In recent years, many groups have devoted a great deal of their investigation to tungsten oxide for gas sensing applications, such as: the Department of Materials Science of the Kyushu University (Prof. Yamazoe), Departments of Physics and Chemistry of Materials, University of l'Aquila (Prof, Santucci), the Laboratory for Surface Science and Technology of the University of Maine (Prof. Vetelino and Prof.
Lad) and sensor group of Assoc. Prof. Nguyen Van Hieu, International Training Institute of Materials Science (ITIMS). Besides, there have been many other contributions from several other groups, what has led tungsten oxide to be highly appreciated in the field of MOX gas sensors.
In practice, these publications have been showing that tungsten trioxide is an interesting material for the detection of mainly three gases: ammonia, hydrogen sulfide and nitrogen dioxide. These investigations indicate that, sensor response of tungsten oxide to these gases can be, at least, as good as the sensor response exhibited by tin oxide, if not higher. Moreover, tungsten oxide has revealed as low sensitive to carbon monoxide and hydrocarbons, what makes it more interesting for the detection of these gases.
Ammonia
Ammonia is a chemical made by both man and nature. The amount of ammonia produced every year by man is very small compared to that produced by nature every year. However, when ammonia is found at a level that may cause concern, it is usually produced either directly or indirectly by man. It is a colorless gas with a very sharp odor. The odor is familiar to most people because ammonia is used in smelling salts,
household cleaners and window cleaning products. Ammonia is very important to animal and human life. It is found in water, soil, and air, and is a source of much needed nitrogen for plants and animals. Most of the ammonia in the environment comes from the natural breakdown of manure and dead plants and animals.
Table 4. Occupational Exposure Standards 2000 of the UK Health and Safety Executive[29].
8-hour Exposure 15-minute Exposure
Gas ppm mg/m3 ppm mg/m3
Ammonia (NH3) 25 17 35 24
Hydrogen sulfide (H2S) 10 14 15 21
Nitrogen Dioxide (NO2) 3 6 5 9
Table 5. Some properties of NH3.
Ammonia has symmetric top structure, with one central N-atom above a tripod of three H-atoms (Figure 22). The A-axis corresponds to the figure axis of a symmetric top, and thus is also the main axis of symmetry. Table 5 lists the rotational constants for NH3 along with some other properties of the molecule. This figure also shows the molecule’s structure as well as its main symmetry axis, which coincides with the figure axis, along which lies the dipole moment. Three rotations of 120º about this axis each produce an indistinguishable change in the molecule. Thus, the main axis is 3-fold symmetric. In addition, NH3 has three planes of symmetry through the molecule at mutual angles of 120º, giving ammonia c3v molecular symmetry.
Eighty percent of all man-made ammonia is used as fertilizer. A third of this is applied directly as pure ammonia. The rest is used to make other fertilizers that contain ammonium. Ammonia is also used to manufacture synthetic fibers, plastics, and explosives. Many cleaning products also contain ammonia. Outdoors, you may be
Property Value Bond length r0(NH) = 1.02 Å
Bond angle 107.8º
A0 = 6.196 cm-1 B0 = 9.944 cm-1 Rotational constants
C0 = 9.944 cm-1 Symmetry number σ = 3
Dipole moment à= 1.471 Figure 22. NH3’s structure and symmetry
axis.
exposed to high levels of ammonia in air from leaks and spills at production plants and storage facilities, and from pipelines, tank trucks, rail cars, ships, and barges that transport ammonia. Higher levels of ammonia in air may occur when fertilizer is applied to farm fields. After fertilizer is applied, the concentration of ammonia in soil can be more than 3000 ppm; however, these levels decrease rapidly over a few days.
Indoors, you may be exposed to ammonia while using household products that contain ammonia. Some of these products are ammonia cleaning solutions, window cleaners, floor waxes, and smelling salts. You can also be exposed to ammonia at work because many of the cleaning products there also contain ammonia. Farmers, cattle ranchers, and people who raise chickens can be exposed to ammonia from decaying manure.
Some manufacturing processes also use ammonia.
Table 6. Requirements for NH3 gas detection equipment.
