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TiO2 nanorods decorated with Pd nanoparticles for enhanced liquefied petroleum gas sensing performance Dattatray S. Dhawale, Tanaji Pralhad Gujar, and Chandrakant D. Lokhande Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02312 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017
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Analytical Chemistry
TiO2 nanorods decorated with Pd nanoparticles for enhanced liquefied petroleum gas sensing performance Dattatray S. Dhawale1*, Tanaji P. Gujar2 and Chandrakant D. Lokhande3 1
Qatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa University (HBKU), Qatar Foundation, P.O. Box 34110, Doha, Qatar.
2
Applied Functional Polymers, Department of Macromolecular Chemistry, University of Bayreuth 95440 Bayreuth, Germany.
3
Centre for Interdisciplinary Studies, D. Y. Patil University, Kolhapur, Maharashtra, India.
KEYWORDS: Pd:TiO2, nanorods, chemical synthesis, gas sensor, LPG ABSTRACT: The development of highly sensitive and selective semiconductor based metal oxide sensor devices to detect toxic, explosive, flammable and pollutant gases is still a challenging research topic. In the present work, we systematically enhanced the liquefied petroleum gas (LPG) sensing performance of chemical bath deposited (CBD) TiO2 nanorods by decorating Pd nanoparticle catalyst. Surface morphology with elemental mapping, crystal structure, composition and oxidation states and surface area measurements of pristine TiO2 and Pd:TiO2 nanorods were examined by high resolution transmission electron microscopy (HR-TEM) equipped with an EDS detector, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption characterization techniques. LPG sensing performance of pristine TiO2 and Pd:TiO2 nanorods have been investigated in different LPG concentration and operating temperature range. The LPG response of 21 % for pristine TiO2 nanorods is enhanced to 49 % after Pd catalyst decoration with response, recovery times are reasonably fast, sensor exhibited long-term stability, and this could be due to the strong metal support (Pd:TiO2) interaction and catalytic properties offered by Pd nanoparticle catalyst. We believe that the work described herein demonstrates general and scalable approach that provides a promising route for rational design of variety of sensor devices for LPG detection.
Over the past few decades, the development of chemosensors has play a vital role which is most important technology and become key research topic with particular attention to characterize the interactions of the gases and chemical species with metal oxide semiconductors. Therefore, development of high responsive sensing devices has attracted continuous interest in the scientific, industrial and in everyday life.1-5 Increasing worldwide concerns towards environmental degradation, health hazards and security have led to an upsurge interest for detecting and monitoring potentially flammable and toxic gases like LPG along with other dangerous gases.6-7 Especially, extensive use of LPG in daily life for cooking as well as a fuel for automobile vehicles requires fast and selective detection of LPG to precise measure of leakage.8-9 In spite of considerable efforts, superior chemical sensors to detect the LPG to prevent an occurrence of accidental explosions have not been found hitherto, the problem being of vital significance to industry as well as general public. Due to such demands, research on fabrication of different types of sensor materials by sophisticated technologies have accelerated over the last few decade.10-11 To meet the demand of high gas responsive and cheap sensing devices, research and development play an important role especially on chemical gas sensor, including efforts to enhance the performance of traditional sensors
through nano-engineering. Chemical gas sensors are chemical systems that convert chemical stimuli into some form of response that can be easily detected, such as an electronic signals produced by interaction of metal oxide surface and injected gas molecules. Out of different types of chemical gas sensors,12-14 metal oxide resistive gas sensor is choice of researchers due to low cost production, reliable and robust nature. A typical resistive gas sensor contains an active sensing layer of metal oxide, the resistance of which is highly sensitive and selective to the nearby surroundings. High gas response, selectivity and fast response/recovery, are the general requirements for a good sensor. Therefore, fabrication of functional nanostructured materials having suitable surface properties is major field of the gas sensor performance improvement. Some of the metal oxide such as ZnO, NiO, TiO2, SnO2, CdO, etc. with special kind of morphologies were successfully used to detect the different kinds of gases.15-20 In recent years, nanomaterials with special morphologies such as nanotubes, nanowires, nanorods and nanobelts etc. with high aspect ratio have controlled the gas sensor research field since such type of nanomaterials offers large number of surface sites to facilitate the surface reaction.21 Among the wide range of metal oxides, nanostructured TiO2 is a versatile material because of its unique optoelec-
