Preparation of Mesoporous and Meso-macroporous Sn 0 2 Powders and Application to H 2 Gas Sensors

Mesoporous SnO2 (mp-SnO2) and meso-macroporous SnO2 (m·mp-SnO2) pellet-type gas sensors were fabricated by the sol-gel method employing SnCl4·5H2O as a Sn source. The mesoporous structure was controlled by C20H37O7SNa, while the macroporous structure was controlled by polymethylmethacrylate (PMMA) microspheres. The introduction of macropores by the addition of PMMA microspheres into mp-SnO2 tends to increase the pore diameter and crystallite size. The large amount of macropores introduced into mp-SnO2 sensors by the addition of PMMA microspheres in the preparation process signifi cantly increased the resistance of all the sensors. Among all those tested, the mp-SnO2 sensor with only 5 wt% Sb2O5 added exhibited the largest response at 400°C. The 70% response and recovery times could be reduced by the introduction of macropores.


Introduction
Sn0 2 is well known to be the most important material for semiconductor gas sensors since it can be used to detect a wide variety of gases with high sensitivity, good stability and at low cost. (1-5)In recent years, particular focus has been directed to mesoporous (mp-) Sn0 2 powders as sensor materials.(6)(7)(8)(9) However, the poor thermal stability of the mesoporous structures of the mp-Sn0 2 powders synthesized so far limits their applications to gas sensors, which are usually operated in the tenlperature range of 250 -500°C.In our previous study, thermally stable mp-Sn0 2 powders were prepared by employing the self-assembly of ~ general surfactant as a template for the mesopore, (6,10) but the gas sensing properties of the mp-Sn0 2 sensors were relatively lower than expected from their large specific surface area.In addition, we have demonstrated that well-developed macroporous ceranlic films, which were prepared by a modified sol-242 Sensors and Materials, Vol.21, No. 5 (2009) gel method employing polymethylmethacrylate (PMMA) microspheres as a ten1plate, showed excellent sensing properties to H 2 .(l1,12)This study is, therefore, focused on preparing thermally stable mp-Sn0 2 powders with submicron-sized macropores (m'mp-Sn0 2 powders) by employing PMMA microspheres, with the aim of improving their H2 sensing properties.In addition, the effects of the addition of Si0 2 and Sb 2 0 s to m'mp-Sn0 2 on the H2 gas sensing properties have also been examined.

Preparation of mp-Sn0 2 and m'mp-Sn0 2 powders
mp-Sn0 2 and m'mp-Sn0 2 powders were prepared employing SnCI 4 '5H 2 0 as a Sn source, and the mesoporous and macroporous structures were controlled by selfassemblies of sodium bis(2-ethylhexyl)sulfosuccinate (aerosol-OT, AOT, Kishida Chemical Co., Ltd.) and PMMA micro spheres with a diameter of 800 nm (MP-1600, Soken Chern.& Eng.Co., Ltd.), respectively.The typical preparation procedures of mp-Sn0 2 and m'mp-Sn0 2 were as follows.A given amount of each constituent listed in Table 1 was mixed in 400 ml of ultrapure water and the pH value of the resulting mixture was adjusted, by adding an NH3 aqueous solution, to be 8.5 in some cases.As for tetraethoxysilane (TEOS) and SbCI 3 , the amounts required to produce the given amounts of Si0 2 and Sb 2 0 s were added to the solution.The mixed solutions were maintained at 20°C for 3 days.Thereafter, the solutions were evaporated in an oven at 80°C overnight.The resultant powders were then treated with a 0.1 mol dm-3 phosphoric acid solution for about 2 h, and then subjected to heat treatment at 650°C for 5 h in air.Hereafter, each sample will be referred to by its abbreviation listed in Table 1.The crystal phases of mp-Sn0 2 and m'mp-Sn0 2 powders were characterized by X-ray diffraction analysis (XRD; Rigaku, RINT2200), and the crystallite sizes were calculated using Scherrer's equation.
The specific surface area and pore size distribution were measured by the Brunauer-Emmett-Teller (BET) method using a N z sorption isotherm (Micromeritics, TriStar3000).The morphology of the powder pellets was observed by scanning electron microscopy (SEM; lEOL Ltd., JCM-S700).
2.2 Fabrication 0/mp-Sn0 2 and m'mp-Sn0 2 sensors and measurement o/their H2 sensing properties mp-Sn0 2 and m•mp-SnO z sensors were prepared as follows.Before heat treatment, the powder was molded into a pellet at a pressure of 1,000 kg cm-2 • Then, the pellet was calcined in air at 600°C for S h.A pair of Pt electrodes was fabricated on the pellet surface by screen printing.The gas sensing properties of the mp-and m•mp-Sn0 2 pellet-type sensors to 1,000 ppm Hz were measured at a flow rate of 0.1 dm 3 min-J in the temperature range of 300-S00°c.The magnitude of the response was defined as the ratio (R/R g ) of sensor resistance in air (R.) to that in 1,000 ppm H2 balanced with air (R g ).