Applications Concentration requirements
Respond time
Working
temperature Action Environment
- Environmental Assessment - In animal cages
0.1ppb - 200 ppm 1ppm -25 ppm
~ Several minutes
~ 1 minute
0-40 oC 10-40 oC
Reducing pollution Animal protection Automation
- Measurement NH3 emissions from vehicles - Controlling gas in the car
4-2000 g/min 50 ppm
~ several seconds
~ 1 second
Up to 300 oC 0 – 40 oC
Improving air quality
Chemistry Gas leak alert Medicines Analysis of breath
20 – 1000 ppm
50 – 2000 ppb
~ Several minutes
~ 1 minute
Up to 500 oC
20 – 40 oC
High concentrations of NH3 gas could be explosive
Diagnosis of ulcers in the digestive system caused by bacteria
Detecting NH3 in the environment at high concentrations is quite easy due to it has high shocked properties. However, at low concentrations, the senses of man can hardly recognize the presence of NH3. Moreover, in some special cases as in the hazardous environments, humans can not directly use their senses to detect gas in the environment, that need the help of gas sensors. Some industries need to detect the presence of NH3 at extremely low concentrations are: Environment, automobile
industry, chemical industry and medical diagnostics, ... The auto industry highly focused on NH3 detecting in low concentrations, especially on air conditioning technology and exhaust gas treatment technology to reduce NO emissions into the environment by using NH3 based on the reaction (14):
(14) However, if uncontrolled NH3 gas emissions into the chamber so that enough, it will cause excess reaction NH3 and environmental release. Thus emissions of cars in the chamber should have a device to control the amount of NH3 emissions. Besides the auto industry, there are many other related industries also need to NH3 gas detecting.
Concentration demand or requirements for each case are given in Table 6.
The NH3 detecting devices
In these time, lots of NH3 gas sensor devices are widely offered for sale in the market. There are two kinds of this device, that is
built inside the sensor and sensor is not integrated.
Figure 23 (the left side one) is second type of this product. It is simply the signal processing and display equipment. This category has the advantages of mobility, that is no need to move the system for examination, just put the sensor head to the scene and connect to the system
processor. However, it causes large signal loss, that make manufacturers should pay attention to. This type is usually not as exactly as the product has a built inside gas sensors.
It is necessary to use mobile sensors for products that do not have integrated sensors. The sensor head is often very compact and easy to assemble a processing system (Figure 24).
Currently, Some gas detecting technique are being commercialized in the world, they are using semiconductor metal oxide materials, conductive polymer or using optical analysis equipment. The conductive polymer is although operating at room temperature but it has some big disadvantages, that is short-lived and not very high sensitivity. Besides, the optical devices accurately analyze but has large size, difficult to install, reducing the mobility of the system and very expensive. So the technique using metal oxide semiconductors materials is still the first choice for this type of sensor, although it has still some drawback but has been gradually being overcome.
Leading in the promising metal oxide semiconductors material, which is used in NH3
Figure 23. Some types of ammonia detector.
gas sensor is WO3 material with the interesting properties such as, it could operate at lower temperatures, low cost, selected with NH3, ...
Yamazoe et al.
[3],[57],[76],[90],[99],[102] proposed that Au-catalyzed WO3 is an excellent sensitive material for ammonia detection. Their sensors have usually a thick-film sensitive layer based on WO3 powder, obtained by a pyrolytic route [57]. This group has analyzed the influence of the thickness of the sensitive layer [3], the addition of many different additives [76],[90] and the interference of NO on ammonia sensor response [99].
Table 7. Selected publications on NH3 gas sensors based on WO3
Materials Sensitive layer Concentration range
Sensor response (Temp., Concentration)
Year/
Reference WO3:Au Powder - dip coat. 5 ppb – 50 ppm ~ 40 (50 ppm, 450ºC) 1992 [57]
WO3 Sputtering 10 ppm ~ 1.1 (300 ºC) 1995 [73]
WO3 Sputtering 1 – 27 ppm ~ 10 (1 ppm) 1995 [61]
WO3 Drop-coating 10 – 1000 ppm ~ 6 (1%, 300 ºC) 2000 [54]
WO3:Mg 7.65 (350 ºC)
WO3:Zn 5.06
WO3:Mo 10.1
WO3:Re
Powder - dip coat. 30 ppm
9.46
2000 [90]
WO3 Sputtering 300 - 1500 ppm 9.7 (1500 ppm, 250 ºC) 2011 [67]
WO3 Drop-coating 25 ppm ~1.11 (400ºC) 2003 [2]
WO3 Sputtering 100 ppm ~7 (400ºC) 2003 [96]
Their Au-loaded WO3 exhibited excellent sensing properties to NH3 in air at 450 ºC, being able to detect 5 ppb of ammonia in air. The authors attribute their high sensor response to the fact that small particles of Au are dispersed on the surface of WO3 grains and oxygen is adsorbed on these particles and on the interface between Au and tungsten trioxide [102]. The adsorbed oxygen, especially that located on the interface, traps electrons and thus forms an electron-depleted layer. This oxygen
Figure 24. Some of commercialized gas sensors head.
species are consumed by ammonia, resulting in a relaxation of the depletion layer.
Meixner at al [61], using a thin-film gas sensor, have also confirmed that tungsten oxide has proved to be the most suitable material for ammonia detection. Finally, Llobet et al [54] built drop-coated WO3 sensors on silicon substrates, showing the compatibility of thick film with new silicon technologies. A summary of these and other papers is provided in Table 7.
As a result of these works, WO3 appears to be a promising material for the detection of ammonia. Some additives have been identified as suitable to enhance sensor response. However, the specific reactions that occur on tungsten oxide have not been completely explained. Besides, the interference of humidity, as well as the role played by additives, merit further investigation.