Nanostructured PdO Thin Film from LangmuirâBlodgett Precursor for

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Nanostructured PdO Thin Film from Langmuir-Blodgett Precursor for Room Temperature H2 Gas Sensing Sipra Choudhury, Chirayath A. Betty, Kaustava Bhattacharyya, Vibha Saxena, and Debarati Bhattacharya ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04120 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 18, 2016

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ACS Applied Materials & Interfaces

Nanostructured PdO Thin Film from Langmuir-Blodgett Precursor for Room Temperature H2 Gas Sensing Sipra Choudhury*a, C. A. Bettya, Kaustava Bhattacharyyaa, Vibha Saxenab, Debarati Bhattacharyac a

Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India. Fax: +91 22 25505151; Tel: +91 22 25590284; E-mail: [email protected]

b

C

Technical Physic Division, Bhabha Atomic Research Centre, Mumbai 400085,India. Solid State Physic Division, Bhabha Atomic Research Centre, Mumbai 400085,India.

1 ACS Paragon Plus Environment


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Abstract: Nano particulate thin films of PdO were prepared using Langmuir–Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)– chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air-water interface was confirmed by surface pressure-area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV-Vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed presence of prominent Pd2+ with a small quantity (18 %) of reduced PdO (Pd0) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, band gap energy of PdO was estimated to be 2 eV which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20-30 nm. These PdO film when employed as a chemiresistive sensor showed H2 sensitivity in the range of 30 ppm to 4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.

Keywords: Langmuir-Blodgett film, interdigitated hydrocarbon, PdO thin film, room temperature H2 sensing, photosensitivity.


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1. Introduction Palladium oxide (PdO) is a p-type semiconductor with band gap energy in the visible light range and has widely been used as modifier/sensitizer to enhance visible light absorption for wide band gap oxides1. It also acts as catalysts for methane and CO oxidations under oxygen environment.2 In addition, it has also been utilized for the detection of hydrogen (H2) gas due to its high selectivity and high reactivity. H2 is a renewable energy source and extensively used in fuel cells and hybrid vehicles since it does not generate any green house gases or pollutants when it is burnt. However, if H2 concentration exceeds 4% in air, it can cause explosions due to its flammability in air. Therefore, there is a high demand for sensors to detect hydrogen at room temperature in many industrial environments. Several wide band gap semiconducting oxides such as SnO2, 3 In2O3 4 and ZnO 5 were studied for the development of hydrogen sensors owing to their fast response and high sensitivity. However, sensors based on these oxides are, in general, operated at high temperature >200° C. In order to lower the operating temperature, oxides doped or coated with catalyst such as Pt, Pd were developed which showed enhanced sensitivity6-9. PdO coated SnO2 nanowire as well as PdO coated TiO2 nanowire shows sensing ability towards H2 gas due to reduction of PdO to Pd metal which affects the charge transfer between PdO and the other metal oxide.8,9 Highest response for H2 gas (1000 ppm) was observed in uncoated as well as Pd coated TiO2 or SnO2 nanowire gas sensors at working temperature of 100°C. Although pure PdO can be an ideal material for H2 sensing at low temperatures, the possibility of developing a hydrogen sensor based on pure PdO remains largely unexplored. Only recently, H2 gas sensing of pure PdO thin film with nano-sized cracks and nanoflakes has been reported.10,11 However, no recovery has been observed for this sensor at room temperature.



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In general, thin films of PdO are prepared either by reactive sputtering12 or by oxidation of Pd metal.13 The shape and size of nanoparticles in thin films can be controlled either by the selforganization process, where a surfactant template14 is used or by layer-by-layer deposition method15 based on exposing a substrate sequentially to cationic and anionic substances forming successive bilayers due to electrostatic attraction. Despite the promising applications of PdO films in photosensitization and gas sensing, well-organized PdO thin films prepared by self organized route have not yet been explored for this purpose. Langmuir-Blodgett (LB) technique is a very useful self organization method where a compact and organized monolayer is formed by the compression of floating monolayer on the water surface and ordered multilayers of amphiphilic long chain molecules are deposited layer by layer by lifting/ immersing a substrate through the air-water interface. This technique has significant advantages such as molecular organization at the monolayer level, immense control on film thickness by varying number of deposited layers and homogeneous deposition over a large area. Thin metal oxide films can be prepared from the thermal decomposition of multilayer LB films containing metal salts. We had previously prepared thin films of metal oxides such as SnO2, TiO2 and WO3 by decomposition of multilayer LB films of their respective anionic salt and ODA complex, which is formed at the air-water interface. These metal oxide films have been used for various potential applications such as gas sensing16,17, photosensitization18 and electrochromism.19 Recently we have prepared composite thin film of TiO2 with 1 % PdO by LB technique, that has been successfully used as photocatalyst for H2 generation.20 Usually it is difficult to deposit uniform divalent metal oxides by heating the LB films of divalent metal carboxylate,

