Peculiarities of gas sensing characteristics of SnO₂-based sensors modified by SnO₂-Au nanocomposites synthesized by SILD method

Peculiarities of gas sensing characteristics of SnO ² -based sensors modified by SnO ² -Au nanocomposites synthesized by SILD method

Ghenadii Korotcenkov, Beongki Cho, Valery Tolstoy * , Larisa B.Gulina * , and Sang Do Han ** ^† Abstract

The problems associated with the synthesis, characterization and application of SnO

-Au nanocomposites for the optimization of conductometric gas sensors have been discussed in this report. Nanocomposites have been synthesized on the surface of SnO

films using successive ionic layer deposition(SILD) method. It has been shown that the proposed approach to surface modification of metal oxide films is an excellent method for the optimization of the operating characteristics of SnO

-based gas sensors, being developed for the detection of reducing gases as well as ozone.

Key Words : SnO

-Au, nanocomposite, characterization; SILD, gas sensors, modification

1. Introduction

At present, the incorporation of a second component in metal oxide such as bulk doping or surface modification is becoming one of the most promising methods for the optimization of gas sensing properties of conductometric solid state gas sensors. For these purposes various addi- tives such as noble metals(Ag, Pd, Pt, Au), transition met- als(Fe, Mn, Co, Ni, Cu), and oxides(SiO 2 , Al 2 O 3 ) can be used ^[1-3] .These additives can act as either promoters, cat- alysts, surface sites for adsorption of oxygen and target gas or as elements promoting improvement of porosity of gas sensing matrix and thermal stability of metal oxide structure. The design of nano-composites is another promising direction for the development of materials for solid state gas sensors. Nano-composite materials, due to their composition and structure peculiarities may possess unique physical–chemical properties. In this report, we discuss the preliminary results of synthesis, characteriza- tion and gas sensor application of nanocomoposites on the basis of SnO 2 -Au system.

For synthesis and deposition of SnO 2 -Au composite on the surface of SnO 2 films, the successive ionic layer

deposition(SILD) method has been used ^[4] . As reported earlier ^[5,6] the SILD technology is a promising method for the deposition of metal oxide nanolayers on different surfaces, for example on the surface of porous materials.

The SILD method essentially consists of repeated successive treatments of the substrate in solutions of various salts, anions and cations that can form upon adsorption poorly soluble compounds. Then the sub- strate is being washed with distilled water to remove excess of the salt. One such treatment comprised only one deposition cycle. This method does not provide a high deposition rate in comparison with other chemical methods of deposition ^[7] . However, high deposition rates are not required for surface modification. Precise monitoring of both the size and the composition of deposited clusters is more important for surface modi- fication. SILD technology permits surface modification with high precision through the composition control of precursor solutions and the number of ionic deposition cycles. Ellipsometric measurements have shown that after each cycle a layer of peroxicomplexes of metals with thickness 0.6~1.5 nm can be deposited on the sur- face of the substrate ^[4] . The SILD method has advan- tages in comparison with many other methods used for the surface modification of metal oxides ^[7] .

2. Experimental Details

The availability of the SnO 2 -Au nanocomposites for

Department of Material Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, Korea

*St. Petersburg State University, St. Petersburg, Russia

**Energy Efficiency and Materials Convergence Research Division, Korea Institute of Energy Research, Daejeon, Korea

†

Corresponding author: [email protected]

(Received : March 2, 2009, Revised : July 17, September 20, 2009,

Accepted : September 4, 2009)

gas sensor applications was estimated using thin film sensors fabricated on the base of SnO ² layers, synthe- sized by SILD ^[6] . Undoped SnO ² layer was deposited using 50 deposition cycles.

The 0.01 M aqueous solutions of SnCl 2 and HAuCl 4

were used as precursors. The duration of treatments in various solutions was equal to 1 min. Different(1 to 16) deposition cycles were used in our experiments. The synthesized layers were studied by FTIR(Fourier Trans- form Infra-Red), UV-Vis(Ultraviolet-Visible), and XPS (X-ray Photoelectron Spectroscopy), as well as by SEM (Scanning Electron Microscopy). For these experiments SnO 2 -Au films deposited on silicon and quartz sub- strates have been used.

In order to study sensor responses in both steady state and transient modes, the sensors were put into a special flow-type reactor and exposed to atmospheres with con- trolled relative humidity and concentration of the target gas. In our experiments we used H 2 , CO(2000 ppm) and ozone(~1 ppm) as tested gases. All measurements were conducted at a steady-state temperature between 25 and 500 ^o C.

3. Structure and Composition Characterization

SEM images(see Fig. 1) of SnO ² : Au nanocomposite have shown that 2D and 3D precipitates are formed dur- ing SILD. The size of these precipitates has large dis- persion and varies from 5 to 40 nm. At that the growth of SnO 2 : Au nanocomposite layers takes place mainly through the growth of 3D agglomerates. The joining of agglomerates in to a continuous layer has been observed after 13 cycles of deposition.

