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Macroscopic and microscopic investigations of the effect of gas exposure on nanocrystalline SnO2 at elevated temperature
Applied Surface Science 234 (2004) 82–85
Macroscopic and microscopic investigations of the effect of gas exposure on nanocrystalline SnO2 at elevated temperature T.G.G. Maffeı¨sa,*, M. Pennya, K.S. Tenga, S.P. Wilksa, H.S. Ferkelb, G.T. Owena a
Multidisciplinary Nanotechnology Centre, School of Engineering, University of Wales Swansea, Faraday Tower, Singleton Park, Swansea SA2 8PP, UK b TU Clausthal, Institut Fuer Werkstoffkunde und Werkstofftechnik Agricolastrasse 6, 38678 Clausthal-Zellerfeld, Germany
Abstract We have investigated the effect of O2 and reducing gases (CH4 and CO) exposure on nanocrystalline SnO2 in vacuum and at elevated temperatures (120 8C) using three different techniques: X-ray photoelectron spectroscopy, in vacuum resistance measurements and scanning tunnelling microscopy and spectroscopy. XPS and resistance measurements showed that O2 chemisorbtion causes an upward surface band bending of 0.2 eVand a resistance increase of 50 MO while CH4 exposure resulted in a 0.1 eV downward band bending and a 20 MO drop in resistance. Stable STM imaging at 120 8C was achieved and clearly resolved the 8 nm particles. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O2 exposure. # 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline particles; SnO2; XPS; STM–STS
Gas sensors based on polycrystalline SnO2 offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime and high selectivity and sensitivity. The sensing mechanism relies on perturbations in surface conductivity induced by chemical reactions between target gases and oxygen species absorbed onto the SnO2 surface. The constant supply of oxygen from the air ensures regeneration of the sensor surface and therefore, long device lifetime. The net effect observed upon exposure to reducing target gases such as CH4 and CO is a drop in the bulk resistance of the polycrystalline sensing film. The sensitivity of the gas sensing layer has been *
Corresponding author. Tel.: þ44 1792 295427; fax: þ44 1792 295676. E-mail address: [email protected] (T.G.G. Maffeı¨s).
shown to increase with decreasing grain size [1,2], therefore nanocrystalline SnO2 is expected to exhibit high sensitivity. In a previous article [3], we demonstrated the effect of O2 and CH4 on the surface electronic properties of SnO2 nanoparticles in ultra high vacuum (UHV) and at elevated temperatures using X-ray photoelectron spectroscopy (XPS). In this article, we compare these previous results with in vacuum resistance measurements done under the same conditions, correlating surface band bending with bulk resistance variations. In addition to these two macroscopic studies of the gas sensing mechanism, we present results from a scanning tunnelling microscopy and spectroscopy (STM– STS) investigation of the same SnO2 nanoparticles at room and elevated temperatures and in the presence of O2 and CO.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.081
T.G.G. Maffeı¨s et al. / Applied Surface Science 234 (2004) 82–85
O1s
486.4
530.25 530.15
486.2
530.05
486.1
529.95
O
Fl as h 2, 10 m O in 2, 20 m O in 2, 30 m O in 2, 60 C m H4 i ,6 n 0m in
486.3
Fig. 1. Binding energy variations of the Sn3d5/2 (left axis) and O1s (right axis) core levels after flash cleaning, incremental O2 exposure and CH4 exposure at 120 8C.
Exposure to O2 at 120 8C reveals core level shifts of 0.2 eV to lower BE, which infers a 0.2 eV increase in surface band bending. This is a consequence of the chemisorbtion of oxidising gases removing electrons from the SnO2, which results in the formation of a depletion layer, upward band bending and, therefore low surface conductivity. This corresponds to the normal operating state of a real gas sensor where oxygen from the air ensures low conductivity. Exposure to CH4 at 120 8C resulted in a 0.1 eV downward band bending at the surface as the reducing agents react with the oxygen. Reducing agents remove the adsorbed oxygen and restore electrons to the conduction band, hence decreasing the resistance of the semiconductor. The changes in the bulk resistance of the SnO2 sample following the same surface treatments as for 60 50 40 30 20 10
2
Fl as 10 h m O in 2 20 s O min 2 30 s O min 2 s 6 C 0m H4 in 60 s m in s
0
O
Fig. 1 shows the variation of the Sn3d and O1s core level binding energy (BE) with the different surface treatments. The fact that the BE of both core levels follows the same trend suggests that these BE shifts are likely to be Fermi shifts corresponding to rigid band bending at the surface, rather than chemical changes.
