 0.1–0.5) was assessed as a function of temperature in the range of 100–700 ◦ C in air. All the compositions showed linear plots, and from the slopes of the plots, the activation energies (Ea ) for the conduction process were calculated (Fig. 4). The activation energy of Cr2 O3 was found to be 0.17 eV. Interestingly the activation energy of Cr1.9 Ti0.1 O3 increased abruptly to 0.45 eV and then decreased with an increase in molar composition of Ti4+ . This may be due to the presence of CrTiO3 having considerable influence on the conductivity of the composites. Jayaraman et

Fig. 3. Cr 2p, Ti 2p and O 1s XPS spectra of CTO (x = 0.2) powder after heat treatment at 1000 ◦ C.

The change in resistance of the Cr2−x Tix O3 (x = 0–0.5) thick films fabricated at 850 ◦ C and tested at 400 ◦ C with 1–5 ppm of acetone is presented in Table 1. It was observed that the change in resistance of Cr2 O3 in presence of imposed acetone was negligible. The relative response of the sensors was found to be maximum for x = 0.1 and minimum for x = 0.5 to all the acetone concentrations tested (Fig. 5a). As aforementioned, the same trend was observed for the DC conductivities of these composites. As a result the inclusion of Ti4+ had a significant effect on the sensor performance (Fig. 5b). The change in resistance (from air to imposed acetone) of all the composites was found to be within 10 s and the same baseline resistance was recovered within the same time after flowing the clean air through the test chamber (Fig. 5c). However, when the sensors were exposed to acetone before attaining the working temperature (400 ◦ C) the baseline resistance was not attained. To overcome this problem, the sensors were kept at 400 ◦ C at least for 3 h before each trial (sensor conditioning and calibrating) during measurements for the baseline stability. In all the trial to trial and measurement to measurement cycles, the baseline resistance was achieved which may be due to dehydration of water molecules adsorbed on the surface, providing the wider surface area for the analyte gas adsorption and desorption process at the working temperature. In addition, it was shown that the sensing performance of these composites was considerably influenced by the humidity of the environment. The sensor relative response of Cr2−x Tix O3 decreased by a factor of 21.7% and 38.7% after exposing to 50 and 80% RH, respectively (Fig. 5d). However, the response and recovery characteristics remained unchanged. Chabanis et al. [14] reported that humidity had no significant effect on the alcohol sensing properties of Cr1.8 Ti0.2 O3 , but in the present investigation we observed the reduction on the sensing potential of these sensors to acetone nearly by 40%. However, even after reduction, the relative response of the sensor was high enough for a good sensor categorization. Thus, these sensors can easily be operated under 5–100% RH conditions. 3.3. Sensor stability

Fig. 4. DC resistance measurements of Cr2−x Tix O3 (x = 0–0.5) and the Arrhenius plots for activation energy determination.

The sensor stability test was performed by placing the sensors in the test chamber with 1 ppm of acetone for 144 h and measuring the changes in resistance after each 24 h. The decrease in sensor relative response was in the form of first order exponential decay (Fig. 6) for all the composites. The sensor relative response was found to decrease by a factor of 34.6, 28.9 and 24.9% for x = 0.1, 0.2 and 0.3, respectively. However, after 144 h, all the sensors after being exposed to1 ppm acetone, showed almost constant response values to acetone. The sensors were

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Table 1 Resistance and sensitivity determination of Cr2−x Tix O3 (x = 0–0.5) thick film sensors at different acetone concentrations Sample

Gas concentration (ppm)

Ra (M)

Rg (M)

Cr2 O3

1 2 3 4 5

65 65 65 65 65

70 71 72 73 74

Cr1.9 Ti0.1 O3

1 2 3 4 5

49 49 49 49 49

388 529 600 650 739

Cr1.8 Ti0.2 O3

1 2 3 4 5

68 68 68 68 68

Cr1.7 Ti0.3 O3

1 2 3 4 5

(Rg − Ra )/Ra 0.07 0.09 0.10 0.12 0.14

Response time (s)