Effects o/introduction o/macropores and/or various additives to mp-Sn0 2 on the H2 sensing properties 3.1.1 Characterization 0/ mp-Sn0 2 and m'mp-Sn0 2 powders
The pore size distribution and specific surface area (SSA) of mp-Sn0 2 (samples A, A-S5 and A-TS5) and m•mp-Sn0 2 (samples A-P, A-PT, A-PS5, A-PTS5) powders are shown in Fig. 1.Powder A, which was prepared without PMMA, TEOS or SbCI 3 , showed a larger SSA of 152.1 m 2 g-I and a larger pore volume of 0.145 cm 3 g-l with a smaller centered pore diameter of ca.2.9 nm than those of a conventional SnO z powder.(2)(3)(4) This means that powder A had a well-developed mesoporous structure.The addition of 5 wt% Sb 2 0 5 , i.e., A-S5, resulted in a slight increase in SSA (164.3 m 2 g-I), while the addition of Si0 2 to the A-S5, i.e., A-TS5, markedly increased the SSA (200.2 m 2 g-I) and reduced the centered diameter of the mesopores (to ca. 2 nm).
On the other hand, the introduction of macropores to powder A, i.e., A-P, slightly reduced the SSA (143.2 m 2 g-l) and broadened the distribution of the mesopores with a larger centered pore diameter (ca. 4 nm), but the pore volume remained unchanged.These results support the finding that the introduction of macropores tends to increase the diameter of the mesopores, although the reason for this phenomenon is not clear at present.The addition of 5 wt% Sb z 0 5 to A-P, i.e., A-PS5, was hardly effective in modifying the mesoporous structure, while the addition of 9 wt% SiOz to A-P, i.e., A-PT, significantly increased the SSA and the pore volume with a smaller centered pore diameter (ca.3.5 nm).The addition of 5 wt% Sb 2 0 5 to A-PT, i.e., A-PTSS, resulted in a decrease in SSA without a marked change in the pore size distribution.Figure 2 shows SEM images of an mp-SnO z (A) pellet and representative m'mp-Sn0 2 (A-PS5 and A-PTS5) pellets after calcination at 600°C for 5 h.For pellet A, a large amount   of mesopores (23-69 nm in diameter) can be observed at the surface, but the similar size of mesopores was not confirmed in Fig. l(a).This means that a large amount of small mesopores with a centered pore diameter of ca.2.9 nm was well-developed inside the oxide walls.On the other hand, the formation of well-developed and spherical macropores in the range of 400-450 run and 380-400 run in diameter was observed at the surface of A-PS5 and A-PTS5, respectively.The n10rphology of such spherical macropores well reflected that of PMMA macrospheres (ca.800 nm in diameter), but the ratio of their shrinkage was markedly high (ca.47 and 59% for A-PS5 andA-PTS5, respectively).
Figure 3 shows XRD patterns of typical mp-Sn0 2 and m'mp-Sn0 2 powders after calcination at 650°C for 5 h.It is clear that all the powders have peaks corresponding to the Sn0 2 crystalline phase (JCPDS 88-0287).The crystallite size (CS) increased with the introduction of n1acropores into mp-Sn0 2 (from comparison of A and A-S5 with A-P and A-PS5, respectively).This is probably because the crystal growth was accelerated by the heat of the con1bustion of PMMA microspheres added as a macropore template.The crystal growth is also responsible for the increase in diameter of the mesopores observed for m'mp-Sn0 2 in comparison with that of mp-Sn0 2 , as shown in Fig. 1.In addition, it was revealed that CS was markedly decreased by the addition of 5 wt% Sb 2 0 s , from comparison of A and A-P with A-S5 and A-PS5, respectively, but no diffraction peaks other than Sn0 2 were observed.This implies that the antimony added was sufficiently incorporated into the Sn0 2 crystal lattice and a very small amount of Sb 2 0 5 andlor antimony-based oxides prevented the crystal growth among Sn0 2 crystallites.(l6-18)

