gas sensing properties of nanocrystalline tungsten oxide synthesized by acid

We confirm that the sensor elements made from 600°C calcined powders doped with platinum sintered at 800°C are highly responsive and selective to ammonia vapour at 350°C operating tempe

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Sensors and Actuators B xxx (2010) xxx-xxx

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Sensors and Actuators B: Chemical

journal homepage: www.elsevier.com/locate/snb

Gas sensing properties of nanocrystalline tungsten oxide synthesized by acid precipitation method

T.D Senguttuvan**, Vibha Srivastava, Jai S Tawal>, Monika Mishra?, Shubhda Srivastava’, Kiran Jain

4 Electronic Materials Division, National Physical Laboratory, Dr K.S Kishnan Marg, New Delhi 110012, India

b Materials Characterization Division, National Physical Laboratory, Dr K.S Kishnan Marg, New Delhi 110012, India

Article history:

Received 9 December 2009

Received in revised form 23 June 2010

Accepted 26 June 2010

Available online xxx

WO3-2H20 samples were prepared by acidic precipitation of sodium tungstate solution Nanocrystalline WO3 powders were obtained after 350 and 600 °C calcinations XRD patterns of these samples showed a diffraction profile similar to that of monoclinic WO3 Calcinations at 600°C yield WO3 powders with particle sizes ranging from 60 to170 nm whereas that for 350°C calcinations are in the range of 30-150 nm The gas sensing properties of these powders in the form of thick film were investigated with and without

Keywords:

WO;

Metal oxide

Gas sensor

Ammonia sensor

Thick film

Chemical synthesis

platinum doping Gas response of thick films sintered at two different temperatures was measured at four different operating temperatures We confirm that the sensor elements made from 600°C calcined powders doped with platinum sintered at 800°C are highly responsive and selective to ammonia vapour

at 350°C operating temperature as compared to sensor elements made from commercial powders

1 Introduction

Gas sensors are used for many applications such as process

controls in chemical industries, detection of toxic environmental

pollutants, and for the prevention of hazardous gas leaks Different

oxide semiconductors such as SnOz, WO3, ZnO, MoOs3, TiOz, In2O3

and mixed oxides have been studied and showed promising appli-

cations for detecting target gases such as NOx, O03, NH3, CO, CO2, HS

and Sox [1-3] The working principle of these sensors is based on the

detection of a change in resistance on exposure to a gas Due to the

constraints of gas permeation only the surface layers are affected

by such reactions Among various oxide sensors, WO3 is responsive

to NOx, H2S, and NH3 [4-6] In order to achieve improvements in the

gas sensing properties such as enhancing their response, selectiv-

ity to a given target species and reduce the operating temperature,

small amounts of noble metals (Pt, Pd and Ag) are added to active

metal oxide layers [7,8] The sensor characteristics strongly depend

on preparation techniques and the resulting material microstruc-

ture of active metal oxide layer [9,10] Cantalini et al reported

positive effects on the response cross-sensitivity by increases in

the annealing time for the WO3 thin films [11,12] Tamaki et al

have shown that the response to NO; increases dramatically with

decreasing grain size of the tungsten oxide for sintered block type

* Corresponding author Tel.: +91 11 45609461; fax: +91 11 25726938

E-mail address: tdsen@mail.nplindia.ernet.in (T.D Senguttuvan)

WO3-based gas sensors [13] This generated a wide interest in synthesizing nanosized powder and investigating gas sensor properties Nanosized powders of WO3 have been prepared by sol-gel, spray-pyrolysis, radio frequency magnetron sputtering and aqueous chemical route etc [9—12,14] Compared with other techniques, aqueous chemical process is attractive because of its cost effective- ness and the ease of material preparation This technique was well reported for NO, sensor fabrication [15] However, it is not widely explored for ammonia gas sensors preparation

In the present work, we investigate ammonia gas sensing properties of nanocrystalline tungsten oxide with and without platinum doping under different processing conditions

2 Experimental Tungsten trioxide powder was prepared by acidic precipitation route 5 g Sodium Tungstate was dissolved in 200 ml of de-ionised water to obtain a transparent solution To this solution 5% dilute HCl

is added drop-wise to obtain yellow tungstic acid precipitate It was then washed with water to remove sodium and chlorine ions till no chlorine ion was detected on AgNO; test At this stage the precipitate was filtered and dried at 100°C These powders were calcined

at 350 and 600°C to obtain powder A and powder B respectively For Pt doping, calcined powders A and B were mixed with 0.4 wt% chloroplatinic acid solution in water and dried to obtain powders C and D X-ray powder diffraction (XRD) analysis on powder