21,22

since it forms a discontinuous oxide

film after decomposition. In this paper we report our successful deposition of a pure divalent metal oxide film, PdO by employing LB technique. Highly ordered PdO thin films were prepared


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by using octadecyl amine with anionic metal complex and characterised in details. The PdO thin films thus prepared have shown room temperature H2 sensing ability in the ppm range (30-4000 ppm) and excellent visible light sensitivity.

2. Experimental Octadecyl amine (ODA) and sodium chloropalladate were purchased from Fluka and Sigma Aldrich, respectively and were used as received. Water used throughout this study was from a Millipore MilliQ filter system and had a resistivity of 18 MΩ cm. Surface pressure-area isotherm measurement of ODA monolayer and deposition of multilayer LB film were carried out using a KSV 5000 Langmuir-Blodgett double-barrier teflon trough. The surface morphology of the monolayer at the air water interface was studied by using Brewster Angle Microscope (BAM; model: Nanofilm_ep3bam, Accurion, Germany). X-ray diffraction (XRD) patterns were recorded with a Laboratory diffractometer using Cu Kα radiation at room temperature. The Raman spectra were recorded with Horiba Jobin Yvon spectrometer (model: Lab RAM HR) using He-Ne laser operating at 632 nm and a 100 X objective. X-Ray Phototelectron Spectroscopy (XPS) was carried out in the SPECS instrument with a PHOBIOS 100/150 Delay Line Detector (DLD) with 385W, 13.85 kV and 175.6 nA (sample current). Al Kα (1486.6eV) dual anode was used as the source and XPS was recorded with pass energy of 50 eV. As an internal reference for the absolute binding energy, the C-1s peak (284.5 eV) was used. UV–visible measurements of PdO deposited on quartz as well as Si wafer wes conducted on a double beam UV-Visible spectrophotometer (JASCO, V 650) and a double-beam spectrometer (V-670, JASCO) in specular reflectance mode, respectively. Two silver coated mirrors were utilized for reference of the spectrum in specular reflectance mode.



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Thicknesses of the PdO films were measured by Sentech ellipsometer (Model: SE4OO adv) using 633 nm laser and was estimated to be 12-14 nm. Atomic force microscopic (AFM) images of the films were recorded using a Scanning Probe Microscope (NT-MDT model-SPM solver P47) in contact mode using Silicon Nitride tips. Field-emission scanning electron microscopy (FESEM) micrographs of the as-deposited and PdO films were recorded using FESEM ATS 2100, SERON Inc. and FESEM Hitachi, S-4700 respectively. For electrical characterization, inter-digitated electrodes (IDE) were deposited by thermal evaporation of gold in vacuum ~10-5 torr. using a shadow mask with the following dimensions: 5 electrodes each side with 0.5 mm electrode width and 0.2 mm separation between the electrodes. The PdO films were fixed in a custom made gas testing chamber (volume ~ 1000 ml) of stainless steel with provisions for gas injection/release. Hydrogen gas sensing and photocurrent measurements were carried out under ambient conditions at room temperature (25°C) using parstat 2273.

3. Results and Discussions

Nano-structured thin PdO films were prepared using LB deposition technique. When chloroform solution of octadecyl amine was spread on sodium chloropalladate solution (10-4 M) at the air-water interface, an expanded area per molecule (~30 Å2) was observed in the surfacepressure area isotherm measurement, as shown in Figure 1. The area per molecule at the airwater interface is always estimated by extrapolating the steepest part of the isotherm to zero surface pressure. The cross section of chloropalladate group is expected to be larger than that of simple amine group. The increase in area per molecule as compared to ODA on pure water (18 Å2),17 is related to increase in the size of the head group23 due to complex formation between the