H2 sensing properties of mp-Sn0 2 and m•mp-Sn0 2 sensors
Figure 4 shows response transients of three types of m-Sn0 2 sensor (A, A-S5, and A-TS5) and four types of m•mp-Sn0 2 sensor (A-P, A-PS5, A-PT, and A-PTS5) to 1,000 ppm H2 at 400°C.The introduction of a large amount of macropores into mp-Sn0 2 sensors by the addition of PMMA microspheres in the preparation process markedly increased the resistance of all sensors (from comparison of A, A-S and A-TS5 with A-P, A-PS5 and A-PTS5, respectively), because these macropores greatly reduced the conductive pathways.In contrast, the addition of 5 wt% Sb 2 0 s reduced the sensor resistances, even though the crystallite size decreased upon Sb 2 0 s addition (see Fig. 3).The resistance decrease can be explained by the valency control, i.e., partial substitution of Sn 4 + sites with Sb s + ions, producing free electrons, as described in eq. ( 1).(13-1S) (1) On the other hand, the simultaneous addition of 9 wt% Si0 2 with Sb 2 0 s increased the sensor resistances.The hydrolysis of TEOS, as a Si0 2 source, is generally slower than that of SnCl 4 and SbCl 3 as Sn0 2 and Sb 2 0 s sources, respectively.Therefore, the Si0 2 probably segregated around the agglomerates of Sn0 2 doped with Sb 2 0 s , and then strictly limited the electron conduction among them.
To investigate the effects of the introduction of macropores and the addition of Sb 2 0 s and Si0 2 into mp-Sn0 2 and m'nlp-Sn0 2 on the H2 sensing properties, the operating temperature dependence of the magnitude of the H2 response is also depicted in Fig. 5. Sensors A and A-P, which were prepared without TEOS or SbCl 3 , showed relatively low H2 responses, while the addition of 5 wt% Sb 2 0 s led to a large increase in H2 response (see A-S5 and A-PS5 in Fig. 5).However, the sinlultaneous addition of 9 wt% Si0 2 with 5 wt%Sb 2 0 s decreased these H2 responses (see A-PT, A-TS5 and A-PTS5).In contrast, the introduction of macropores does not seem to contribute to improving their responses sufficiently, from comparison of A, A-S5 and A-TS5 with A-P, A-PS5 and A-PTS5, respectively.Among these, it is apparent that some m'mp-Sn0 2 sensors (A-P and A-PTS5) showed a slightly larger H2 response than the mp-Sn0 2 sensors (A and A-TS5), probably because the m•mp-Sn0 2 powders had larger pore diameters than the mp-Sn0 2 powders (see Fig. 1), and thus a larger number of active sites than the mp-Sn0 2 powders.On the other hand, A-S5 showed a relatively larger H2 response than A-PS5, even though the pore dian1eter was smaller.This suggests that the existence of the additives (Sb 2 0 5 or Si0 2 ) significantly affects the activity of gas reaction sites in m•mp-Sn0 2 .
The 70% response and recovery times of these sensors could be effectively reduced by the introduction of macropores, as summarized in Table 2.These results support the finding that the macropores markedly improved gas diffusivity in the m'mp-Sn0 2 films and all gas molecules, such as H2 as a reactant and H 2 0 as a product, easily access or more away from the active sites on the m•mp-Sn0 2 •