530.35 Sn3d
Bulk resistance (MOhm)
Flash heating for 1 min at 400 8C, then the temperature was stabilised at 120 8C. Exposure to O2 at 5 105 mb for 10, 20, 30 and 60 min at 120 8C. Exposure to CH4 at 5 105 mb for 60 min at 120 8C.
486.5
Binding energy (eV)
The SnO2 nano-powder was produced by laser ablation of a ceramic rod. The fabrication details are published elsewhere [4]. Two sets of samples were prepared from the nano-powder, one for the macroscopic experiments (XPS and in vacuum resistance measurements) and one for the STM–STS experiments. The samples used in the macroscopic experiments were fabricated by simply evaporating a suspension of the powder onto alumina substrates, followed by annealing in air at 400 8C for 20 min. The substrates are standard gas sensing substrates with two platinum electrodes on the topside used to measure the resistance of the SnO2 layer and a platinum heater on the backside. The sensor was mounted on a specially designed sample holder, allowing heating of the sensor via its own heater [3] during either XPS or resistance measurements. These measurements were conducted at about 108 mb in a VG ESCAlab Mk II using un-monochromated Al Ka radiation (1486.6 eV). The samples used in the STM experiments were fabricated by depositing the 8 nm particles onto Si substrates directly from the nano-powder production plant [5]. The samples were loaded into our Omicron STM/SEM-HC (base pressure: 1011 mb) and scanned with electrochemically etched W tips. The system allows resistive heating of the samples and is equipped with a gas inlet pipe that can be positioned within 1 mm of the sample. This set-up means that the gas pressure on the sample itself is much higher than the pressure measured by the ion gauge. For the XPS and resistance measurements, the samples were subjected to the following surface treatments:
83
Fig. 2. Variation of the bulk resistance of the SnO2 layer after flash cleaning, incremental O2 exposure and CH4 exposure at 120 8C.
T.G.G. Maffeı¨s et al. / Applied Surface Science 234 (2004) 82–85
Fig. 3. 250 nm 250 nm constant current STM image of 8 nm SnO2 particles acquired at a tip voltage of 3.4 V, tunnelling current of 0.6 nA and at a temperature of 120 8C. The grey scale ranges from 0 to 5.1 nm.
(a)
(b) After CO
Log (tunnelling current (nA))
the XPS experiments are shown in Fig. 2 and can be directly compared to the BE shifts. Increasing the temperature from RT to 120 8C causes the resistance to decrease by 8 MO as expected for a semiconductor. Incremental O2 exposure shows a resistance increase of approximately 50 MO that is supported by the 0.2 eV increase in surface band bending detected using XPS and also by the widely accepted theory that adsorbed O2 will remove electrons from the surface, creating a potential barrier and hence increasing the surface resistance. Exposure to CH4 for 60 min decreases the resistance by approximately 20 MO. As mentioned previously, the removal of surface oxygen donates electrons into the conduction band of the semiconductor, lowering the resistance. This finding concurs with the presence of the downward band bending of 0.1 eV revealed from the XPS study. These results convincingly link the variation of surface band bending measured by XPS with the overall resistance of the SnO2 sensing layer. The sample used in the STM–STS experiment were subjected to similar surface treatments as for the XPS and resistance measurements: after loading in the STM chamber, the sample was scanned at RT then heated to 120 8C and scanned before and after exposure to 5 109 mb of O2 for 1 h, and after exposure to 5 108 mb of CO for 1 h. As mentioned before, the pressure at the sample itself is much higher than
dlnI / dln V (a.u.)