A and B were catried out using Bruker Analytical X-ray diffrac-

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tometer equipped with graphite monochromatized CuKa radiation

(A = 1.5418 A) The nano-scale characterization was carried out by

a transmission electron microscope (TEM, model JEOL JFM 200x)

The morphologies of thick films were observed by using scanning

electron microscope (LEO 440 SEM)

Pure powders A and B, platinium doped powders C and D, micron

sized commercial powder (Sigma-Aldrich) and platinum doped

commercial powders were used to make thick film paste Thick film

paste was prepared in butyl carbitol medium containing a small

amount of ethyl cellulose and terpineol oil These pastes were then

screen printed on alumina substrates that had gold finger contacts

Sensor films were coated over the gold finger contacts, and sintered

at 600°C and 800°C for 10 min Thus we have obtained eleven dif-

ferent sensor elements They are designated as AP_GOO (for sensor

prepared from pure powder A sintered at 600 °C), BP_600 (for sensor

prepared from pure powder B sintered at 600 °C), AP_800 (for sensor

prepared from pure powder A sintered at 800 °C), BP_800 (for sensor

prepared from pure powder B sintered at 800°C), CPt_G00 (for sen-

sor prepared from Pt doped powder C sintered at 600°C), DPt_600

(for sensor prepared from Pt doped powder D sintered at 600°C),

CPt_800 (for sensor prepared from Pt doped powder C sintered at

800 °C), DPt_800 (for sensor prepared from Pt doped powder D csin-

tered at 800°C), Comm Powder (sensor prepared from commercial

powder Sintered at 800°C, Comm-600 (sensor prepared from Pt

doped commercial C sintered at 600°C) and Comm-800 (sensor

prepared from Pt doped commercial C sintered at 800 °C) Resistiv-

ity and gas response for CNG, NOz, NH3, CO, LPG and Ethanol gas

was measured using Keithley 2000 multimeter in a static system

The sensor element was placed onto an externally heated sample

holder and the working temperature of the thick films was deter-

mined with a thermocouple attached near the sensor element The

gas or vapour of required amount was injected into the chamber

using a 1 ml syringe The sample’s resistance was measured in air

and after exposure to targeted gas or vapour After completing the

measurement, the gas was leaked out To get uniform temperature

distribution throughout the sensor elements, they were heated to

targeted temperature in dry air for 1h before measurements were

carried out Gas sensing properties of the thick films were carried

out at operating temperatures ranging from 250 to 450°C The gas

response (S) was defined as the ratio Ra/R¢g or R¢g/Ra, for reducing and

oxidizing gases, respectively; where R, is the resistance in pure air

and Rg is the sensor resistance in the presence of a species diluted

in air

3 Results and discussion

Fig 1 shows the XRD pattern of powders A and B The charac-

teristic peaks observed at 20 values 23.2, 28.88 and 34.17 confirms

that these powders are monoclinic WO3 [JCPDS 43-1035] The sharp

diffraction peaks imply good crystallinity of WO3 powders Absence

of characteristic peaks corresponding to other impurities such as W

or W(OH)., indicates the phase purity of WO3 The tungsten triox-

ide can exists in several polymorphic forms such as monoclinic,

hexagonal and pyrochlore around room temperature Choi et al

have shown in the case of WO3 derived from sol prepared by ion

exchange method both hexagonal and pyrochlore crystals switch

completely to monoclinic phase if it is heated to more than 500°C

and cooled down to room temperature [16,17] However in our case

both the powders A and B are calcined at 350 and 600°C show only

monoclinic WO3

Fig 2 shows the TEM microstructure of the powders A and

B respectively It is evident from the micrographs that the pow-

der A has particles in the size range of 30-150nm and powder

B has particles in the range of 60-170 nm This increase in parti-

cle size is understandable since higher calcination temperatures

300

N oO oO

350°C

100

600°C

caida ated dJ sd An hs

29 (deg)

0

Fig 1 XRD pattern of powders calcined at 350 and 600°C

lead to increased diffusion rate that in turn coalesce the adjacent grains

3.1 Gas Response—pure WO3

Response to ammonia gas was measured for AP_600, BP_6OO,

AP_800 and BP_800 sensor elements The sensor response was eval- uated as a function of operating temperature We could not observe

600° C

,

1 im

Fig 2 TEM micrograph of powders calcined at 350 and 600°C

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—a— A-350°C

|—®— B-600°C

r + T r T r T T T 1

Temperature (°C)