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tronic and dielectric properties and broad range of applications including gas sensors, dye-sensitized solar cells, photocatalysis, anti-reflection coatings, and so on.22-25 To meet the demand of diverse applications, nanostructured TiO2 has opened up the possibilities of producing sensing electrodes in the form of nanorods, nanowires, nanotubes, etc.26-28 Among the nanostructures, nanorods provide a higher surface to volume ratio and can be potentially used to fabricate large area, highly sensitive and stable gas sensors. More et al.29 reported the LPG sensing performance of web like TiO2 nanostructure and achieved maximum LPG sensitivity of 35 %. Osnat et al.30 have evaluated the gas responses of TiO2 nanofibers towards CO and NO2. However, these sensors suffer from relatively low sensitivity or slow response. Therefore to boost the gas response and lower the operating temperature for reduction in power consumption and safe detection of explosive gases, catalyst such as Pd, Pt and Ag used to decorate the surface of metal oxide semiconductors.31,32 Although, Pd catalyst loaded over different kind of TiO2 nanostructures used to detect various dangerous gases,3337 the systematic studies on the effect of Pd nanoparticle decoration on TiO2 nanorods for LPG sensing performance have not yet been undertaken and we demonstrate this approach by decoration of Pd nanoparticle catalyst on TiO2 (Pd:TiO2) nanorods. Therefore, present work concentrates mainly to enhance the LPG sensing performance and stability of TiO2 nanorods by decoration of Pd nanoparticles. With this motivation, in this work, research efforts have been directed towards developing high-performance LPG sensor which operates at low temperature at less fabrication costs. We employed a low temperature chemical route to synthesize pristine TiO2 and Pd decorated TiO2 (Pd:TiO2) nanorods for LPG sensing performance. The prepared pristine TiO2 and Pd:TiO2 nanorods were subjected to HRTEM, XRD, EDS, XPS and N2 adsorptiondesorption characterizations. The proposed Pd:TiO2 nanorods have lot of advantages such as easy to synthesis which results in low cost and offers high LPG sensitivity of 49 % because of change in carrier concentration and distribution of oxygen component induced by the decoration of Pd nanoparticles uniformly on TiO2 nanorods. Additionally, the optimum temperature where the sensitivity has its maximum value also shifts to lower temperature for Pd:TiO2 compared with that of pristine TiO2 sensor. Finally, the LPG sensing mechanism has been discussed. EXPERIMENTAL SECTION Synthesis of TiO2 nanorods. TiO2 nanorods were synthesized by chemical bath deposition (CBD) method at low temperature. Specifically, for the deposition of TiO2 nanorods, 2.5 mL of titanium (III) chloride (TiCl3) (30 wt% in HCl) and 1 M urea (NH2CoNH2) was slowly added to the beaker containing 50 mL aqueous solution and resultant pH was ~ 1. The reaction bath was kept on constant stirring at 300 K for 30 min to become homogenous solution. Finally stirring was stopped, kept under unstirred condition and glass substrate was dipped in a reac-
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tion bath. Then bath temperature was raised from 300 K to 353 K and kept it for another 2 hrs. Once bath temperature reached to 353 K, precipitation starts in reaction bath and heterogeneous reaction begins and resulted into deposition of Ti(OH)4 on the glass substrate. Finally, substrate coated with Ti(OH)4 nanorods were taken out and cleaned with distilled water and calcined at 723 K in air for 2 hr and converted to TiO2. The whitish colored uniform coated TiO2 films are strongly adherent to glass substrate. For the Pd nanoparticle decoration, the calcined TiO2 nanorods were inserted in to a beaker containing 10 mM PdCl2 alcoholic solution of methanol for 5-10 sec and dried in air flow and such 10 cycles were performed and heat treated at 473 K and used for characterization and gas sensing. Characterization. The AMBIOS XP-1 surface profiler was used to measure the thickness of the film and resulted thickness was 715 nm. N2 adsorption-desorption study was performed using an ASAP-2010 surface area analyzer to calculate the surface area and pore diameter. For the structural determination, X-ray diffraction (XTD) pattern of TiO2 thin film was performed using PW3710, Philips, CuKα radiation, (λ = 0.154 nm). HRTEM images of TiO2 were collected by using TEM JEOL JSM-2000EX2 that was equipped with an EDS detector. First of all, TEM sample preparation was performed by adding small quantity of specimen into ethanol solvent and sonicated for 5 min and dropped on the carbon coated Cu grids. The elemental mapping of Ti, O, and Pd was acquired with a post-column energy-filter and furthermore so called 3windoe method was utilized to generate these elemental maps. To analyze the state and type of the chemical binding, XPS was performed using a thermos-electron corporation Kα system. Gas sensor study. To study the LPG sensing performance, we placed TiO2 and Pd:TiO2 sensor devices in a sealed chamber and measured electrical resistance in air for pristine TiO2 and Pd:TiO2 films (i.e. Ra). We, then injected LPG into the chamber and measured electrical resistance of TiO2 and Pd:TiO2 films having LPG inside the chamber i.e. Rg and the LPG response, S (%) was calculated using following relation, S %=