84
After O2
120°C
After CO After O2
120°C
RT
RT
-4
-2
0
2
Sample bias (V)
4
-4
-2
0
2
4
Sample bias (V)
Fig. 4. (a) Normalised conductivity spectra of the 8 nm SnO2 particles acquired at RT, 120 8C before gas exposure, 120 8C after O2 exposure at 5 109 mb for 1 h and 120 8C after CO exposure at 5 108 mb for 1 h. The spectra have been shifted vertically for clarity and the horizontal lines indicate the local zeroes; (b) corresponding log(I) vs. sample bias curves. The vertical lines indicate the position of the band edges.
that measured by the ion gauge because of the experimental set-up. Fig. 3 shows a constant current STM image of the 8 nm SnO2 particles acquired at 120 8C. The SnO2 particles are clearly resolved and the STM operation remained stable for days, which allowed the acquisition of local I(V) curves after O2 and CO exposures. The I(V) curves were averaged (average over 10–50 individual curves) and transformed into the normalised conductivity dln(I)/dln(V), which gives a good representation of the surface density of state [6]. Fig. 4 shows the variation of the normalised conductivity and the raw I(V) curves (on a semilog scale), as a function of surface treatments similar to those used for the XPS and resistance measurements. The bottom spectrum (a) was acquired at RT and presents an apparent surface band gap of about 2 eV (0.2 eV), with the Fermi level located at mid gap. The band edges were estimated using both normalised conductivity spectra and the log(I) versus sample bias curves. This value is much smaller than the band gap of bulk SnO2 (3.6 eV) [7] but is in reasonable agreement with previous STS results on nanocrystalline SnO2 [8]. The difference between the surface electronic band gap of nanocrystalline SnO2 as determined by STS and the bulk band gap of single crystal SnO2 can be attributed to surface states caused by defects and surface contamination.
T.G.G. Maffeı¨s et al. / Applied Surface Science 234 (2004) 82–85
This would also account for the position of the Fermi level at mid gap even though SnO2 is naturally n-type because of oxygen vacancies. The normalised conductivity at 120 8C does not show any significant changes from the RT data, except for an increase in the conduction band density of states, which could be caused by an increase in thermally excited carriers. Following O2 exposure, the STS measured surface band gap appears to have become larger (2.5 eV (0.2 eV)), due to an increase in the estimated position of the conduction band edge. This could be a consequence of the increased resistance as chemisorbed oxygen species remove electrons from the SnO2, and is comparable to the XPS measured shift of 0.2 eV following O2 exposure. However, the STS data did not reveal any change in the valence band position, which would be expected from a Fermi shift. Additionally, a large feature appeared at about 3.1 eV in the conduction band, which could indicate a possible change in the surface electronic structure caused by oxygen chemisorbtion. Subsequent CO exposure did not induce any further changes, possibly because the temperature was too low for the CO molecules to react efficiently with the chemisorbed oxygen.
85
In conclusion, we have shown that the increase in band bending at the surface of nanocrystalline SnO2 gas sensing layers caused by O2 exposure is accompanied by a large increase in the resistance of the SnO2 layer. Likewise, the decrease in surface band bending observed after exposure to CH4 is correlated to a resistance drop. Additionally, we have obtained the first STM images of 8 nm SnO2 particles at 120 8C. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O2 exposure.
References [1] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271–276. [2] G. Zhang, M. Liu, Sens. Actuators B 69 (2000) 144–152. [3] T.G.G. Maffeı¨s, G.T. Owen, M.W. Penny, T.K.H. Starke, S.A. Clark, H. Ferkei, S.P. Wilks, Surf. Sci. 520 (2002) 29–34. [4] J. Naser, H. Ferkel, Nanostruct. Mater. 12 (1999) 451. [5] J. Naser, H. Ferkei, W. Riehemann, B.L. Mordike, Laser Eng. 9 (1999) 195. [6] N.D. Lang, Phys. Rev. B 34 (1986) 5947. [7] K. Reimann, M. Steube, Solid State Commun. 105 (1997) 649. [8] J. Arbiol, P. Gorostiza, A. Cicera, A. Cornet, J. Morantes, Sens. Actuators B 78 (2001) 57–63.