Effects ojSb 2 0 S addition to m'mp-Sn0 2 on their sensor properties
The effects of the amount of Sb 2 0 5 addition to m'mp-Sn0 2 on the H2 sensing properties were further investigated in detail, as shown in Fig. 6, since the addition of 5 wt% Sb 2 0 5 markedly improved the H2 responses of sensors A, A-P and A-PT.The addition of Sb 2 0 5 up to 10 wt% was found to markedly reduce the sensor resistance in air.Therefore, it is recognized that the antimony was doped into the Sn0 2 • Beyond the addition of 10 wt%, the addition of Sb 2 0 5 led to an increase in resistance with increasing  amount of SbzOs addition, probably owing to segregation of SbzOs-based compounds due to the solubility limit of SbzOs into SnO z , although these impurities were not confirmed by XRD due to low crystalline and/or small amounts thereof.Generally, it was reported that the solubility limit of SbzOs into SnO z was ca. 5 wt% and that 5 wt% doping results in the smallest CS(lS-18) and the lowest resistance.The higher solubility limit of SbzOs observed in this study may result from localized heat from the combustion of the PMMA microspheres added in the preparation process, as discussed above.
Figure 7 shows the operating temperature dependence of the H2 response of all Sb 2 0s-added m'mp-SnO z sensors (the amount of Sb 2 0 s addition: 0-50 wt%).A-PTS5 showed the highest Hz response among them, irrespective of its low sensor resistance.This suggests a large acceleration of the combustion of H2 with chemisorbed oxygen on the surface by the SbzOs doping.In addition, the response and recovery times of A-PTS5 were also the shortest among all the sensors (see Table 2).However, the sensor response of A-PTS5 was lower than those of A-S and A-PS5, as also shown in Fig. 5.This may arise from the fact that the existence of Si0 2 in m•mp-Sn0 2 contributes to the increase in specific surface area, but also a reduction of the number of active sites on the SnO z surface.More detailed investigation on the surface chemistry of m•mp-SnO z added with SbzOs and SiOz will be carried to clarify the changes in their gas sensing properties.

Conclusions
mp-Sn0 2 and m•mp-Sn0 2 powders with and without the addition of Si0 2 and/or Sb 2 0 s were prepared employing AOT and PMMA micro spheres as templates and their H2 sensing properties were investigated.
From the results, it was revealed that the introduction of macropores by the addition of PMMA microspheres into the mp-Sn0 2 tends to increase the pore diameter and CS.The addition of 5 wt% Sb 2 0 s reduced sensor resistance and the simultaneous addition of 9 wt% Si0 2 with Sb 2 0 s increased sensor resistance.The large amount of macropores introduced into mp-Sn0 2 sensors by the addition of PMMA micro spheres in the preparation process markedly increased the resistance of all sensors.The addition of Sb 2 0 s up to 10 wt% was found to reduce the sensor resistance in air, but beyond that led to an increase in sensor resistance.Among the sensors tested, the mp-Sn0 2 with only 5 wt% Sb 2 0 s added showed the largest response at 400°C.The 70% response and recovery times could be reduced by the introduction of macropores.

Table 2
Response and recovery times of representative mp-Sn0 2 and m•mp-Sn0 2 sensors.