R a -R g Ra
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(1)
RESULTS AND DISCUSSION Morphological, Crystal structural and Surface Characteristics. The TiO2 film synthesized using CBD is whitish in color, uniform, peculiarly reflecting as well as highly adherent to glass substrate as shown in inset of Figure 1 (A). Figure 1 (A) and (B) shows the high resolution transmission emission scanning electron micrographs (HRTEMs) of pristine TiO2 and Pd:TiO2 nanorods, respectively. From HRTEM images, it is seen that the TiO2 film contains numerous branched array of nanaorods and almost all of them shows the same nature with 10-20 nm in diameter and a length typically ranges from 200-300 nm. The HRTEM image of Pd:TiO2 as shown in Figure 1(B) confirms successful decoration of TiO2 nanorods by
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and pore size of TiO2 nanorods are found to be 31 m2g-1 and 9.45 nm, respectively. Such a high surface area and micro-pore diameter can provide more active sites which help to facilitate the transport of interacting gases resulting improved the gas sensing performance. 6000
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Pd nanoparticles with a narrow particle-size distribution and particle size around 7 ± 1 nm (Figure S1 in supplementary information). The Pd nanoparticles are appeared as dark dots with homogeneous deposition throughout the TiO2 support. The EDS analysis was performed for the confirmation of Pd nanoparticles and surface coverage on TiO2 nanorods. EDS mapping under the STEM mode of the Pd:TiO2 sample (Figure 1 (C)) further verified the locations of Ti, O and Pd atoms. An element mapping confirms that O (green color) is completely covered the Ti (white color) throughout the whole sample as would be expected from an all oxide material. Small Pd nanoparticles (red color) are well anchored to the TiO2 nanorod. Additionally, as shown in Figure S2, the EDS spectra displays the sharp peaks of Ti, O, and Pd confirming Pd:TiO2
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Figure 2. XRD pattern of pristine TiO2 nanorods.
composite. As shown in Figure 1 (D), selected area electron diffraction (SEAD) pattern shows set of rings of Pd:TiO2 indicated that the nanorods are polycrystalline structure which is agreement with XRD results. Figure 1. HRTEM images of pristine TiO2 (A) Pd:TiO2 (B), Elemental mapping of Pd:TiO2 (White-Ti, Green-O and Red-Pd) (C) and SEAD pattern of Pd:TiO2 nanorods (D). Figure 2 shows XRD pattern of pristine TiO2 nanorods confirms polycrystalline nature of nanorods which exhibits high intense diffraction peaks belongs to tetragonal rutile TiO2 phase according to the JCPDS card no. 01-0716411. In addition to this, low intense three diffraction peaks with (101), (004) and (200) planes could be attributed to the anatase TiO2 form (JCPDS card no. 01-086-1157) similar to the reported literature.38 The pore size distribution and surface area of pristine TiO2 nanorods was performed and corresponding isotherm and pore size distribution plots are shown in Figure S3 (A) and (B), respectively. The TiO2 nanorods exhibits type IV isotherm with H2 hysteresis loops according to Brunauer-DemingDeming-Teller (BDDT) classification.39 The surface area
Further to prove the existence of Pd, Ti, O and know the chemical state of components in Pd:TiO2 nanorods, XPS studies have been performed. The survey XPS spectra of pristine TiO2 and Pd:TiO2 nanorods are presented in Figure 3 (A) which confirm the presence of Pd, Ti, and O elements. The Ti 2p, O 1s and Pd 3d spectra and their corresponding curve fittings are shown in Figure 3 (B-D), respectively. The position of Ti 2p3/2 peak at the binding energy 465.5 eV and the low intense Ti 2p1/2 peak at 458.0 eV indicate the presence of Ti4+ oxidation states similar to reported in literature.40 The O 1s band having one peak at the 529.6 eV of the Pd:TiO2 ascribed to the oxygen atoms of TiO2 and two additional peaks at 531.1 and 532.9 eV, related to carboxy groups. The Pd 3d spectrum as shown in Figure 3 (D) represents two peaks with one at 342.7 eV (Pd 3d3/2) and other at 336.8 eV (Pd 3d5/2) confirms the Pd2+ oxidation state in the Pd:TiO2 nanorods. From all of these XPS analysis confirms the nanorods are composed of Pd, Ti and O. The atomic compositions of the Ti 2p, O 1s and Pd 3d in the Pd:TiO2 nanorods are found to be 14 %, 57 % and 4 %, respectively confirming very high content of oxygen in the present Pd:TiO2 nanorods which is beneficial for gas sensor application.