Macroscopic and microscopic investigations of the effect of gas exposure on nanocrystalline SnO2 at elevated temperature T.G.G. Maffeı¨sa,*, M. Pennya, K.S. Tenga, S.P. Wilksa, H.S. Ferkelb, G.T. Owena a
Multidisciplinary Nanotechnology Centre, School of Engineering, University of Wales Swansea, Faraday Tower, Singleton Park, Swansea SA2 8PP, UK b TU Clausthal, Institut Fuer Werkstoffkunde und Werkstofftechnik Agricolastrasse 6, 38678 Clausthal-Zellerfeld, Germany
Abstract We have investigated the effect of O2 and reducing gases (CH4 and CO) exposure on nanocrystalline SnO2 in vacuum and at elevated temperatures (120 8C) using three different techniques: X-ray photoelectron spectroscopy, in vacuum resistance measurements and scanning tunnelling microscopy and spectroscopy. XPS and resistance measurements showed that O2 chemisorbtion causes an upward surface band bending of 0.2 eVand a resistance increase of 50 MO while CH4 exposure resulted in a 0.1 eV downward band bending and a 20 MO drop in resistance. Stable STM imaging at 120 8C was achieved and clearly resolved the 8 nm particles. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O2 exposure. # 2004 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline particles; SnO2; XPS; STM–STS
Gas sensors based on polycrystalline SnO2 offer many advantages over current technologies for detecting reducing gases, such as low cost, long lifetime and high selectivity and sensitivity. The sensing mechanism relies on perturbations in surface conductivity induced by chemical reactions between target gases and oxygen species absorbed onto the SnO2 surface. The constant supply of oxygen from the air ensures regeneration of the sensor surface and therefore, long device lifetime. The net effect observed upon exposure to reducing target gases such as CH4 and CO is a drop in the bulk resistance of the polycrystalline sensing film. The sensitivity of the gas sensing layer has been *
Corresponding author. Tel.: þ44 1792 295427; fax: þ44 1792 295676. E-mail address: [email protected] (T.G.G. Maffeı¨s).
shown to increase with decreasing grain size [1,2], therefore nanocrystalline SnO2 is expected to exhibit high sensitivity. In a previous article [3], we demonstrated the effect of O2 and CH4 on the surface electronic properties of SnO2 nanoparticles in ultra high vacuum (UHV) and at elevated temperatures using X-ray photoelectron spectroscopy (XPS). In this article, we compare these previous results with in vacuum resistance measurements done under the same conditions, correlating surface band bending with bulk resistance variations. In addition to these two macroscopic studies of the gas sensing mechanism, we present results from a scanning tunnelling microscopy and spectroscopy (STM– STS) investigation of the same SnO2 nanoparticles at room and elevated temperatures and in the presence of O2 and CO.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.081
T.G.G. Maffeı¨s et al. / Applied Surface Science 234 (2004) 82–85
O1s
486.4
530.25 530.15
486.2
530.05
486.1
529.95
O
Fl as h 2, 10 m O in 2, 20 m O in 2, 30 m O in 2, 60 C m H4 i ,6 n 0m in
486.3
Fig. 1. Binding energy variations of the Sn3d5/2 (left axis) and O1s (right axis) core levels after flash cleaning, incremental O2 exposure and CH4 exposure at 120 8C.
Exposure to O2 at 120 8C reveals core level shifts of 0.2 eV to lower BE, which infers a 0.2 eV increase in surface band bending. This is a consequence of the chemisorbtion of oxidising gases removing electrons from the SnO2, which results in the formation of a depletion layer, upward band bending and, therefore low surface conductivity. This corresponds to the normal operating state of a real gas sensor where oxygen from the air ensures low conductivity. Exposure to CH4 at 120 8C resulted in a 0.1 eV downward band bending at the surface as the reducing agents react with the oxygen. Reducing agents remove the adsorbed oxygen and restore electrons to the conduction band, hence decreasing the resistance of the semiconductor. The changes in the bulk resistance of the SnO2 sample following the same surface treatments as for 60 50 40 30 20 10
2
Fl as 10 h m O in 2 20 s O min 2 30 s O min 2 s 6 C 0m H4 in 60 s m in s
0
O
Fig. 1 shows the variation of the Sn3d and O1s core level binding energy (BE) with the different surface treatments. The fact that the BE of both core levels follows the same trend suggests that these BE shifts are likely to be Fermi shifts corresponding to rigid band bending at the surface, rather than chemical changes.
