Page 390 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
which is a typical oxide semiconductor gas sensor sensor having the mesoporous structure is clear.
material [1]. The measurement was performed at Incidentally, since the SnO ceramics used for the
2
300 C in air containing 800 ppm H or CO. The sen- measurement had large secondary particles, it is
2
sitivity increases significantly with decreasing par- questionable whether mesoporous structure con-
ticle size lower than 10 nm. It has been reported that tributed to the enhancement of gas detection effec-
the thickness of the space charge layer of SnO is tively. Therefore, it is expected that the sensitivity of
2
approximately 3 nm [2]. From these results, the sen- the oxide semiconductor gas sensor with ordered
sitivity of gas detection is considered to be mesoporous structure increases further with decreas-
improved dramatically when the space charge layer ing size of the secondary particles.
occupies almost all the inside of each particle.
Moreover, it has been indicated that not only the (2) Characteristic of nanosized metal sensors using
sensitivity but also the response improves by
decreasing the particle size of CuO-doped SnO to a surface plasmon resonance
2
few nanometers [3]. Thus, the gas detection is based It is known that a surface plasmon resonance which is
on the chemical reaction on the surface of oxide an excitation of plasmon by light occurs on the sur-
semiconductor particles, and the characteristics can face of metal, and light with a specific wavelength is
be remarkably improved by controlling the particles absorbed. For this reason, the metal nanoparticles
size in the nanometer scale. used for coloring of the stained glass in church and
In order to increase contact frequency between the ornament glass exhibit a vivid color for a long time.
surface of the particle and the gas to detect, the The chemical sensors using the surface plasmon res-
oxide semiconductor gas sensors are commonly onance attract attention in recent years.
made to have porous microstructure. For this reason, Since plasmon which is the quantum of plasma
it is very important in the improvement of the sens- oscillations is vibration of electron density, it nor-
ing characteristics to control the porosity of sensor mally does not interact with light which is a trans-
structure in the nano/meso-size. Recently, ordered verse wave. However, for example, if a nanosized thin
mesoporous ceramics, such as SnO and TiO which metal deposited on a prism will be irradiated with p-
2
2
have stable structure even at high operating temper- polarized light through the prism under the conditions
atures in the range of 200–400 C, has been synthe- of total internal reflection, an exponentially decaying
sized and their gas-sensing characteristics have been wave called an evanescent wave will be generated on
evaluated. Fig. 6.5.11 describes the sensitivity to the metal surface, and the excitation of plasmon will
500 ppm H for ordered mesoporous SnO ceramics take place.
2
2
2
–1
having a large specific surface area of 305m g [4]. The penetration depth of the evanescent wave is
Compared with the ceramics having a specific comparable with a wavelength of light, and the wave
–1
2
surface area of 8.4 m g , the advantage of the number k is given by the following formula:
e
⎛ ⎞
k ⎜ ⎝ c ⎠ ⎟ n sin (6.5.3)
p
e
where is the angular frequency of light, c the
speed of light in vacuum, n the refractive index of
p
the prism and an incident angle of light. The sur-
face plasmon resonance arises when the incident
light is absorbed under a coupling condition that k e
is equal to the wave number k of the surface plasma
sp
oscillation:
⎛ ⎞
ms
k ⎜ ⎝ c ⎠ ⎟ + s (6.5.4)
sp
m
where and are dielectric constants of the metal
m
s
and thin layer on the metal surface, respectively. If a
laser is used as a light source, the coupling condition
Figure 6.5.11 will depend on the incident angle . Therefore, if is
The sensitivity to 500 ppm H of SnO with various changed, reflected light will be absorbed at a certain
2
2
–1
2
2
–1
specific surface areas [4]. : 305 m g ; : 8.4 m ·g . incident angle.
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