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amine and the chloropalladate ion at the air-water interface. It is important to note at this point that the area per molecule for ODA monolayer on pure water has been reported to vary in the range of 0.18 – 0.24 nm2. 18,24,25 Such variation in the area per molecule is a consequence of varying content of the dissolved CO2 which changes the pH of the subphase water. The BAM images of surface monolayer of ODA-chloropalladate complex at various surface pressures were shown in the insets of Figure 1. The image 1 (see Figure 1) shows BAM image recorded at zero surface pressure. The bright region with a sharp and clearly defined edge in this image indicates the formation of two-dimensional condensed domains at the air-water interface, while the dark region corresponds to gas phase. This suggests the coexistence of the condensed and gas phases of ODA monolayer, even at a very low surface pressure (π = 0 mN/m). Similar observation has been reported by Serra et al26 for ODA monolayer on titanyl oxalate solution. As the pressure increases, the bright domains coalesce to form a homogeneous monolayer (image 2). At surface pressure of 30 mN/m, a uniform monolayer (image 3) is observed and it is transferred to solid substrate by vertical dipping method. A number of small bright spots observed in the BAM images are due to multilayer granules formed by ejection of matter from the monolayer due to localized oscillation27 much before the collapse pressure. Multilayers of ODA-chloropalladate (91 layers) complex were successfully deposited on a solid substrate by LB technique. Transfer ratio of the monolayer in upward and downward movement of substrate was around one indicating Y-type of deposition. As-prepared LB films thus prepared on various substrates were decomposed at 300°C for 2 hours in air to prepare thin metal oxide films. In order to confirm the formation of PdO after thermal decomposition of multilayer LB film, XRD measurements and Raman studies were carried out. The XRD pattern (Figure 2) of asdeposited LB film of ODA-chloropalladate shows (00l) lines in the 2θ range 1.5-10ᵒ. The 7 ACS Paragon Plus Environment


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enlarged pattern corresponding to 2θ values in the range of 3-9ᵒ is shown in the inset A. As evident from this Figure, a crystalline ordered structure was observed in the as-deposited LB film with a layer spacing of 42 Å. Chloropalladate ion has a square planar structure and the amine group of ODA is co-ordinated from opposite sides of the planar structure. Two hydrocarbon chains arranged as one on top of another should lead to 54 Å separation23 between the two consecutive planes containing the head groups. The lower d-spacing (42 Å) is due to the expanded head groups that allowed the interdigitation of hydrocarbon chains as shown schematically in inset B of Figure 2. Interdigitation of hydrocarbon chains in the LB film of ODA-chloroplatinate was earlier observed by Ganguly et al23. However, when the XRD measurements were carried out after thermal decomposition of the LB film, no peak corresponding to ordered multilayer structure was observed (upper curve of Figure 2). An additional peak appeared at 2θ = 33.8° corresponds to characteristic (101) line of tetragonal PdO. The crystallite size as calculated by X-ray line broadening analysis using the Scherrer formulae was found to be 16 nm. The oxide formation after the thermal decomposition of multilayer LB films was further confirmed by Raman spectroscopy study (Figure S1). A distinct peak in the Raman spectra at 649 cm-1for thermally decomposed LB film can be assigned to the B1g mode of single crystal PdO.28,29 This peak is absent in the as-deposited LB film, which is composed of only chloropalladate salt with long chain ODA. To analyse the surface composition and the oxidation state of the surface elements of the as-prepared LB film and the PdO thin film, XPS measurements were carried out. The XPS spectrum of the as-deposited LB film shows the presence of nitrogen peak at 399.5 eV and chlorine peak at 198 eV as shown in Figure 3a and 3b respectively. On the other hand, nitrogen (N 1s) and chlorine (Cl 1s) peaks are absent in the spectra of PdO thin film, confirming the 8 ACS Paragon Plus Environment

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formation of PdO film after thermal decomposition (shown in the upper curve of Figure 3a and 3b respectively). The surface composition of the PdO thin film and the oxidation state of Pd in PdO ware analyzed using XPS spectra of O and Pd. Figure 3 c shows the Pd 3d spectrum which indicates Pd2+ state in PdO thin film with a small quantity of metallic Pd (Pd0). After the deconvolution of the Pd 3d5/2 spectrum, compositions of Pd2+ and Pd0 were estimated to be 82% and 18%, respectively. The observed binding energy values of Pd2+ and Pd0 were 336.7 eV and 335.0 eV respectively which are in consistent with the reported values.30 Figure 3d shows Pd 3p3/2 and O1s XPS spectrum of the PdO thin film. There is overlapping of binding energy of Pd 3p3/2 with that of various O1s species. Though it is difficult to deconvolute all peaks properly to resolve all the chemical components of O1s, we have resolved various components at 529.9 eV, 532.9 eV and 537.3 eV (inset of Figure 3d). The binding energy difference between Pd 3p3/2 and Pd 3p1/2 is 28.1 eV. From the Figure 3d it is clear that Pd 3p1/2 is 561.2 eV, therefore main peak resolved at 532.9 eV is assigned to be Pd 3p3/2. The resolved component at 529.9 eV corresponds to the lattice oxygen of PdO. This value is similar to the reported value.12 Another component at higher binding energy at 537.3 eV corresponds to adsorbed oxygen species on the PdO surface. Depending on crystallinity and purity, reported band gap of PdO has a wide range which varies from 0.8 to 2.2 eV.28,31,32 Nanocrystalline PdO thin film is further characterized by UV-vis and specular reflectance UV-vis spectroscopy to estimate the band gap energy. Figure 4a shows the absorption spectra of as-deposited LB film and PdO thin film. The optical band gap energy of PdO film was estimated based on the equation: (αhν) = α0(hν - Eg)n Where, α is absorption coefficient, hν is the photon energy, Eg is the band gap of the material, α0 is absorption constant and the exponent ‘n’ depends on the type of transition, for allowed direct