Recovery time (s)

Activation energy (eV)

– – – – –

– – – – –

0.17

6.9 9.8 11.2 12.3 14.0

9 9 9 8 9

9 8 8 9 9

0.45

519 643 716 776 830

6.6 8.5 9.5 10.4 11.2

10 5 7 6 9

10 5 9 10 7

0.44

64 64 64 64 64

357 467 538 579 611

4.6 6.3 7.4 8.0 8.5

9 8 9 8 9

8 8 9 8 9

0.37

Cr1.6 Ti0.4 O3

1 2 3 4 5

13 13 13 13 13

86 94 103 109 118

5.6 6.2 6.9 7.4 8.1

9 9 8 9 9

9 9 9 9 9

0.32

Cr1.5 Ti0.5 O3

1 2 3 4 5

64 64 64 64 64

343 408 457 487 530

4.4 5.4 6.1 6.6 7.3

7 8 9 9 9

9 9 9 8 8

0.31

Fig. 5. (a) The plot of sensor relative response as a function of acetone concentration for Cr2−x Tix O3 (x = 0–0.5). (b) The multiple plots of sensor relative response and activation energy changes vs. the molar composition of titanium. (c) Change in resistance as a function of time exhibited by Cr1.8 Ti0.2 O3 exposed to 2 and 3 ppm of acetone. (d) The change in resistance as a function of time exhibited by Cr1.8 Ti0.2 O3 exposed to 4 ppm of acetone in the presence of air, 50 and 80% RH.

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O(ads) ↔ O(ads) − + h+

(c)

H2 O ↔ OH(ads) + H+ + e−

(d)

(CH3 )2 CO(g) + H+ ↔ (CH3 )2 COH(ads) +

(e)

(CH3 )2 COH(ads) + + H2 O + O(ads) − ↔ CH3 COHOH(ads) + CH3 OH

Fig. 6. The first order exponential decay of the sensor relative response of Cr2−x Tix O3 (x = 0.1, 0.2 and 0.3) exposed to 1 ppm of acetone for 144 h.

(f)

CH3 COHOH(ads) ↔ CH3 CO∗ + H+ + OH(ads)

(g)

CH3 CO∗ + OH(ads) ↔ CH3 COOH(ads)

(h)

2OH(ads) ↔ H2 O(g) + O(ads) − + h+

(i)

CH3 COOH(ads) ↔ CH3 COOH(gas) + vacant sites

(j)

4. Conclusion conditioned before the test and it was observed that the sensors were able to attain their baseline resistance. This suggests that these sensors could be operated at high/low concentration with continuous monitoring of acetone. 3.4. Sensing mechanism The high sensing performance of the CTO thick film to acetone adsorption and desorption is explained with the following mechanism. (a) Ti-doped chromia is a p-type electronic conductor and generates near surface chromium vacancies with Ti4+ dissolution into Cr2 O3 in the solid solution [21]. (b) The adsorption of molecular oxygen takes place on the surface either through surface oxygen vacancies or with near-surface chromium vacancies. (c) The charge is held on the adsorbed oxygen originating from the metal center (Cr) and covers the oxide surface developing a potential difference on the surface due to charge migration across the interface between the oxygen and the sensor surface. (d) The water molecule approaching the surface produces adsorbed hydroxyls with the liberation of an electron. (e) The protonation of acyl oxygen accentuates the positive charge on the acyl carbon, making it more attractive to a water molecule. The water molecule will attach itself to the acyl carbon using a lone pair on the oxygen atom of water, making it more positively charged. (f and g) Dissociative chemisorption of this charged species with surface oxygen forms a surface geminal diol which undergoes internal rearrangement to form an unstable ethanoyl radical and strongly chemisorbed surface hydroxyl species, releasing surface trapped electrons and methanol [28] to neutralize bulk holes, and hence giving electrical response. (h) The nucleophilic attack of surface hydroxy species with ethanoyl radicals produces acetic acid [29]. (i) The surface hydroxys are assumed to be neutral species with respect to the lattice because the observed electrical effect of an increase in water vapor pressure on the surface is to increase the resistance, i.e., to release surface-trapped electrons. (j) Desorption of weakly bound CH3 COOH from the surface, providing a new vacant site for the surface approaching acetone molecule. These reactions processes can be expressed as below: O2(g) ↔ O(ads) + O(ads)