530.35 Sn3d
Bulk resistance (MOhm)
Flash heating for 1 min at 400 8C, then the temperature was stabilised at 120 8C. Exposure to O2 at 5 105 mb for 10, 20, 30 and 60 min at 120 8C. Exposure to CH4 at 5 105 mb for 60 min at 120 8C.
486.5
Binding energy (eV)
The SnO2 nano-powder was produced by laser ablation of a ceramic rod. The fabrication details are published elsewhere [4]. Two sets of samples were prepared from the nano-powder, one for the macroscopic experiments (XPS and in vacuum resistance measurements) and one for the STM–STS experiments. The samples used in the macroscopic experiments were fabricated by simply evaporating a suspension of the powder onto alumina substrates, followed by annealing in air at 400 8C for 20 min. The substrates are standard gas sensing substrates with two platinum electrodes on the topside used to measure the resistance of the SnO2 layer and a platinum heater on the backside. The sensor was mounted on a specially designed sample holder, allowing heating of the sensor via its own heater [3] during either XPS or resistance measurements. These measurements were conducted at about 108 mb in a VG ESCAlab Mk II using un-monochromated Al Ka radiation (1486.6 eV). The samples used in the STM experiments were fabricated by depositing the 8 nm particles onto Si substrates directly from the nano-powder production plant [5]. The samples were loaded into our Omicron STM/SEM-HC (base pressure: 1011 mb) and scanned with electrochemically etched W tips. The system allows resistive heating of the samples and is equipped with a gas inlet pipe that can be positioned within 1 mm of the sample. This set-up means that the gas pressure on the sample itself is much higher than the pressure measured by the ion gauge. For the XPS and resistance measurements, the samples were subjected to the following surface treatments:
83
Fig. 2. Variation of the bulk resistance of the SnO2 layer after flash cleaning, incremental O2 exposure and CH4 exposure at 120 8C.
T.G.G. Maffeı¨s et al. / Applied Surface Science 234 (2004) 82–85
Fig. 3. 250 nm 250 nm constant current STM image of 8 nm SnO2 particles acquired at a tip voltage of 3.4 V, tunnelling current of 0.6 nA and at a temperature of 120 8C. The grey scale ranges from 0 to 5.1 nm.
(a)
(b) After CO
Log (tunnelling current (nA))
the XPS experiments are shown in Fig. 2 and can be directly compared to the BE shifts. Increasing the temperature from RT to 120 8C causes the resistance to decrease by 8 MO as expected for a semiconductor. Incremental O2 exposure shows a resistance increase of approximately 50 MO that is supported by the 0.2 eV increase in surface band bending detected using XPS and also by the widely accepted theory that adsorbed O2 will remove electrons from the surface, creating a potential barrier and hence increasing the surface resistance. Exposure to CH4 for 60 min decreases the resistance by approximately 20 MO. As mentioned previously, the removal of surface oxygen donates electrons into the conduction band of the semiconductor, lowering the resistance. This finding concurs with the presence of the downward band bending of 0.1 eV revealed from the XPS study. These results convincingly link the variation of surface band bending measured by XPS with the overall resistance of the SnO2 sensing layer. The sample used in the STM–STS experiment were subjected to similar surface treatments as for the XPS and resistance measurements: after loading in the STM chamber, the sample was scanned at RT then heated to 120 8C and scanned before and after exposure to 5 109 mb of O2 for 1 h, and after exposure to 5 108 mb of CO for 1 h. As mentioned before, the pressure at the sample itself is much higher than
dlnI / dln V (a.u.)
