PdO Thin Film Research Articles

Reliable NO2 sensing and alert system based on Pd-PdO thin film with sub ppm level detection at room temperature

Human and environmental health is greatly affected due to toxic gas emissions in the environment from power generation, transportation and other industries. For real time monitoring of polluting gases, gas sensor should be reliable, robust and stable in all environmental conditions. We report, nanostructured PdO thin film based portable and low voltage (< 5 V) operating NO2 sensor for sensing sub-ppm level of NO2, with limit of detection (LOD) 80 ppb at room-temperature. To optimize the sensor response of PdO thin film prepared by cost effective self-assembly technique films were calcined at 200 °C, 300 °C and 400 °C. Effect of calcination temperatures on film’s surface morphology, composition and energy band gap have been studied using various techniques. The charge carrier concentration, charge carrier transport mechanism, sensitivity, selectivity, repeatability, reproducibility, reliability, linearity etc. have been studied. PdO thin films calcined at 400 °C are most sensitive with approx. 60% and 100% response for 500 ppb and 1 ppm NO2, respectively at room-temperature. After stable response upto 6 months and more was obtained for these, a portable, pocket size electronic device of size (8 × 5 cm) was fabricated and tested successfully, coupling with the sensor film to prepare a wearable sensor.

Correlation of surface processes with characteristic sensing responses of PdO thin films to ethanol

Gas sensing characteristics of PdO nanoflake thin films exposed to ethanol (EtOH) at temperatures below 250 °C was studied. The PdO thin film responses distinctly to 0.15 ppm EtOH in dry air at temperatures above 100 °C. A characteristic valley-shaped response feature develops in the early stage of the exposure to EtOH at 150 °C and above. Formation of the valley feature is a result of the combined effects of two types of surface processes, which successively modify the sensor conductance in the opposite way. Dehydrogenation of adsorbed EtOH induces reduction of preadsorbed oxygen anions and the PdO substrate, resulting in the drop of the conductance. On the other hand, subsequent adsorption of EtOH and oxygen on newly growing Pd nanoclusters, which develop due to the reduction of the PdO substrate, causes the increase in the conductance. The Pd nanoclusters can be later reoxidized in the EtOH gas mixture, thereby modifying the sensing behavior of the PdO sensor. X-ray photoelectron spectroscopy (XPS), diffusive reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature programmed desorption (TPD) were used to study adspecies and gaseous species formed in the EtOH gas sensing reactions. On the basis of the characteristic electrical response and chemical characterizations, we proposed the formation mechanism for the characteristic response feature.

On-chip growth of patterned ZnO nanorod sensors with PdO decoration for enhancement of hydrogen-sensing performance

In this study, we used a low-temperature hydrothermal technique to fabricate arrays of sensors with ZnO nanorods grown on-chip. The sensors on the glass substrate then were sputter decorated with Pd at thicknesses of 2, 4, and 8 nm and annealed at 650 °C in air for an hour. Scanning electron microscopy, high resolution transmission microscopy, X-ray diffraction, and surface analysis by X-ray photoelectron spectroscopy characterization demonstrated that decoration of homogenous PdO nanoparticles on the surface of ZnO nanorods had been achieved. The sensors were tested against three reducing gases, namely hydrogen, ethanol, and ammonia, at 350, 400, and 450 °C. The ZnO nanorods decorated with PdO particles from the 2 and 4 nm layers showed the highest responses to H2 at 450 and 350 °C, respectively. These samples also generally exhibited better selectivity for hydrogen than for ethanol and ammonia at the same concentrations and at all tested temperatures. However, the ZnO nanorods decorated with PdO particles from the 8 nm layer showed a reverse sensing behaviour compared with the first two. The sensing mechanism behind these phenomena is discussed in the light of the spillover effect of hydrogen in contact with the PdO particles as well as the negative competition of the PdO thin film formed between the sensor electrodes during sputter decoration, Pd–Zn heterojunction that forms at high temperature and thus influences the conductivity of the ZnO nanorods.

