ing open and low density material with a "house-of- cards" structure [1]. This porous structure is formed by lowering the pH of suspension to a specific and a ...
Appl. Phys. A 63, 271-275 (1996)
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A highly sensitive and selective hydrogen gas sensor from thick oriented films of MoS2 Bijan K. Miremadi, Ravi C. Singh, S. Roy Morrison, Konrad Colbow
Department of Physics, Simon Fraser University,Burnaby, B.C. V5A 1S6 Received: 29 March 1995/Accepted: 28 May i996 Abstract. A new process is developed to fabricate a highly sensitive and selective hydrogen sensor by depositing a partially crystalline and highly oriented film of Mog2 from its single layer suspension on an alumina substrate. When these films are promoted with some catalysts selected from Pt-group metals (Pt, Pd, Ru or any combination of these metals) they exhibit a high sensitivity and selectivity to hydrogen gas. Unlike other metal oxide sensors which are sensitive to many reducing and oxidizing gases and operate at a temperature of 350 °C or higher, this sensor is highly selective to hydrogen gas and its operating temperature is from 25 to 150°C. The lower operating temperature enhances safety when dealing with hydrogen gas. The sensor response to hydrogen at 120 °C is linear in concentration from 30 to 104 ppm with a 10 to 30 second response time and a 45 to 90 second recovery time. Above 104 ppm the sensor is still linear but the slope of conductance versus hydrogen concentration changes.
As the use of hydrogen becomes ever more ubiquitous in oil upgrading, semiconductor processing, and as transportation fuel, there is an increasing demand for sensors which are sensitive to hydrogen gas only, rather than detecting hydrogen and other residual gases. We have reported in an earlier paper [-1] the formation of highly oriented films of layered compounds on A1203 substrates, using exfoliated disulfides such as MoS2 [2] or WS2 [3] in the form of single layers in suspension. To exfoliate the material, the layered compound powder is intercalated with lithium atoms, and then immersed in water where lithium reacts violently with water. The hydrogen generated between the layers of the disulfide, forces the layers apart and single molecular layers in suspension of aqueous LiOH are produced. These single layers can be restacked in many useful ways. For example, we can make inclusion compounds, where metal ions are deposited between the layers, expanding the c-spacing, as determined by X-ray diffraction, or we can produce
a structure with alternating single layers of for example MoS2 and WS2 [4]. One can deposit the layers as monolayers on a support such as alumina, providing a stable support for catalysts such as Pt-group metals [5], or one can attach the single layers together in the form of edge to basal planes, producing open and low density material with a "house-ofcards" structure [1]. This porous structure is formed by lowering the pH of suspension to a specific and a critical pH at which the edges of the single layers in suspension are charged opposite (OH-) to the charges on the basal planes (H ÷), and thus the attraction between the opposite charges forms a porous material. With a small increase in pH to a second critical value the attached edge-to-basal planes in the suspension can be detached, thus freeing the layers back into the suspension. The layers at this stage apparently find a charge matrix susceptible to form an oriented film when a few drops of this suspension are deposited on a substrate and dried. The oriented films prepared by this two step process have many unique physical and chemical characteristics, one of which, is their change of electrical conductivity when exposed to hydrogen gas, which makes them suitable for use in the fabrication of hydrogen gas sensor. The conductivity change can be enhanced many times when Pt or Pd catalysts are dispersed onto the surface of the films, followed by sintering in air. The following hydrogen reaction kinetics for the Pt-Mo system is the most probable mechanism of the conductivity change: H2(gas) + 2Pt ° ~ 2Pt: H - ~ Pt ° + H + + ek2
(1)
where kl and ka are the rate constants of the reaction. Equation (1) represents the catalytic dissociative chemisorption of hydrogen molecules on the surface. Following the spillover phenomena, the adsorbed atoms then donate electrons to the conduction band of the sintered n-type polycrystalline semiconductor film. The donated electrons provide the required change in the electrical conductivity of the sensing material which in turn can be
272 monitored as a measure of sensor sensitivity. The adsorption/desorption of hydrogen is likely a fast reaction. Adsorption of oxygen may extract conduction electrons like O+e-
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If in the steady state for (1), the fractional hydrogen coverage (0) at the surface is small, then the rate of conductance change is directly proportional to the concentration of adsorbed atoms. The activation energy for adsorption [6] is then given by
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where E, is the activation energy for adsorption of hydrogen and k3 is the adsorption constant. Since the change in hydrogen coverage AO/At, is proportional to the rate of conductance change AG, the activation energy Ea can be obtained from a plot of AG versus 1/T. We found, as it will be discussed in the results, that the activation energy is low ( ~ 3.2 kcal/mol), corresponding to a temperature range of 80 150°C, where the sensor is highly active.