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transition, n = 1/2 and for indirect transition, n = 2. The band gap energy of the PdO film was estimated to be 2 eV from the Tauc plot of direct transition, i.e., plot of (αhν)2 vs. hν (Figure 4b), by extrapolating the linear portion from higher photon energy to zero absorption coefficient (α = 0). Specular reflectance spectra of as-deposited LB film and PdO thin film are shown in Figure 5a. The optical absorbance can be correlated with the reflectance (R) using the Kubelka-Munk (KM) function12 KM(R) = (1-R)2/2R, which is often used for evaluating band gap of a semiconductor material. By plotting the KM function vs. energy (Figure 5b) and by fitting with linear plot (inset of Figure 5b) in the visible range, the band gap of PdO thin film was estimated to be 1.6 eV which is close to that obtained from the absorption spectrum. The variation in the estimated value of the band gap could be due to different techniques used. Such a variation in band gap energy due to the experimental technique (optical density measurement, photoconductivity measurements and diffuse reflectance measurements) used, was also observed in earlier reports.33,34

In order to study the surface morphology of PdO thin films, AFM and SEM microscopy studies were carried out. Figure 6 shows AFM images of as-deposited ODA-chloropalladate LB film and PdO prepared by thermal decomposition of as-prepared LB film. Section profile (Figure 6c and 6d) of AFM images clearly indicate the formation of larger grains in the as-deposited LB film (Figure 6a) whereas smooth film containing nanocrystalline grains of size 20-30 nm was formed after thermal decomposition of LB film (Figure 6b). This is also reflected in recorded SEM images for as-deposited LB and nanocrystalline PdO film. As shown in Figure 7a, grains of average size ~ 100 nm are observed in as-deposited LB films. On the other hand, nanocrystalline


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grains of size 20-30 nm are observed in PdO thin film (Figure 7b). The histogram given in the inset clearly indicates average particle size as 22 nm and larger grains of size more than 30 nm could be the aggregates of smaller grains. Electrical characterization of PdO thin film was also carried out using DC current-voltage measurement. A linear I-V characteristic (Figure S2) of PdO thin film shows Ohmic characteristics between the Au electrode (inset of Figure S2) and the PdO thin film. The resistance calculated from the inverse slope of the linear curve is 7.87 MΩ.

H2 gas sensitivity of PdO film deposited on quartz was studied by measuring the current vs. time as shown in Figure 8. Current changes due to the change in resistance of the PdO film with H2 adsorption/ desorption. Figure 8a shows repeated response of the PdO film with 2000 ppm H2 in the gas testing chamber. After H2 gas exposure, no instantaneous recovery was observed by opening the stop cocks of the testing chamber (Figure 8b). This indicates that PdO surface is chemically modified after H2 exposure. Recovery was obtained only after light exposure by opening the lid or the carrier gas (air) flow. Reproducible response towards H2 gas and sensor recovery within one minute by shining light at room temperature is observed (Figure 8a). Similar response graph was obtained after passing air in the sensing chamber without opening the lid. It was observed that the range of the PdO thin film sensor for H2 gas sensing is quite large ranging from 30 ppm (0.003%) to 4000 ppm (4%). Inset of Figure 8b shows response of H2 gas for concentration as low as 30 ppm. Room temperature operation, wide sensing range, and low detection limit show the potential of nanocrystalline PdO thin films for H2 sensor as compared to that based on other oxides. Sensors prepared by using oxides such as SnO2 nanowires,35 In2O3 nanopushpin4 and ZnO nanopillars5 were shown to have H2 gas sensing properties at operating temperature higher than 200° C. Nevertheless, H2 sensor based on



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polycrystalline ZnO

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is worth noting which shows sensing at room temperature and could be

recovered under UV illumination. Different models were proposed for explaining the gas sensing mechanism of PdO films. Lee at al.