(b)

An extensive investigation on acetone sensing characteristics of CTO powders (x ≤ 0.5) prepared by combustion technique was been carried out. The linear increase in the sensor response as a function of concentration, and very quick response and recovery mark these materials to be excellent candidates for acetone monitoring. Though they have humidity dependency to almost 40%, the relative response exhibited is greater than 6 in the concentration range of 1–5 ppm. The exponential decay of the sensor relative response comes to a constant value after 80 h exposure to 1 ppm of acetone. This result indicates the probability of the sensor operation for several hundred hours with outstanding performance and continuous usage. The gas response is believed to be due to surface catalytic reaction of acetone with the O(ads) − forming a geminal diol and an unstable ethanoyl radical as an intermediate product and then desorbing as an acetic acid at the working temperature of 400 ◦ C. The results on the CTO obtained through combustion technique are very promising for the preparation of a sensitive and low cost acetone sensor. Acknowledgements We thank the support from the National Natural Science Foundation of China (20101003), Key Project of Natural Science of Heilongjiang Province (ZJG0602-02) and Fund for Outstanding Youth of Heilongjiang Province. References [1] D.E. Williams, Semiconducting oxides as gas-sensitive resistors, Sens. Actuators B 57 (1999) 1–16. [2] D. Niemeyer, D.E. Williams, P. Smith, K.F.E. Pratt, B. Slater, C. Richard, A. Catlow, A.M. Stoneham, Experimental and computational study of the gas-sensor behavior and surface chemistry of the solid-solution Cr2−x Tix O3 (x ≤ 0.5), J. Mater. Chem. 12 (2002) 667–675. [3] S.C. Naisbitt, K.F.E. Pratt, D.E. Williams, I.P. Parkin, A microstructural model of semiconducting gas sensor response: The effects of sintering temperature on the response of chromium titanate (CTO) to carbon monoxide, Sens. Actuators B 114 (2006) 969–977. [4] N. Bˆa.rsan, U. Weimar, Understanding the fundamentals principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity, J. Phys. Condens. Matter. 15 (2003) R813–R839.

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Biographies Suman Pokhrel graduated as a physical chemist in 1996 from Tribhuwan University, Nepal and PhD in material chemistry in 2005 from University of Madras, India. He was a Jawaharlal Nehru Research Fellow during his doctoral work. He then joined the School of Chemistry and Materials Science, Heilongjiang University, Harbin, China as overseas postdoctoral researcher in 2005–2006. Presently he is a Georg Foster Research Fellow under Alexander von Humboldt Foundation in the Department of Theoretical and Physical Chemistry, T¨ubingen University, Germany. His research interest includes nano-structured solid-state oxide materials for various applications such as chemical and optical sensor, fuel cells and designing the precursors for CVD applications through thermal characterization. Yang Ming is a graduate student in the School of Chemistry and Materials Science, Heilongjiang University. His research deals with the preparation and investigation on the sensing property of composite oxides. Lihua Huo received her PhD in physical chemistry from Fuzhou University, China, in 1997. She is a professor of School of Chemistry and Materials Science, Heilongjiang University. Her research interests are mainly directed towards the development of nano-particle and organic–inorganic composite materials and their application as gas sensors. Hui Zhao received his PhD from Department of Chemistry, Jilin University, China, in 1999. He is a professor of chemistry at School of Chemistry and Materials Science, Heilongjiang University. His working interests are the study of oxide electrolyte and electrode materials for SOFC and organic–inorganic catalytic materials. Shan Gao finished his postdoctoral work from Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, China, in 2000. He is a professor of School of Chemistry and Materials Science, Heilongjiang University. His research interests are mainly directed towards the aspect of structural chemistry and nano-material.