Flame-Made Pt-Loaded TiO2 Thin Films and Their Application as H2 Gas Sensors

1 Department of Chemistry, Faculty of Science, Chiang Mai University, 239 Huay Kaew Road, Muang District, Chiang Mai 50200, Thailand 2Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, 239 Huay Kaew Road, Muang District, Chiang Mai 50200, Thailand 3Nanoelectronics and MEMS Laboratory, National Electronics and Computer Technology Center, Klong Luang, Pathumthani 12120, Thailand 4Materials Science Research Center, Faculty of Science, Chiang Mai University, 239 Huay Kaew Road, Muang District, Chiang Mai 50200, Thailand


Introduction
Metal oxide nanopowders offer promising research materials because of their wide range of applications.Many studies have focused on enhancing the performance of gas sensors.They were synthesized by different techniques such as the modified sol-gel method [1], thermal plasma [2], hydrothermal [3], and flame spray pyrolysis [4,5].Flame spray pyrolysis (FSP) is one-step synthesis method that is suitable for large-scale production of noble metal-metal oxide nanocomposites [6][7][8].
Hydrogen gas is an important gas for clean energy sources, highly flammable, and burnable in air at a very wide range (4-75% by volume) [9][10][11].However, its presence due to leakage at a sufficient high concentration together with oxygen in air will cause explosion.Moreover, H 2 cannot be detected by human senses when it leaks.Therefore, semiconducting Pt-loaded TiO 2 is one of the most promising candidates for flammable gas detection due to its advantages including low cost, high sensitivity, fast response, simplicity of use, and ability to detect a large number of gases.
In this study, we report the synthesis of unloaded TiO 2 and Pt-loaded TiO 2 by FSP and study the effect of Pt catalyst dispersion in TiO 2 nanopowder surface on hydrogen gas-sensing behaviors due to the fact that Pt nanoparticles dispersed on the TiO 2 surface can act as electron sinks and result in a decrease in electron-hole recombination [12].In addition, Pt is known to be a very effective catalyst for several reducing gases on metal oxide supports [13,14].
The crystallinity phases of unloaded TiO 2 and Pt-loaded TiO 2 were analyzed by X-ray diffraction (XRD, Philips X' Pert MPD) using CuK radiation at 2 = 20-80 ∘ with a scanning speed of 5 ∘ /min.The morphology and size of the nanoparticles were characterized by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM).The existence of Pt was confirmed by energy-dispersive X-ray spectroscopy (EDS).
The unloaded TiO 2 and Pt-loaded TiO 2 sensing films were fabricated according to a previous report [9].The gassensing characteristics of unloaded TiO 2 and Pt-loaded TiO 2 sensing films in a stainless steel chamber were characterized over a high-concentration range of H 2 gas (150-10,000 ppm).The standard flow through technique was used to test the gas-sensing properties of unloaded TiO 2 and Pt-loaded TiO 2 films.A constant flux of synthetic air of 2 L/min as gas carrier was flowed to mix with the desired concentration of hydrogen gas dispersed in synthetic air.The sensors were tested for hydrogen gas at the operating temperature of 300-400 ∘ C. It should be noted that the operating temperature could not be below 300 ∘ C since the resistances of Pt-loaded TiO 2 films were too high and could not be measured below 300 ∘ C. By monitoring the output current through the sensor, the resistances of the sensor were continuously recorded to evaluate the sensor response, response time and recovery time.The sensor response () is defined as the resistance ratio   /  (  is a resistance in dry air,   is a resistance in the test gas) [9].Response time is defined as the time required until 90% of the response signal is reached while recovery time is the time to recover to 90% of baseline resistance.

Particle and Sensing Films Properties.
The XRD spectra of unloaded TiO 2 and 0.25, 0.50, 0.75, 1.00, 2.00, and 3.00 mol% Pt-loaded TiO 2 nanopowders are shown in Figure 1.The diffraction peaks of the unloaded TiO 2 and Pt-loaded TiO 2 samples showed that both anatase and rutile phases in the seven samples are very similar, which matched well with the * JCPDS file no.21-1272 and =JCPDS file no.21-1276, respectively.The XRD peak of Pt peaks was not found in these patterns (JCPDS file no.87-0640) because Pt was loaded in the range of very low concentration when comparing with the TiO 2 nanoparticles [8,9,15].
The bright-field TEM imaging was used to investigate the 2D accurate size and morphology of the nanopowders as