The EDX(Energy Dispersive X-ray) and XPS analy- sis(see Fig. 2) revealed that the Sn: Au ratio in the layer is 0.6~1.0. At that the analysis of XPS spectra indicates that Au in nanocomposite phase after annealing is in metal state.

The FTIR spectra of Au-SnO 2 layers(see Fig. 3) show that the nanocomposite is amorphous or a very finely dispersed phase, as the maximum of the Sn–O band is at 580 cm ⁻¹ . After heating at 400 ^o C, the 1640 cm ⁻¹ band, assignable to the bending vibrations of incorporated water molecules and possibly Sn-OH groups, disappears.

The disappearance of the peroxide band undoubtedly indicates that the peroxide compound, synthesized dur-

ing SILD process, decomposes. At the same time the Sn–O band broadens and develops additional compo- nents at 605 and 480 cm ⁻¹ . Nevertheless, complete split- ting of this Sn–O band into two components, which is typical for the crystalline form, is not being observed. It means that nanocomposite layers keep their finely dis- persed structure even after annealing at T> 400 ^o C.

Fig. 1. SEM images of SnO

:Au nanocomposite films formed by SILD method on Si substrate: (a) 1 deposition cycle; (b) –13 deposition cycles, annealing at T=600

C.

Fig. 2. Full-range XPS spectra of Au-SnO

nanocomposites

deposited by SILD method using 13 deposition

cycles(T

=600

C).

4. Gas Sensor Characterization It has been observed from the results presented in Figs. 4 and 5, the surface modification by SnO 2 -Au nanocomposite improves the operating parameters of both ozone sensors and sensors of reducing gases(CO, H ² ). Sensor response of modified SnO ² films exceeded the response of undoped SnO ² films synthesized by

SILD method. At that the conductivity response to the reducing gases(R air /R gas ) increased with increasing number of deposition cycles. However, after 16 cycles of Au- SnO ² deposition, SnO ² sensor response(R ^air /R ^gas ) to CO(2000 ppm) and H 2 (2000 ppm) reached 20~25(see Fig. 4).

The optimum sensor response to reducing gases takes place at operating temperatures ~450~500 ^o C. However, for ozone detection the maximum sensor response was observed at considerably lower operating temperatures around ~150 ^o C(Fig. 5). For comparison, the SnO 2 films deposited by spray pyrolysis has been reported to show maximum response to ozone at operating temperatures 200~320 ^o C ^[8,9] . In the temperature range of maximum sensitivity to reducing gases(T~450 ^o C), the sensor response to ozone(R ^ozone /R ^air ) is very, close to 1.0. For sensors aimed for reducing gases the control of such behavior is an important advantage, because the above mentioned effect could be used for the improvement of selectivity of sensor response to reducing gases.

In contrast to sensing effects with reducing gases, during ozone detection the optimization effect takes place for samples after 1~4 cycles of SnO 2 -Au deposi- tion. An increase of the number of deposition cycles has been observed to strongly decrease the sensor response to ozone.

With regard of SnO 2 -based sensor applications in real devices, considerable decrease of recovery time after interaction with ozone is one of the important conse-

Fig. 3. FTIR spectra of Au

SnO

nano-layers synthesized at the surface of silicon after 20 SILD cycles. 1 - initial state; 2 – after heating in air at a temperature equal to 400

C; t

=20 min.

Fig. 4. Influence of the number of Au-SnO

nanocomposites deposition cycles on sensor response of SnO

- based sensors to reducing gases (1) H

and (2) CO:

T

=450

C.

Fig. 5. Influence of surface modification by Au-SnO

nanocomposite on the temperature dependences of

sensor response to ozone: 1-initial; 2-1 M; 4-16 M.

quences of SnO ² surface modification by Au-SnO ² nanocomposites. As reported in earlier ^[8,9] , the long time of recovery process is one of the main disadvantages of ozone sensors designed on the base of SnO 2 metal oxides. Therefore, the decrease of recovery time after interaction with ozone is already in 10 folds(see Fig. 6).

One can consider an important favorable result of SnO 2

surface modification by Au-SnO 2 nanocomposites. At that observed increase of response time after modifica- tion(see Fig. 6) cannot be considered as essential wors- ening of operating characteristics, because even after such increase of response time, the speed of response remains acceptable for various practical applications.

For sensors of reducing gases, the decrease of time con- stants takes place for both response and recovery proc- esses.