1 Experimental MoS2 powder (98 % purity) was from Johnson Matthey, n-butyllithium in hexane (2.5 M) from Aldrich Chemical and Pd and Pt chlorides from BDH Chemical. A model ISI-DS130 scanning electron microscope (SEM) with an EG&G ORTEC energy-dispersive X-ray attachment (EDX) was used for micrographs and to analyze the elemental compositions of the sample. The X-ray powder diffraction system (XRD) used was a Philips diffractometer with 1.5405 °A CuK~ radiation and Ni filter.
cates a "house-of-cards" structure [1]. To prepare the oriented films, the pH of the collected liquor is raised to a pH of 3.3 __+0.1, by washing in distilled water (pH ~6) just enough to return the house-of-cards structure back to a single layers form in suspension. A drop of this suspension is deposited onto a glass substrate and dried; the X-ray pattern indicates an ordered restacking of single layers in the form of an oriented film [1]. The sensors are then prepared by depositing a few microliter of the new suspension onto an alumina substrate between two electrical gold contacts to obtain a film ( = 1 btm thickness), to which the desired catalysts (Pt, Pd, etc.) are added. This is followed by sintering in air at ~ 340 °C. For the purpose of heating and controlling the temperature of the substrate, a thin film heater and thermistor were deposited in a prior silk screen process on the back of the alumina substrates.
3 Results Fig. 1 shows the crossectional view of a hydrogen sensor, indicating the substrate, the gold electrical contacts and the sensing material. The heater and thermistor are on the bottom of the alumina substrate (not shown). Fig. 2 is a graph of sensitivity ( G - Go)/Go versus hydrogen concentration in air at 20 °C and 120 °C, where Go is the conductance in absence of hydrogen, and G the Sensing Material
Gold Electrical Contact
Rlumina substrate
Fig. 1. Crossectional view of a hydrogen sensor, indicating the substrate, the gold electrical contacts and the sensing material. The heater and thermistor are inbedded in the substrate but are not shown
2 Sample preparation The preparation of MoS2 single layers in suspension is described in detail in the references [1-5]. In brief, the as-received powder is immersed in a hexane containing n-butyllithium in a glove box. The suspension is left for about 48 hours to intercalate the Li into the MoS2. The excess n-butyllithium is washed off with hexane, the solvent decanted and the powder dried while still in the glove box. The powder is then removed and immersed in water, resulting in the exfoliation of the Li intercalated powder into single layers in suspension of aqueous LiOH (pH value of 12 14). The high pH suspension is poured into a larger container of pH 1.8 HNO3 sotution while the solution is stirred at high speed. The equilibrium pH of the solution at this stage is about 2 _+ .05, and within a few minutes the suspension of layers will begin to flocculate, and in a few hour it clears, with all the newly formed particles settled at the bottom of the container. When the low pH solution is decanted and the collected liquor is dried, X-ray diffraction examination of the powder indi-
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conductance when the sensor is exposed to different concentrations of hydrogen. While the sensitivity at 20 °C is high, the response and the recovery times are a few minutes, compared to 25-30 seconds at 120 °C (Fig. 3). In general these sensors show a response more than 100 times smaller to other reducing and oxidizing gases than to hydrogen gas. At 275 ppm hydrogen concentration, Fig. 3 shows the response time (time to reach 90% of peak value) of the same Pt-based hydrogen sensor as a function of temperature and Fig. 4 the activation energy of the sensor. Fig. 4 shows two different slopes: In the low temperature range (20-85 °C), where the sensor has low sensitivity and a slow response and recovery, the activation energy is 44.5 Kcal/mol. Between (85-150 °C), where the sensitivity