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LPG Sensing Performance. The LPG sensing performance of pristine TiO2 and Pd:TiO2 sensor has been investigated in homemade gas sensor unit. But, before exposing to LPG, both the TiO2 and Pd:TiO2 electrodes were allowed to stabilize their electrical resistance in ambient air to ensure stable zero level and was taken as a resistance in air Ra. Typical initial stabilization curve of resistance from non-equilibrium to equilibrium in ambient air for pristine TiO2 at 673 K and Pd:TiO2 at 598 K have been recorded and presented in Figure 4 (A). For both the sensors that is; pristine TiO2 and Pd:TiO2, we observed the initial rapid decay within few seconds and then stable value of electrical resistance have been attained. But the electrical resistance and operating temperature in air for Pd:TiO2 nanorod sensor is lower compared to pristine TiO2 and this could be due to the loading of Pd nanoparticles on to top of TiO2 surface layer. This Pd catalyst is helpful to release the electron easily from the surface of TiO2 and increase the rate of reaction and thus decreasing the operating temperature.41 Figure 4 (B) shows the response of pristine TiO2 and Pd:TiO2 to 2600 ppm LPG at various operating temperatures. For pristine TiO2, the LPG response increased up to 21 % when temperature reaches to 673 K and when again further temperature increased to 698 K, LPG response start to decrease. For Pd:TiO2 on the other hand, the LPG response reached to 35 % at 598 K, resulting lowering in operating temperature for Pd:TiO2 electrode. Both the pristine TiO2 and Pd:TiO2 sensors exhibited a peak with increasemaximum-decrease tendency at 2600 ppm LPG in the examined temperature range similar to reported literature.41 As the operating temperature keeps on increasing, thermal energy increases which help to overcome the activation energy barrier resulting linear increase in LPG response. However, when temperature continuously increased, gas adsorption ability limited by the speed of diffusion of LPG molecules resulting decrease in LPG response. Based on above results, operating temperature for pristine TiO2 and Pd:TiO2 are 673 and 598 K, respectively and applied hereinafter. Compared to pristine TiO2,
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Pd:TiO2 electrode exhibits lower operating temperature because of Pd loading which forms weakly bonded complex of oxygen molecules that dissociated at lower temperature. Figure 4 (C) represents the variation of LPG response with different LPG concentration for pristine TiO2 and Pd:TiO2 nanorods at an operating temperature of 673 K and 598 K, respectively. From the graph, we can see that the LPG response increasing steeply with increasing LPG concentration up to 5200 ppm which indicates still sufficient numbers surface states are available to interact with LPG gas molecules. The pristine TiO2 nanorods exhibited maximum LPG response of 27 % which is significantly enhanced to 49 % after Pd nanoparticle decoration upon exposure of 5200 ppm of LPG. This indicates Pd:TiO2 electrode shown enhanced LPG response performance over pristine TiO2 which can be explained as fallows. The nanorods structure of TiO2 having high accessible surface area could provide more reactive sites. Additionally, as per the well know spillover effect,42 Pd:TiO2 nanorod electrode could accelerate the ion sorption of oxygen species (O2-, O- and O2-). In addition to this, Pd nanoparticles strongly connected to the TiO2 nanorod support may help to improve the sensor performance as shown in XPS, HRTEM and EDS results. Figure 4 (D) shows the dynamic responses of Pd:TiO2 nanorods at 598 K upon exposure of 5200 ppm LPG. From the Figure 4 (D), one can easily see that the LPG response increases immediately when LPG turn on and when LPG take out from the test chamber, LPG response drops down to its original state. The corresponding response and recovery times for Pd:TiO2 electrode is 100 s and 200 s, respectively. Such a fast response and recovery response of the sensors attributed due to the loading of Pd nanoparticle catalyst which help to promote the electron transfer rapidly and nanorod like morphology of TiO2 support which provides easy path for the LPG to react with surface of Pd:TiO2.