Hydrogen Gas-Sensing Properties.
The interaction of the resistive sensors with analyzing H 2 gas caused a change in the resistance of the unloaded TiO 2 and Pt-loaded TiO 2 sensors.Figure 4(a) shows the sensor resistance versus sensing time of H 2 sensing characteristics performed at the operating temperature of 300 ∘ C with different H 2 gas concentrations (0.015-1 vol%) and Pt loading levels (0-3.00 mol%).The interaction of the resistive sensors with a target gas resulted in a change in electrical conductance upon the gas exposure.The original baseline in resistance was quite stable after recovering from multiple gas exposures.When a reducing gas was detected using n-type semiconductors (TiO 2 ), the resistance decreased due to the increased number of electrons on the semiconductor surface [16].In addition, the resistance change was enhanced significantly with Pt loading and was monotonically increased with gas concentration.The sensing behaviors were subsequently analyzed in terms of sensor response (S), response time, and recovery time as illustrated in Figures 4(b  up to 2.00 mol% but decreased as the loading level increased further to 3.00 mol% at all gas concentrations.In addition, the sensor response increased linearly in log-log scale, indicating that the hydrogen response followed the well-known power law. The hydrogen detection limits at 300 ∘ C of Pt-loaded TiO 2 sensors were then estimated by power-law fitting the response curve and calculating the concentration at which the response was 1.1 (10% change of resistance).The calculated detection limits of unloaded and 0.25, 0.5, 0.75, 1.00, 2.00, and 3.00 mol% Pt-loaded TiO 2 sensors were estimated to be 1, 0.3, 0.1 and 0.05, 0.013, 0.004, and 0.009 vol%, respectively.The detection limits of TiO 2 sensors were greatly improved with Pt loading and the 2.00 mol% Pt-loaded TiO 2 sensor exhibited a low detection limit of 40 ppm, which can potentially be used for hydrogen leak detection.
In terms of response time (Figure 4(c)), the response time decreased rapidly with increasing H 2 concentration and was substantially improved with increasing Pt loading level.The response time naturally decreased with gas concentration due to faster gas dynamic at high concentration while Pt reduced the response time via catalytic effect, in which the hydrogen was quickly dissociated by Pt nanoparticles.The reaction rate monotonically increased with amount of Pt as 3.00 mol% Pt resulted in shorter response time than 2.00 mol% Pt.The 2.00 and 3.00 mol% Pt-loaded TiO 2 sensors exhibited very short response times of ∼8 and ∼5 seconds at H 2 concentration of 1 vol%.Regarding the recovery time (Figure 4(d)), the recovery time increased relatively slowly with increasing H 2 concentration and was rather insensitive to Pt loading level.The recovery time naturally increased with gas concentration due to longer time required to desorb larger amount of adsorbed gas species at high concentration while Pt does not play a direct role in desorption of hydrogen.The Pt-loaded TiO 2 sensors require rather long recovery time in the range of 700-1000 seconds, which could be due to longer time required for gas to diffuse out of the thick and porous TiO 2 layer.
Operating temperature generally has an important impact on the performance of semiconductor gas sensors.Typically, the sensing response of the metal oxides tends to increase with increasing temperature [17,18].Figure 5 shows the electrical resistance in air of 2.00 mol% Pt-loaded TiO 2 thick-film sensor as a function of time at different temperatures of 300 ∘ C, 350 ∘ C, and 400 ∘ C for the thick films of 2.00 mol% Pt-loaded TiO 2 .It was shown that the baseline resistance of the Pt-loaded sensor tends to decrease with increasing operating temperature while the resistances under 1 vol% hydrogen exposure are quite similar.Thus, the resistance change and corresponding response are decreased with increasing operating temperature.The effects of Pt loading level and operating temperature on gas-sensing response of 1 vol% H 2 performed at 300-400 ∘ C are shown in Figure 6.Pt-loaded TiO 2 sensors exhibited relatively high responses at 300 ∘ C and the response at the optimal operating temperature was particularly pronounced at 2.00 mol% Pt loading concentration.At higher operating temperature (350-400 ∘ C), Pt loading provided relatively lower enhancement of sensor response.Thus, the suitable operating temperature and concentration of Pt loading can greatly improve H 2 response of TiO 2 sensors.The best sensing performance with a high sensor response to 1 vol% of H 2 concentration of 470 was obtained at the optimal Pt-loading level of 2.00 mol% and optimal operating temperature of 300 ∘ C.