is high, the activation energy is 3.2 Kcal/mol. Above 150°C the sensitivity drops rapidly, but the response is faster. Fig. 5 shows a comparison of X,ray diffraction patterns of as received MoS2 powder (Fig. 5a), the edge to basal planes MoS2 "house-of-cards" structure (Fig. 5b), and MoS2 oriented films of about 1 pm thickness (Fig. 5c). Fig. 6 shows an X-ray diffraction comparison of a Ptdispersed oriented film at different sintering temperatures. The oriented structure does not change at sintering up to 300 °C (Fig. 6a). At 340 °C, the pattern indicates conversion of MoS2 into MoO3 (Fig. 6b). At 375 °C almost 90% of MoS2 film is converted into MoO3 (Fig. 6c). The sintered film at 425 °C (Fig. 6d) is totally converted into the oxide MOO3. Platinum acts as a strong oxidation catalyst at these temperatures, which in turn enhances the oxidation rate. The lack of any line due to Pt or its oxide indicates a high degree of Pt dispersion in the film and thus a strong Mo-Pt bond. Fig. 6e shows for comparison the X-ray lines of M e oxide from a pure as-received M o O 3 powder.
4 Discussion On formation of the oriented films, it should be noted that if the pH value of single layer suspension (pH 12) is
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lowered by using HNO3 solution toward the point of zero charge (pzc), or even lowering it further below this point, the X-ray diffraction pattern of the dried powder shows signs of random restacking of the layers. In random restacking, the diffraction pattern shows a strong (0 0 2) line and less stronger (0 0 l) lines (1 = 4, 6, 8, --. ), in addition to the mixed (h k/) lines (Fig. 1 in Ref. 1). On the other hand, if the pH is lowered, going through the pzc at pH 2 (formation of house-of-cards structure) and then back up to pH 3.5 (re-dispersion of the layers into suspension), the X-ray pattern of the dried layers indicates an ordered restacking of the layers. In this case one observes a strong (0 0 1) line and only less stronger (0 0 l) lines (l = 2, 3, 4, ..- ) without any mixed (h k l) line (Fig. 5c). If the pH is raised
further to pH > 3.5, the dried layers show random restacking again. The promotion of the films with Pt or Pd catalyst and sintering at 340 °C has three functions: At this temperature Pt atoms bond to Mo by replacement of sulfur atoms, forming strong Mo-Pt bonds [7]. Thus, the X-ray diffraction pattern of the sintered fihn Fig. 6b shows no line due to Pt or its oxides, although laser ablation analysis of the sintered films showed a high concentration of Pt atoms with a Pt/Mo atomic ratio of 0.24. This indicates high dispersion of Pt on the MoS2 films with a Mo charge transfer of Mo 6÷ ~ Mo 5+, as a result of Pt electron donation and formation of a strong crystal field Mo-Pt bond [5]. The second function of the catalysts is their dissociative and reactive characteristics (Eqs. 1-3) in which they can dissociate hydrogen molecules into atoms for absorption and electron donation, or the reaction between 2H, H2, O - and 02- species to form H 2 0 and electrons. This results in a high conductivity change when the films are exposed to hydrogen molecules. The third function of the catalysts is to permit a low operating temperature by lowering the sensor response and recovery times. These times are also determined by the degree of oxidation of the noble metals during the high temperature sintering. Pt promoted sensors operate at temperatures as low as 20 °C, but with slower response and recovery. Pd promoted sensors, on the other hand, operate only above 100 °C due to their thicker oxide layer and show faster response and recovery. In the case of Ru catalyst, the oxide layer formed is relatively thick and the sensor operates only above 300 °C. The laser ablation analysis of the films without catalyst and before sintering shows a S/Mo atomic ratio of 1. This indicates that during exfoliation and acid treatment (pH adjustment), almost half the sulfur atoms, most likely the loosely bonded atoms on the