Fig 3 Response of AP_800 (sensor prepared from pure powder A), BP-800 (sensor

prepared from pure powder B) and Comm Powder (sensor prepared from commer-

cial powder) all sintered at 800-4000 ppm NH3 gas

noticeable sensor response for AP_600 and BP_GOO sensor elements

Fig 3 shows the sensor response for AP_800 and BP_800 sensor

elements to 4000 ppm NH3 gas For both the sensors elements

maximum response was observed at 400°C operating tempera-

ture, where as for the sensor elements prepared from micron sized

commercial highest response was observed at 400°C operating

temperature WO3 powders calcined at 600°C better response as

compared to powders calcined at 350°C The TEM results have con-

firmed lower particle size for powder A than for powder B As per

linear extrapolation one would expect lower grain size for AP_800

sensor element and hence a higher response which is contrary to

our results The reason for this discrepancy can be understood by

seeing the SEM micrographs of AP_800 sensor element (Fig 4) and

BP_800, sensor element (Fig 5) AP_800 sensor element showed

polycrystalline grain morphology consisting of spherical grains of

grain size 100 nm together with plate like grains of grain size in the

range 300-500 nm The average grain size of this sensor element

was calculated by linear intercept method (EN 623-3) and it is found

f=

EMS, NPL

te

EHT=15.88 kỤ UD: 12nn

Detector= SE1 Photo No.=2938 380nn —

Fig 4 SEM micrograph of AP_800 (sensor prepared from pure powder A sintered

¬ oat

EHT=15 66 kV Detector= SE1 Photo No =293°

ag= 18.88 K X EHS, NPL

380nn — ct

—mmmmmm NÊU DELHI

Fig 5 SEM micrograph of BP_800 (sensor prepared from pure powder B sintered at

800°C)

to be 680 nm BP_800, sensor element showed polycrystalline grain morphology consisting of mostly of spherical grains with grain sizes ranging from 130 to GOO0nm The average grain size of this sensor element is 300nm We observe better a response in smaller grain size sensor element These results are similar to increase in response of tungsten oxide sensor with decrease in grain size as reported by Tamaki et al [13] The sensing properties of WO3 film are also controlled by the surface defects [18] The higher response

to NH3 for BP_800 sensor element can also be because of its large irregular voids Further, more oxygen vacancies are expected on the surface of tungsten oxide since it is calcined at higher temperature

in accordance with the results obtained by Liu et al [19]

3.2 Gas response—Pt doped WO3 The highest magnitude of response to 4000 ppm NH3 observed for BP_800 sensor element was 9 We have tried to improve the response by doping with platinum as explained earlier Response to

ammonia gas was measured for sensor elements CPt_600, DPt_600,

100 ¬

80 -

60 -

40 -

20 +

T qT

Temperature (°C)

T

200

Fig 6 CPt.600 (sensor prepared from Pt doped powder C sintered at 600°C), CPt_800 (sensor prepared from Pt doped powder C sintered at 800°C) sensor, Comm-600 (sensor prepared from Pt doped commercial C sintered at 600°C) and Comm-800 (sensor prepared from Pt doped commercial C sintered at 800°C)

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—— naman

©,

40 +

0 4

Temperature (°C)

Fig 7 DPt.600 (sensor prepared from Pt doped powder D sintered at 600°C)

DPt_800(sensor prepared from Pt doped powder D sintered at 800°C) Comm-600

(sensor prepared from Pt doped commercial C sintered at 600°C) and Comm-800

(sensor prepared from Pt doped commercial C sintered at 800°C) response for to

4000 ppm NH3 gas

120 +

100 +

80 ¬

Response 60 +

40 -

Concentration (ppm)

Fig 8 The response of DPt_800(sensor prepared from Pt doped powder D sintered

at 800°C) to NH3 gas operating at 350°C

CPt_800 and DPt_800 Fig 6 shows the sensor response for CPt_600,

CPt_800, Comm-600 and Comm-800 sensor element to 4000 ppm

NH3 gas The maximum response was observed at 350°C for all

sensor elements except for Comm-600 However, the response

magnitude was better for CPt_800 sensor element and it was found

to be 82 as compared to all other elements Fig 7 shows the

sensor response for DPt_600 sensor element and DPt_800 sensor

element along with Comm-600 and Comm-800 sensor elements to

4000 ppm NH3 gas The maximum response was observed at 350°C,

and was highest for sensor annealed at 800°C Above 400°C oper-

ating temperature both sensor elements (DPt_600 and DPt_800)