11

have proposed reduction of PdO to Pd0 on interaction with H2 gas since increase in

current was observed due to H2 gas interaction. Another mechanism has been suggested based on the oxygen ionosorption model. According to the oxygen ionosorption model,37,38 the adsorbed oxygen on the PdO surface at room temperature increases the conductivity of the p-type PdO forming superoxide (O2-) or peroxide (O22-). Below 150⁰C the most probable species is super oxide. The surface adsorbed oxygen species is removed by H2 gas interaction forming OH species while donating the electrons to the PdO conduction band which decreases the conductivity of the p type PdO film.10 Therefore, upon interaction with H2 gas the following reaction is more probable in PdO films at room temperature.

O2- + H2 ↔ 2 OH + e-

Thus the surface hydroxylation will reduce the conductivity of the PdO sensor. As desorption of the hydroxyl surface complex is not feasible from the PdO surface at room temperature, the surface hydroxyl remains on the PdO sensor after H2 exposure. Nevertheless, sensor recovery was observed by passing carrier gas or by shining light. Upon exposure to light, electron and holes are created while the surface hydroxyl attracts the photo-generated electrons creating negatively charged species while the holes increase the conductivity. Once the lid is closed, current decreases as the photo-generated excess electrons and holes recombine with an exponential decay (Figure 8a) and then it gets stabilized (inset of Figure 8b).


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It is suggested that when H2 adsorbed PdO film is exposed to light, generated electron-hole pairs will interact with the surface species in the following way:

e- + h+ + OH ↔ OH- + h+

On further H2 exposure, the surface adsorbed negatively charged hydroxyl complex interacts with hydrogen giving back the electrons to the metal oxide conduction band thereby decreasing the conductivity and the process is repeated (Figure 8a). The probable reaction is given below.

2OH- + H2 ↔ 2H2O +2 e-

Similarly, when ambient air used as the carrier gas for recovery after H2 exposure, the oxygen present in the air, being highly electronegative, will replace the surface adsorbed OH. Adsorbed oxygen will further attract electrons from the Pd metal, thereby increasing the conductivity.

In order to delineate the effect of ambient light on the PdO sensor, we have carried out multiple closures and exposures of the sensor towards ambient light. It is observed that photocurrent of PdO thin film increases in presence of ambient light after opening the lid of sensing chamber as shown in Figure 9. The current decreases as soon as lid is closed. In presence of visible light, electron hole (e-h) pairs are generated for a low band gap energy material like PdO and it causes increase in photocurrent. It is also observed (Figure S3) that the photocurrent increased many folds with exposure to light for longer time. The response of PdO thin film upon exposure to ambient light explains the instantaneous recovery of the H2 adsorbed PdO sensor by



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shining visible light. Subsequently, when the lid of the gas sensing chamber is closed, excess e-h pairs recombine with a single exponential decay (Figure 9 & Figure S3).

4. Conclusions

Nanostructured PdO thin films have been prepared by self-organized LB technique. Multilayer LB precursor forms PdO thin films after thermal decomposition at 300°C. The XRD and Raman studies indicated the formation of PdO thin film after heating the LB films. XPS studies have confirmed the complete removal of nitrogen and chlorine in PdO film after thermal decomposition of LB precursor. Microscopic techniques revealed the closely packed PdO nanoparticle formation in the film with grain size of 20-30 nm. Nanosrtuctured PdO thin film shows H2 gas sensitivity at room temperature in a wide range of 30-4000 ppm concentration with fast recovery either on exposure to ambient light or by carrier gas flow. Further studies on the PdOs film showed orders of increase in photocurrent on exposure to ambient visible light, suggesting its plausible applications in solar energy conversion.

References 1. Huang, C. J.; Pan, F. M.; Chang, I. C. Enhanced Photocatalytic Decomposition of Methylene Blue by the Heterostructure of PdO Nanoflakes and TiO2 Nanoparticles. Appl. Surf. Sci. 2012, 263, 345-351. 2. Datya, A. K.; Bravo, J.; Nelson, T. R.; Atanasova, P.; Lyubovsky, M.; Pfefferle, L. Catalyst Microstructure and Methane Oxidation Reactivity during the Pd↔PdO Transformation on Alumina Supports. Appl. Catal. A 2000, 198, 179-196. 14 ACS Paragon Plus Environment