On the basis of carried out research, including addi- tional conductivity measurements in various environ- ments, we concluded that the observed effects are the result of Au clusters influence on SnO 2 interaction with ozone(T ^oper <300 ^o C) in the case of ozone detection and with air(T ^oper >300 ^o C) in the case of reducing gas detec- tion. For example, it was found that during reducing gas detection, surface modification influences on the film conductivity measured in air. In other words, the increase of sensitivity is attained due to a decrease in the film’s conductivity in initial state, i.e. before inter- action with a target gas. This means that observed opti- mizing effect could be ascribed to the increase in the concentration of oxygen chemisorbed on the surface of

SnO 2 modified by the SnO 2 -Au nanocomposite. Taking into account that surface modification does not change the initial structure of grain’s network and bulk proper- ties of SnO 2 grains, this assumption is realistic. According to current ideas about gas sensing mechanisms ^[10-12] , the conductivity of nanocrystalline SnO ² films is deter- mined by the resistance of inter-grain contacts control- led by oxygen chemisorbed on the surfaces of SnO 2

grains ^[10] . In the case of reducing gas detection, the growth of chemisorbed oxygen concentration could be ascribed to more effective interaction between the mod- ified surfaces with oxygen from the surrounding atmos- phere. The obtained results are in accordance with the supposition made earlier ^[13-15] , that the gold/metal-oxide interface could play an important role in the oxygen activation. As is known, ionoabsorbed oxygen partici- pates in the detection reactions of both CO and H 2 . These oxidation reactions are more effective with the participation of atomic oxygen, so there is no doubt that the growth of chemisorbed oxygen concentration should be accompanied by an increase in sensor response.

5. Conclusions

SnO 2 films with surface modification by SnO 2 -Au has high sensitivity to such reducing gases as CO, H ^2, and to such oxidizing gases as ozone, which exceeded the gas sensitivity of undoped SnO 2 films synthesized by SILD method. At that the sensor response to CO and H ² increased with increase of the number of deposition cycles. For ozone detection maximum response was observed after 1~4 cycles of SnO 2 -Au deposition. An increase in the number of deposition cycles strongly depressed the sensor response to ozone.

Maximum sensor response to reducing gases takes place at temperatures ~450~500 ^o C. For ozone detec- tion, maximum sensor response was observed at con- siderably lower operating temperatures of ~150 ^o C.

In the temperature range of maximum sensitivity to reducing gases(450 ^o C), the sensor response to ozone is very close to 1.0. For sensors for reducing gases such behavior is an important advantage.

Acknowledgements

This work was supported in KIER by the Korea Research Foundation and the Korean Federation of Sci-

Fig. 6. Influence of the number of deposition cycles on

response and recovery times of SnO

-based sensors

during ozone detection.

ence and Technology Societies Grant funded by Korea Government(MOEHRD, Basic Research Promotion Fund) and by World Class University(WCU) Program at the Gwangju Institute of Science and Technology (GIST) through a grant(Project No.R31-20008-000-10026- 0). G. Korotcenkov is also thankful to the Korean BK21 Program for support of his research.

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Ghenadii Korotcenkov

• 1976:PhD in physics and technology of semiconductor materials and devices from Technical University of Moldova, Chisinau, USSR

• 1990:Dr. Sci. degree (Habilitat) in physics and mathematics from Academy of Science of Moldova, Chisinau, USSR.

• 1974~2007: from scientific researcher to main scientific researcher in Technical University of Moldova, Chisinau, Moldova

• 2007~2008: visiting scientist in Korea Institute of Energy Research, Daijeon, Korea.

• 2008~present: research Professor in Department of Material Science and Engineering at Gwangju Institute of Science and Technology, Gwangju, Korea.

Valeri P. Tolstoy

• 1980: PhD in chemistry from St- Petersburg Institute of Technology, St.

Petersburg, Russia

• 2009: Dr. Sci. in chemistry from St- Petersburg State University, St. Petersburg, Russia

• 1981~1994:scientific researcher and then senior scientific researcher, Institute of Chemistry, St. Petersburg State University, St. Petersburg, Russia

• 1994~present: Associate Professor in Institute of Chemistry, St. Petersburg State University, St. Petersburg, Russia

Sang-Do Han

• See p. 10 of 『J. Kor. Sensors Soc ., vol.

16, no. 1 』

• (present) President of The Korean Sen- sors Society

Beongki Cho

• 1995: PhD in Physics and Astronomy from Iowa State University, U.S.A.

• 1995~1996: Postdoc position in Cornell University, USA

• 2000~present: Professor in Departments of Nanobio materials and Electronics and Materials Science and Engineering at Gwangju Institute of Science and Technology (GIST), Gwangju, Korea

Larisa B. Gulina

• 2000:PhD in solid state chemistry from University of St-Petersburg St. Petersburg, Russia

• 2001~2005: research staff in Institute of Chemistry, St. Petersburg State University, St. Petersburg, Russia

• 2005~present: senior scientific researcher

in Institute of Chemistry, St. Petersburg

State University, St. Petersburg, Russia