Nanostructured PdO Thin Film from Langmuir–Blodgett Precursor for Room-Temperature H2 Gas Sensing

Nanoparticulate thin films of PdO were prepared using the 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 the 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 the presence of prominent Pd(2+) with a small quantity (18%) of reduced PdO (Pd(0)) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the 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 30-4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light.

Defining Switching Efficiency of Multilevel Resistive Memory with PdO as an Example

Resistive random access memory (RRAM) is the most promising candidate for next generation nonvolatile memory. In this article, resistive switching in PdO thin film is investigated. The fabricated in‐plane devices showed voltage pulse induced multilevel resistive switching (MRS) with as many as five states under ambient conditions with high degrees of retention and endurance. The I–V characteristics of the different memory states are linear and only a small reading voltage (≈10 mV) is necessary. Raman mapping of PdO (B1g mode, 650 cm–1) and temperature‐dependent electrical transport measurements provide an insight into possible redox mechanism involving PdO/Pd particles. For the first time, the switching efficiency of a MRS device is uniquely defined in terms of a parameter called “multiplex number (M),” which is the sum of the total number of memory states and the ratio between the number of switching events observed in a device and the total number of possible switching events. The present PdO MRS device exhibits the highest M value compared to the values evaluated from the literature examples. Such high performance MRS in PdO devices makes them potential candidates for RRAM and neuromorphic circuit applications.

Bis(β-diketonato)- and allyl-(β-diketonato)-palladium(ii) complexes: synthesis, characterization and MOCVD application

Halogen-free allyl- and β-diketonate-functionalized Pd(ii) compounds were synthesized and successfully applied as MOVCD precursors for Pd and PdO thin film formation.

Pd(001)面上に作成したPｄO薄膜での窒素分子の吸着挙動に関するDFTによる検討

Pd(001)面上に作成したPｄO薄膜での窒素分子の吸着挙動に関するDFTによる検討

PdO Nanoflake Thin Films for CO Gas Sensing at Low Temperatures

We prepared PdO nanoflake thin films by reactive sputter deposition and studied their gas-sensing response to CO at temperatures below 250 °C. The PdO thin film, which has a large open surface area allowing for rapid and extensive CO adsorption, exhibits a complicated CO sensing behavior. At temperatures below 150 °C, the electrical conductance of the thin film drops upon the exposure of the CO/dry air gas mixture. The CO sensing mechanism in this temperature range is described by the oxygen ionosorption model. At 150 °C, oscillatory response occurs when the CO concentration is larger than 2000 ppm. The oscillatory response is due to the reactivity switching between the PdO reduction and reoxidation reactions. In addition, the PdO thin film shows a rapid increase in the conductance before and after the oscillatory response regime. The conductance increase is ascribed to the positive charge accumulation at the interface between the PdO thin film and metal Pd nanoislands, which are formed during the PdO reduction. At temperatures higher than 150 °C, the conductance drops when the PdO thin film is exposed to CO. Besides the ionosorption model, the oxygen vacancy model is used to account for the CO sensing response of the thin film in the high-temperature range. On the basis of the inverse change in the CO sensing sensitivity at 150 °C, a CO sensor of high selectivity, which integrates two PdO sensing elements operating independently at 150 °C and a lower temperature, is proposed.

Effects of the PdO nanoflake support on electrocatalytic activity of Pt nanoparticles toward methanol oxidation in acidic solutions

We prepared PdO nanoflake thin films on carbon cloths by reactive sputtering deposition, and studied electrocatalytic performance of Pt nanoparticles electrodeposited on the PdO thin film in methanol oxidation reaction (MOR) in acidic media. The PdO nanoflake thin film exhibits a cyclic voltamperometric (CV) behavior similar to metallic Pd electrodes because a metallic Pd surface layer is formed on the PdO thin film electrode under the CV measurement condition. The nanoflake morphology of the PdO electrode provides a large surface area for Pt nanoparticle loading, resulting in a large electrochemical active surface area (ESA). The Pt/PdO electrode has a high electrocatalytic activity toward MOR, and the good electrocatalytic performance of the Pt/PdO electrode is ascribed to a high CO tolerance and the large ESA. We suggest that the high CO tolerance of the catalyst on the Pt/PdO electrode is a result of the synergism of the bifunctional mechanism and the electronic effect. XPS analyses indicate that negative charge transfer occurs from the PdO support to the Pt nanoparticles, indicating the presence of the electronic effect. Pt nanoparticles on the PdO support can greatly alleviate the nanoflake damage during the CV measurement, which results from anodic dissolution of metal Pd from the PdO support in acidic media.