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3. Shirahata, N.; Shin, W.; Murayama, N.; Hozumi, A.; Yokogaw, Y.; Mameyama, T.; Masuda, Y.; Koumoto, K. Reliable Monolayer Template Patterning of SnO2 Thin Films from Aqueous Solution and their Hydrogen Sensing Properties. Adv. Func. Mat. 2004, 14, 580-588. 4. Qurashi, A.; Yamazaki, T.; El-Maghraby, E. M.; Kikuta, T. Fabrication and Gas Sensing Properties of In2O3 Nanopushpins. Appl. Phys. Lett. 2009, 95, 153109. 5. Bie, L. J.; Yan, X.; Yin, J.; Duan Y.; Yuan, Z. Nanopillar ZnO Gas Sensor for Hydrogen and Ethanol. Sens. Actuators B 2007, 126, 604-608. 6. Kolmakov, A.; Klenov, D. O.; Lilach, Y.; Stemmer, S.; Moskovits, M. Enhanced Gas Sensing by Individual SnO2 Nanowires and Nanobelts Functionalized with Pd Catalyst Particles. Nano Lett. 2005, 5, 667-673. 7. Wang, H. T.; Kang, B. S.; Ren, F.; Tien, L. C.; Sadik, P. W.; Norton, D. P.; Pearton, S. J.; Lin, J. Hydrogen-selective Sensing at Room Temperature with ZnO Nanorods. Appl. Phys. Lett. 2005, 86, 243503. 8. Meng, D.; Yamazaki, T.; Kikuta, T. Preparation and Gas Sensing Properties of Undoped and Pd-doped TiO2 Nanowires. Sens. Actuators B 2014, 190, 838-843. 9. Shen, Y. B.; Yamazaki, T.; Liu, Z. F.; Meng, D.; Kikuta, T.; Nakatani, N.; Saito, M.; Mori, M. Microstructure and H2 Gas Sensing Properties of Undoped and Pd-doped SnO2 Nanowires. Sens. Actuators B 2009, 135, 524-529. 10. Chiang, Y. J.; Li, K.C.; Lin, Y. C.; Pan, F. M. A Mechanistic Study of Hydrogen Gas Sensing by PdO Nanoflake Thin Films at Temperatures below 250° C. Phys. Chem. Chem. Phys. 2015, 17, 3039-3049.



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11. Lee, Y.T.; Lee, J. M.; Kim, Y. J.; Joe, J.H.; Lee, W. Hydrogen Gas Sensing Properties of PdO Thin Films with Nano-sized Cracks. Nanotechnology 2010, 21, 165503. 12. Haung, C. J.; Pan, F. M.; Chen, H. Y.; Chang, L. Growth and Photoresponse Study of PdO Nanoflakes Reactive-sputter Deposited on SiO2. J. Appl. Phys. 2010, 108, 053105. 13. Serrano, O.G.; Rodriguez, C. L.; Adame, J. A. A.; Paredes, G. R.; Sierra, R. P. Growth and Characterization of PdO Films Obtained by Thermal Oxidation of Nanometric Pd Films by Electroless Deposition Technique. Mater. Sci. and Engg. B, 2010, 174, 273-278. 14. Lin, J.; Lin, Y.; Liu, P.; Meziani, M. J.; Allard, L. F.; Sun, Y. P. Hot-fluid Annealing for Crystalline Titanium Dioxide Nanoparticles in Stable Suspension. J. Am. Chem. Soc. 2002, 124, 11514-11518. 15. He, J. A.; Mosurkal, R.; Samuelson, L. A.; Li, L.; Kumar, J. Dye-sensitized Solar Cell Fabricated by Electrostatic Layer-by-Layer Assembly of Amphoteric TiO2 Nanoparticles, Langmuir, 2003, 19, 2169-2174. 16. Choudhury, S.; Betty, C. A.; Girija, K. G.; Kulshreshtha, S. K. Room Temperature Gas Sensitivity of Ultrathin SnO2 Films Prepared from Langmuir-Blodgett Film Precursors. Appl. Phy. Lett., 2006, 89, 071914. 17. Choudhury, S.; Betty, C. A.; Girija, K. G. On the Preparation of Ultra-thin Tin Dioxide by Langmuir-Blodgett Film Deposition. Thin Solid Films, 2008, 517, 923-928. 18. Choudhury S.; Betty, C. A. A Heterostructured SnO2-TiO2 Thin Film Prepared by Langmuir-Blodgett Technique. Mat. Chem. Phys., 2013, 141, 440-444. 19. Kondalkar, V. V.; Mali, S. S.; Kharade, R. R.; Mane, R. M.; Patil, P. S.; Hong, C. K.; Kim, J. H., Choudhury, S.; Bhosale, P. N. Langmuir–Blodgett Self Organized


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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