show same response to 4000 ppm NH3 gas The response magni-

tude of 125 was observed for DPt_800 sensor element as compared

to a magnitude of 9 for BP_800 sensor element Fig 8 shows the

linear response of DPt_800 sensor element to NH3 gas within the

concentration range 100-4000 ppm operating at 350°C

The response time represents the time required by the response

factor to undergo 90% variation with respect to its equilibrium value

following a step increase in the test gas concentration The recovery

time represents the time required by the sensitivity factor to return

to 10% below its equilibrium value in air following the zeroing of

Fig.9 Resistance vs time graph for DPt_800 (sensor prepared from Pt doped powder

D sintered at 800°C) at different operating temperatures

N co)

800 ppm

Response 3

Fiz 10 The response of DPt_800 (sensor prepared from Pt doped powder D sintered

at 800°C) to 800 ppm of different gases (NH:, LPG, CNG, CO ethanol and NOa.)

the test gas Fig 9 shows response time graph for DPt_800 sensor element at different operating temperatures It took less than 20s

at an operating temperature of 400°C and 60s for 200 °C operating temperature It should also be noted that tungsten oxide nanopar- ticles prepared by gas deposition yield films that can be used even

at room temperature with similar and greater sensitivities for H2S gas [20] Our results indicate the limitations posed by fluid-based chemistry that was used to fabricate the nanoparticle films The Pt-W0O3 (DPt_800 sensor element) sensor was highly selective towards ammonia at an operating temperature of 350°C

Fig 10 shows the response to NH3, LPG, CNG, CO ethanol and NO2

were recorded

4 Conclusions

Nanocrystalline tungsten oxide powder was produced by an acid precipitation of sodium tungstate suitable for ammonia gas sensors These powders were monoclinic WO3 The particles with size distribution of 30-150 and 60-170 nm resulted from 350 and 600°C calcinations respectively Sensor element obtained from 350°C calcined powders showed a polycrystalline grain morphology consisting of spherical grains of size 100 nm together with plate like grains of size 300-500 nm Whereas 600°C calcined powder resulted in a sensor elements with polycrystalline grain morphology consisting only of spherical grains of size ranging from 130 to

600 nm Sensor elements made with pure WO3 powders calcined

at 600°C are better than those calcined at 350°C Platinum doping resulted in better response irrespective of calcination temperature The maximum response to NH3 was achieved at an operating temperature of 350°C for all the platinum doped sensor elements and at 400°C for undoped sensors The best response for NH3 is observed

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in the sensor element annealed at 800 after it was green formed

from Pt doped powder calcined at 600°C The response magni-

tude of 125 observed for the best Pt doped WO3 sensor element

was significantly than a magnitude 9 for the best pure WO3 sensor

element

Acknowledgements

We are thankful to Mr Jain, CEERI Pilani for providing the sensor

substrates We are also thankful to Dr S.K Halder for measurement

of XRD One of the authors (VS) thanks Council of scientific and

industrial research for the award of Research Associateship

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Biographies

T.D Sengutavan obtained his MLE from REC Trichy in the field of Materials Science with specialization in Ceramics and Ph.D in Sol-gel processing from IIT Delhi He is working as scientist in National Physical Laboratory, New Delhi, for the past 13 years His research interest includes ceramic structures, powder processing, microwave sintering and metal oxide gas sensors

Vibha Srivastava obtained her Ph.D in 2004 from Gorakhpur University in materials science Presently she is working as research associate at National Physical Labo- ratory, New Delhi Her current research interests are nanoscience, nanostructure mesoporous materials and gas sensors

Jai S.Tawala obtained his M.Sc in 2006 from Nagpur University in Physics Presently

he is working as technical assistant at National Physical Laboratory, New Delhi His current research interests are nanostructure materials and characterization

Monika Mishra obtained her M.Sc in 2006 from Bundelkhand University in Physics Presently she is working as research intern at National Physical Laboratory, New Delhi Her current research interests are nanostructure materials and gas sensors

Shubhda Srivastava obtained her M.Sc in 2005 from Avadh University in Electron- ics Presently she is doing her M.Tech Project at National Physical Laboratory, New delhi Her current research interests are nanostructure material and gas sensors

Kiran Jain did her Ph.D in the field of high Tc superconductivity from Delhi Uni- versity She is working as scientist in National Physical Laboratory, New Delhi, for the past 25 years on the diverse area of research material science such as high Tc superconductors, ceramics, nanocrystalline semiconductors, thin film photovoltaics and metal oxide gas sensors etc

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