edges, are lost. Sintering of the films without catalyst at 340 °C shows no significant change in this ratio since pure MoS2 films or powder remain stable below 400 °C in air. The overall model suggested by observation is then as follows: The highly dispersed Pt is a strong oxidizing agent and at 340 °C catalyses the oxidation of MoS2 in part to an oxide/sulfide mixed phase. It is well known that a mixture of phases can lead to a very active catalyst [7], since with various sites exposed in a mixed phase there is a wide spectrum of active sites, some of which are active even at low temperature. For example a Pt atom in a particularly active site can be visualized as itself providing the needed activity or it could be that, on these sites the Pt is weekly bonded to the neighboring Mo, allowing adsorption of H2 and its dissociation on a single Pt. As indicated from the X-ray analysis (Fig. 6b), it is found empirically that 340 °C yields the mixed phase, the temperature is high enough for oxidation with the help of deposited Pt, but not high enough for complete oxidation to single-phase MOO3. As both MoO3 and MoS2 are n-type semiconductors, the mixed phase will also be a n-type semiconductor. We suggest a small fraction of surface sites, those that are extremely active, are active enough to dissociate hydrogen even at 20°C and inject electrons as in (1). Possibly
275 spillover occurs, magnifying the effect of the active sites, but this is not necessary for a significant change in conductivity. Figure 6.b shows that the most active sensors, sintered at 340 °C, maintain the layered structure to a significant extent. It is possible that the layered structure is a requirement for low temperature response, possibly Pt is on the edge sites on the layers and when hydrogenated acts as a layer-to-layer bridge for electrons. However, further work is needed to develop the model in such detail.
beneficial in two ways. First, the single layer material provides the high dispersion of the Pt despite the large Pt/Mo atomic ratio. Such an atomic ratio on powdered MoS2, for example, would be impossible without the Pt forming crystallites and probably leading to metallic conductivity. Second, the single layer MoS2 has clearly defined sites that are easily oxidized (edges of the layers) and sites that are passive (basal planes). Thus oxidation at 340 °C is reasonably well defined and reproducible.
References 5 Conclusion A new Pt-MoS2/MoO3 sensor, highly sensitive to hydrogen at low temperature, has been described. From the above discussions and the presented results, the high sensitivity and selectivity of the sensor may be reasonably explained. The stability of both MoS2 and M o O 3 permits the mixed phase to be stable at low temperature. The strong bond between Mo and Pt as discussed in reference [5], results in high dispersal of Pt on stable oxysulfide sites. The mixed phase leads to sites with varying character, corresponding to high and low activation energy that permits hydrogen dissociation even at room temperature. The use of single layer MoS2 as the support for Pt, prepared using various steps outlined above, is probably
1. Bijan K. Miremadi, Timmothy Cowan, S. Roy Morrison: New structure from exfoliated MoS2, the house of cards. J. Appl. Phys. 69, 6373, 1991 2. Bijan K. Miremadi, S. Roy Morrison: High activity catalyst from exfoliated MoS2. J. Catal. 103, 334, 1987 3. Bijan K. Miremadi, S. Roy Morrison: The intercalation and exfoliation of WS2. J. Appl. Phys. 63, 4970, 1989 4. Bijan K. Miremadi, S. Roy Morrison: A new superlattice structure from alternate single layers of MoS2 and WS2. J. AppI. Phys. 67, t515, 1990 5. Bijan K. Miremadi, S. Roy Morrison: MoS2 single layers for stabilization and activation of Pt oxidation catalysts. J. Catal. 131, 127, t991 6. S. Roy Morrison, M.J. Madou: In Chemical Sensing with Solid State Devices Academic Press, INC. Page 69, 1988 7. S. Roy Morrison: The Chemical Physics of Surfaces, 2nd. ed. Plenum Press Pages 141,158 and 283, 1990