Hydrogen Gas-Sensing Mechanisms.
From experimental results, unloaded TiO 2 sensor exhibited very low response to H 2 at operating temperatures 300-400 ∘ C, which is in accordance with other reports indicating that direct dissociation of hydrogen on metal oxides will be effective above 400 ∘ C [19].With Pt loading in the range between 0.25 and 3.00 mol%, hydrogen response increased substantially.It has been widely accepted that Pt enhances sensitivity and response rate of a metal oxide sensor via chemical sensitization with hydrogen gas.Pt can dissociate H 2 into H atoms, which interact with TiO 2 support via the spillover process according to the reaction [20]: The adsorbed hydrogen molecules will interact with the preadsorbed oxygen species including O which were thermally activated on Pt-loaded TiO 2 surface, according to [21][22][23][24]: These reactions produce more electrons and thus increase the conductivity of n-type semiconductor (TiO 2 ) upon exposure to hydrogen with preadsorbed oxygen which leads to freeing of previously trapped electrons.
In order for the catalyst to be effective, there must be a good dispersion and appropriate amount of the catalysts; thus, catalyst particles are available near most interparticle contacts as illustrated in Figure 7(a).The spillover species must be able to migrate to most interparticle contacts in order to dominate the metal oxide resistance.The spillover mechanism will become less effective when catalyst particles are agglomerated or poorly dispersed as depicted in Figure 7(b).The agglomeration and poor dispersion of Pt nanoparticles reduce the number of spillover hydrogen species on TiO 2 support, thereby reducing the hydrogen response.Such aggregation is highly likely when the Pt loading concentration increases to a level as high as 3.00 mol%.Thus, 2.00 mol% is the optimal Pt loading level that yields high amount of Pt with good dispersion on FSP-made TiO 2 nanoparticle supports, which results in an optimal hydrogen response.
For the effect of operating temperature, Pt-loaded TiO 2 films exhibited optimum hydrogen response at temperature of 300 ∘ C and higher operating temperature leads to considerably degraded response.An explanation of sensor response of the temperature-induced change may be formulated by considering the occurrence of different types and concentrations of ionosorbed surface reactive oxygen species (O 2 − , O − , or O 2− ) on the Pt-loaded TiO 2 surface together with spillover effect [25][26][27][28][29][30][31].At a high temperature (i.e., 350-400 ∘ C) (Figure 7(c)), the number of adsorbed oxygen species, particularly O − or O 2− , increased significantly and some of them adsorbed around the Pt nanoparticles.The adsorbed oxygen species will shield Pt particles from H 2 and reduce ability of H 2 dissociation by Pt.Thus, hydrogen response decreases considerably because the spillover effect by Pt is substantially hindered while the rate of direct hydrogen reduction by TiO 2 is still very low.It should be noted that the shielding effect is significant in this case because the Pt particle size of ∼2 nm (from TEM data) is only a few times larger than that of oxygen species.The noble metal on the surface of the metal oxide with the role to enrich its surface or the interface with reactive species such as oxygen ions [32,33].Therefore, the responsible sensing reaction takes place at the surface of the supporting metal oxide, as shown in Figure 7.

Conclusion
In summary, the H 2 sensing process is strongly related to the surface reaction.Different concentrations of noble metal of the highly crystalline unloaded TiO 2 and (0.25, 0.50, 0.75, 1.00, 2.00, and 3.00 mol%) Pt-loaded TiO 2 were synthesized by FSP route.FSP has been shown to be a promising technique for the synthesis of metal oxide and noble loadedmetal oxide in a single step.The structure and morphology of as-prepared products were characterized by HRTEM, SEM, and XRD.Pt-loaded TiO 2 composite particles were deposited on sensor electrode by spin-coating technique.It can be concluded that the fast responses and highest sensor response to H 2 gas were obtained by the incorporation at the optimal 2.00 mol% Pt-loaded TiO 2 at 300 ∘ C (to 1 vol%,  = 470).
The enhanced sensor performance of the Pt-loaded TiO 2 sensors can be attributed to spillover effect, which is most effective at optimally dispersed Pt loading level of 2.00 mol% and moderate operating temperature of 300 ∘ C.

Figure 4 :
Figure 4: (a) Change in resistance, (b) the sensor response, (c) response time, and (d) recovery time of unloaded TiO 2 and Pt-loaded TiO 2 sensors upon exposure to H 2 at operating temperature of 300 ∘ C.
Figure 5: Electrical resistance change induced at 300 ∘ C, 350 ∘ C, and 400 ∘ C for the thick films of 2.00 mol% Pt-loaded TiO 2 .Figure 6: Variation of the sensor response to 1 vol% of H 2 with different operating temperatures ranging from 300 to 400 ∘ C.