Nanocrystalline Tungsten Oxide Thin Films for Electrochromic Performance. RSC Adv., 2015, 5, 26923-26931. 20. Choudhury. S.; Sasikala, R.; Saxena, V.; Aswal, D. K.; Bhattacharya, D. A New Route for the Fabrication of Ultrathin Film of PdO-TiO2 Composite Photocatalyst. Dalton Trans. 2012, 41, 12090-12095. 21. Taylor, D.M.; Lambi, J. N. On the Preparation of Thin Metal Oxides by LangmuirBlodgett Film Deposition. Thin Solid Films 1994, 243, 384-388. 22. Brandl, D.; Schoppmann, Ch.; Tomaschko, Ch; Markl, J., Voit, H. Preparation of Ultrathin Ferric Oxide Layers Using Langmuir-Blodgett Films. Thin Solid Films 1994, 249, 113-117. 23. Ganguly, P.; Paranjape, D. V.; Sastry. M. Novel Structure of Langmuir-Blodgett Films of Chloroplatinic Acid Using n-Octadecylamine: Evidence for Interdigitation of Hydrocarbon Chains. J. Am. Chem. Soc. 1993, 115, 793-794. 24. Choudhury, S.; Bagkar, N.; Dey, G. K.; Subramanian, H.; Yakhmi, V. Crystallization of

Prussian Blue Analogues at the Air-Water Interface Using an Octadecylamine

Monolayer as a Template. Langmuir 2002, 18, 7409-7414. 25. Gur, B.; Meral, K. Preparation and Characterization of Mixed Monolayers and Langmuir−Blodgett films of Merocyanine 540/Octadecylamine Mixture. Colloids and Surfaces A: Physicochem. Eng. Aspects 2012, 414, 281– 288. 26. Serra, A.; Genga, A.; Manno, D.; Micocci, G.; Siciliano, T.; Tepore, A. Synthesis and Characterization of TiO2 Nanocrystals Prepared from n-Octadecylamine-Titanyl Oxalate Langmuir-Blodgett Films. Langmuir 2003, 19, 3486-3492.



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

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27. Galvan-Miyoshi, J.; Ramos, S.; Ruiz-Garcia, J.; Castillo, R. Localized Oscillations and Fraunhofer Diffraction in Crystalline Phases of a Monolayer. J. Chem. Phys. 2001, 115, 8178–8182. 28. McBride, J. R.; Hass, K.C.; Weber, W. H., Resonance-Raman and Lattice-dynamics Studies of Single-crystal PdO. Phys. Rev. B 1991, 44, 5016-5028. 29. Remillard, J.T.;. Weber, W. H.; McBride, J.R.; Soltis, R.E. Optical Studies of PdO Thin Films. J. Appl. Phys. 1992, 71, 4515-4522. 30. Pilloy, T.; Zimmermann, R.; Steiner, P.; Hufner, S. The Electronic Structure of PdO Found by Photoemission (UPS and XPS) and Inverse Photoemission (BIS). J. Phys.: Condens. Matter, 1997, 9, 3987-3999. 31. Okamoto, H.; Aso, T. Formation of Thin Films of PdO and Their Electrical Properties. Jpn. J. Appl. Phys., 1967, 6, 779. 32. Arai, T.; Shima, T.; Nakano, T.; Tominaga, J. Thermally-induced Optical Property Changes of Sputtered PdOx Films. Thin Solid Films, 2007, 515, 4774-4777. 33. Nilsson, P. O.; Shivaraman, M. S. Optical Properties of PdO in the Range of 0.5-5.4 eV. J. Phys. C: Solid State Phys. 1979, 12, 1423-1427. 34. Park, K. T.; Novikov, D. L.; Gubanov, V. A.; Freeman, A. J. Electronic Structure of Noble-metal Monoxides: PdO, PtO, and AgO. Phys. Rev. B 1994, 49, 4425-4431. 35. Kaciulis, S.: L. Pandolfi, E. Comini, G. Faglia, M. Ferroni, G. Sberveglieri S. Kandasamy, M. Shafieic, and W. Wlodarskic, Nanowires of Metal Oxides for Gas Sensing Applications Surf. Interface Anal. 2008, 40, 575-578. 36. Fan, S. W.; Srivastava, A. K.; Dravid V. P. UV-Activated Room-Temperature Gas Sensing Mechanism of Polycrystalline ZnO. Appl. Phys. Lett. 2009, 95, 142106.


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37. Franke, M. E.; Kopline, T. J.; Simon, U. Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter? Small 2006, 2, 36. 38. Wang, C. X.; Yin, L. W.;. Zhang, L. Y.; Xiang, D. Gao, R. Metal Oxide Gas Sensors: Sensitivity and Influencing Factors. Sensors 2010, 10, 2088-2106.