84
After O2
120°C
After CO After O2
120°C
RT
RT
-4
-2
0
2
Sample bias (V)
4
-4
-2
0
2
4
Sample bias (V)
Fig. 4. (a) Normalised conductivity spectra of the 8 nm SnO2 particles acquired at RT, 120 8C before gas exposure, 120 8C after O2 exposure at 5 109 mb for 1 h and 120 8C after CO exposure at 5 108 mb for 1 h. The spectra have been shifted vertically for clarity and the horizontal lines indicate the local zeroes; (b) corresponding log(I) vs. sample bias curves. The vertical lines indicate the position of the band edges.
that measured by the ion gauge because of the experimental set-up. Fig. 3 shows a constant current STM image of the 8 nm SnO2 particles acquired at 120 8C. The SnO2 particles are clearly resolved and the STM operation remained stable for days, which allowed the acquisition of local I(V) curves after O2 and CO exposures. The I(V) curves were averaged (average over 10–50 individual curves) and transformed into the normalised conductivity dln(I)/dln(V), which gives a good representation of the surface density of state [6]. Fig. 4 shows the variation of the normalised conductivity and the raw I(V) curves (on a semilog scale), as a function of surface treatments similar to those used for the XPS and resistance measurements. The bottom spectrum (a) was acquired at RT and presents an apparent surface band gap of about 2 eV (0.2 eV), with the Fermi level located at mid gap. The band edges were estimated using both normalised conductivity spectra and the log(I) versus sample bias curves. This value is much smaller than the band gap of bulk SnO2 (3.6 eV) [7] but is in reasonable agreement with previous STS results on nanocrystalline SnO2 [8]. The difference between the surface electronic band gap of nanocrystalline SnO2 as determined by STS and the bulk band gap of single crystal SnO2 can be attributed to surface states caused by defects and surface contamination.
T.G.G. Maffeı¨s et al. / Applied Surface Science 234 (2004) 82–85
This would also account for the position of the Fermi level at mid gap even though SnO2 is naturally n-type because of oxygen vacancies. The normalised conductivity at 120 8C does not show any significant changes from the RT data, except for an increase in the conduction band density of states, which could be caused by an increase in thermally excited carriers. Following O2 exposure, the STS measured surface band gap appears to have become larger (2.5 eV (0.2 eV)), due to an increase in the estimated position of the conduction band edge. This could be a consequence of the increased resistance as chemisorbed oxygen species remove electrons from the SnO2, and is comparable to the XPS measured shift of 0.2 eV following O2 exposure. However, the STS data did not reveal any change in the valence band position, which would be expected from a Fermi shift. Additionally, a large feature appeared at about 3.1 eV in the conduction band, which could indicate a possible change in the surface electronic structure caused by oxygen chemisorbtion. Subsequent CO exposure did not induce any further changes, possibly because the temperature was too low for the CO molecules to react efficiently with the chemisorbed oxygen.
85
In conclusion, we have shown that the increase in band bending at the surface of nanocrystalline SnO2 gas sensing layers caused by O2 exposure is accompanied by a large increase in the resistance of the SnO2 layer. Likewise, the decrease in surface band bending observed after exposure to CH4 is correlated to a resistance drop. Additionally, we have obtained the first STM images of 8 nm SnO2 particles at 120 8C. STS measurements indicate a change in the surface electronic properties of the SnO2 particles following O2 exposure.
References [1] S.G. Ansari, P. Boroojerdian, S.R. Sainkar, R.N. Karekar, R.C. Aiyer, S.K. Kulkarni, Thin Solid Films 295 (1997) 271–276. [2] G. Zhang, M. Liu, Sens. Actuators B 69 (2000) 144–152. [3] T.G.G. Maffeı¨s, G.T. Owen, M.W. Penny, T.K.H. Starke, S.A. Clark, H. Ferkei, S.P. Wilks, Surf. Sci. 520 (2002) 29–34. [4] J. Naser, H. Ferkel, Nanostruct. Mater. 12 (1999) 451. [5] J. Naser, H. Ferkei, W. Riehemann, B.L. Mordike, Laser Eng. 9 (1999) 195. [6] N.D. Lang, Phys. Rev. B 34 (1986) 5947. [7] K. Reimann, M. Steube, Solid State Commun. 105 (1997) 649. [8] J. Arbiol, P. Gorostiza, A. Cicera, A. Cornet, J. Morantes, Sens. Actuators B 78 (2001) 57–63.