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Figure 4. Typical initial resistance stabilization of pristine TiO2 and Pd:TiO2 nanorods at 673 K and 598 K, respectively from non-equilibrium to equilibrium stage (A), LPG response upon exposure of 2600 ppm LPG as a function operating temperature (B), Variation of LPG response with the LPG concentration at 673 K and 598 K of pristine and Pd:TiO2 nanorods (C) and Dynamic response at 5200 ppm LPG at 598 K of Pd:TiO2 nanorods (D). For the stability studies of pristine TiO2 and Pd:TiO2 nanorod sensor, repeated experiments were performed by
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Analytical Chemistry exposing a fixed LPG concentration of 5200 ppm for 60 days with 10 days interval at an operating temperature of 673 K and 598 K, respectively as illustrated in Figure 5. The stable LPG performance was observed with 10 % initial decay indicating that sensor being studied can stand as a reliable device for LPG detection. 60 TiO2 Pd:TiO2
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lattice of metal oxide. The schematics of the gas sensing mechanism for TiO2 sensor in air and under LPG exposure is illustrated in Figure 6. The ionic species such as O2− (ads), O−(ads) and O2−(ads) forms due to the adsorption of oxygen from the ambient air on the film surfaces and such ionic species extract the electron form the valence band (top most) by trapping. Therefore, these adsorbed oxygen species can capture the electrons from ntype TiO2 and causes a reduced conductivity. The adsorption of oxygen species can be described as fallows,43 O2 gas ↔ O2 adsorbed
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Figure 5. The LPG sensing stability of pristine TiO2 and Pd:TiO2 nanorods at an operating temperature of 673 K and 598 K, respectively. LPG sensing mechanism. The LPG sensing properties of transition metal oxide is depend on the change in barrier height or resistance when target gas e.g. LPG interact. The change in electrical resistance or barrier height is controlled by amount of chemisorbed oxygen on surface and charged species and other various process such as gas adsorption, desorption and diffusion of gases through crystal
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Thus, the process of stabilization of surface resistance is complete by equilibration of the chemisorption and when any process disturbs equilibrium results in change in resistance or barrier height of the TiO2.17 The reducing gas such as LPG consists of CH4, C3H8, C4H10, etc. and because of several intermediate steps, the LPG gas sensing mechanism becomes more complex which is not yet fully understood.44 When LPG gas exposed on TiO2 sensor, chemisorbed oxygen will return the trapped electrons to the TiO2 film surface which causes drastically decrease in the electrical resistance and thus decreases the barrier height. The interaction of chemisorbed oxygen species with injected LPG molecules can be expressed as fallows, Cn H2n2 2O- → H2 O Cn H2n- Oe-
(5)
where, CnH2n+2 represents the CH4, C3H8 and C4H10. The gas response can be boost by decorating noble metal s to metal oxide surface.41 In the present study, when Pd nanoparticles decorated on TiO2 surface, LPG response enhances over pristine TiO2 is primarily due to
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change in surface energy and spillover effect caused by additional Pd on TiO2 surface.32 In addition to this, Pd:TiO2 sensor requires relatively low temperature to dissociate because of weak bonding of Pd atom with oxygen gas.45-47 When LPG exposed and interacted with adsorbed oxygen in the very similar manner as described above but remarkably large number of electrons were re-injected back to the top most conduction band of TiO2 resulting increase conductivity by decreasing barrier height. The active Pd nanoparticle catalyst might accelerates the reaction and provides more active sites to interact with LPG which could be responsible for the enhancing the LPG response. CONCLUSION In summary, we systematically enhanced the LPG sensing performance of TiO2 nanorods by decorating Pd nanoparticle catalyst and achieved the maximum LPG response of 49 %. We also found that the combination of the TiO2 nanorods and Pd decoration helps to achieve stable LPG response (49 %) with decreasing operating temperature (598 K). Based on these findings, we are confident that this study opens the new perspectives on fabricating sensor devices based on TiO2 nanorods for detecting LPG using low temperature wet chemical approaches.
ASSOCIATED CONTENT Supporting Information The Supporting is available free of charge on the ACS Publications website at http://pubs.acs.org Pd particle size distribution over TiO2 nanorods, EDS spectrum of Pd:TiO2 nanorods and N2 adsorption-desorption and pore size distribution of pristine TiO2 nanorods (PDF).
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected] Notes The authors declare no competing financial interest.
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Table of Contents 60 50
LPG response, S(%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
40 30 20 10 0
TiO - 2
Pd:TiO 2
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