Legends of Figures Figure 1. Surface pressure-area isotherm of ODA on Na-chloropalladate solution (10-4 M). Insets: BAM images recorded with surface pressure (1) 0 mN/m, (2) 3.0mN/m and (3) 30 mN/m (Scale bar in the image represent 50 µm) Figure 2. XRD pattern of as-deposited LB film of ODA-palladate and PdO thin film. Insets: magnified pattern of weak peaks (A); schematics of interdigitated bilayer packing in as-deposited LB film (B). Figure 3. XPS spectra of as-deposited LB film and PdO thin film (a) N 1s (b) Cl 1s; XPS spectra of PdO thin film (c)Pd3d5/2 (d) Pd3p with O1s. Inset: Resolved after deconvolution. Figure 4. (a) Absorption spectra of as-deposited LB film and PdO thin film (b) Plot of (αhν)2 vs. hν to calculate band gap energy of PdO. Figure 5. (a) Specular reflectance spectra of as deposited LB film and PdO thin film (b) Kubelka Munk plot of PdO thin film. Inset: linear fit plot in the visible range. Figure 6. AFM images of (a) as deposited ODA-chloropalladate LB film and (b) PdO thin film deposited on Si.(c) and (d) section profiles of the line indicated in figure (a) and (b) respectively.



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Figure 7. SEM image of (a) as deposited and (b) PdO thin film. Inset: Histogram of PdO nanoparticles. Figure 8. Response of PdO sensor with H2 gas of 2000 ppm (a) with recovery and (b) without recovery. ↓ indicates H2 injection and ↑ indicates opening of the lid of the chamber. * indicates the closing of the lid before next injection. ∆ represent the opening of stopcock. Inset: H2 response in ppm level and recovery with light. Figure 9. Photoresponse of PdO thin film with ambient light. ↑ represents the chamber lid is opened. ↓ indicates the lid is closed.


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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


Figure 1



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

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Figure 2


Page 23 of 29

PdO

Intensity (c.p.s)

N1s

Cl 1s

As-deposited

As-deposited

408

404 400 396 392 Binding energy (eV)

206 204 202 200 198 196 194 192 Binding energy (eV)

(a)

(b)

Pd 3d 5/2

340

338 336 334 Binding Energy (eV)

532.9 537.3

529.9

Intensity (c.p.s)

Pd3p3/2

540 538 536 534 532 530 528 526

Binding Energy (eV)

561.2

335.0

Intensity (c.p.s)

336.7

Pd3p and O1s

532.9

Intensity (c.p.s)

PdO

Intensity (c.p.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


O1s

Pd3p1/2

332

570

560 550 540 530 Binding Energy (eV)

(c)

520

(d)

Figure 3



0.5 PdO

0.0

As-depos ited

200

400 600 Wavelength (nm)

800

(a)

100

50

(αhν)

Absorbance

1.0

2

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

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0 1

2 3 4 Photon energy (eV)

5

(b)

Figure 4


Page 25 of 29

Intensity

1.5

1.0

PdO

0.5

0.0

As-deposited

200 300 400 500 600 700 800 Wavelength (nm) (a)

KM

0.008

0.000

KM

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


1.6eV

2.2

2.1

2.0 1.9 1.8 Energy (eV)

1.7

1.6

6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 Energy (eV)

(b) Figure 5



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

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Figure 6


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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


(a)

(b) Figure 7



Current (µ Α)

3.4

2000 ppm H2

*

*

*

3.2 3.0 2.8 2.6 2.4

0

2000

4000 6000 Time (sec)

8000

(a)

Current (µA)

3.4 3.2

Current (µA)

3.6

∆

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

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3.7 30 ppm H2

3.6

*

3.5 3.4 3.3

0

1000

2000

3000

Time (sec)

3.0 2.8 2.6

0

3000

6000 9000 Time (sec)

12000

(b) Figure 8


Page 29 of 29

Current (µ A)

4.4 4.2 4.0 3.8 3.6 3.4 0

3000

6000

9000

12000

Time (sec)

Figure 9

Graphical abstract 2000 ppm H2

3.4 (101)

PdO

42 Å

(001)

Current (µ Α)

Intensity (arb. unit)

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


3.2 3.0 2.8 2.6

(002) (003) (004)

2

4

6

8

LB film

2.4 0

32 34 36 38 40

2θ

2000

4000 6000 Time (sec)

8000

Preparation and characterization of nanostructured PdO thin film deposited by Langmuir-Blodgett technique for using it as H2 gas sensor at room temperature. 29 ACS Paragon Plus Environment

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