Surface structural evolution during the thermal decomposition of a PdO(101) thin film

We used scanning tunneling microscopy (STM) to characterize PdO(101) thin films grown on Pd(111), and the structural changes that occur during isothermal decomposition. We find that the PdO(101) thin films have high-quality surface structures that are characterized by large, crystalline terraces with low concentrations of point defects. Small domains of single-layer oxide are also present on the top layer of relatively thick PdO(101) films grown at 500 K. The thinner PdO(101) films exhibit negligible quantities of such domains, apparently because new domains agglomerate rapidly as the film thickness decreases. We find that the isothermal decomposition rate of a PdO(101) film at 720 K exhibits an autocatalytic regime in which the rate of oxygen desorption increases as the oxide decomposes. Our STM results demonstrate that reduced sites created during oxide decomposition catalyze further PdO decomposition, and reveal strong kinetic anisotropies in the decomposition. The kinetic anisotropies produce one-dimensional reaction fronts that propagate preferentially along the atomic rows of the PdO(101) surface, resulting in the formation of long chains of reduced sites. We also find that reduced sites promote oxygen recombination in neighboring rows of the Pd(101) structure, causing loops and larger aggregates of reduced sites to form. The promotion of decomposition across the atomic rows can qualitatively explain the autocatalytic desorption kinetics. Finally, the STM images provide evidence that underlying PdO(101) layers transfer oxygen to reduced surface domains, thus producing large domains of PdO(101) islands that coexist with reduced domains as well as the larger PdO(101) terraces of the initial surface. Re-oxidation of the surface acts to sustain the autocatalytic decomposition kinetics, and provides a mechanism for oxygen atoms to ultimately evolve from the subsurface of the PdO(101) film.

Oxidation of Methanol on a PdO(101) Thin Film

We investigated the oxidation of methanol on a PdO(101) thin film using temperature-programmed reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). We find that the complete oxidation of methanol occurs almost exclusively on PdO(101) during TPRS experiments conducted at low initial methanol coverages. Partial dehydrogenation of the adsorbed methanol gives rise to a desorption-limited H2O peak at 335 K, while a reaction-limited process causes CO2 and H2O to desorb concurrently at 450 K. The desorption yield of CO2 increases as the methanol coverage rises to ∼0.12 ML (monolayer) but remains steady with increasing coverage thereafter. At high methanol coverage, we observe the desorption of methyl formate, formaldehyde, H2, and H2O in TPRS peaks between ∼285 and 305 K in addition to complete oxidation products. The yields of the partial oxidation products increase with increasing methanol coverage above ∼0.12 ML and start to plateau above a methanol coverage of about 0.5 ML. At saturation ...

Precursor-mediated dissociation of n-butane on a PdO(1 0 1) thin film

We investigated the molecular adsorption and dissociation of n-butane on a PdO(1 0 1) thin film using temperature-programmed reaction spectroscopy (TPRS) experiments and density functional theory (DFT) calculations. At low coverage, n-butane adsorbs on PdO(1 0 1) in a molecular state that is more strongly bound than n-butane physisorbed on Pd(1 1 1). This molecularly adsorbed state of n-butane on PdO(1 0 1) corresponds to a σ-complex that forms on the rows of coordinatively unsaturated (cus) Pd atoms of the oxide surface. TPRS results show that a fraction of the n-butane layer undergoes C–H bond cleavage below ∼215 K and that the resulting fragments are completely oxidized by the surface upon continued heating. The evolution of product yields with the n-butane coverage as well as site blocking experiments provide strong evidence that the n-butane σ-complex serves as the precursor to initial C–H bond cleavage of n-butane on PdO(1 0 1). DFT calculations confirm the formation of an n-butane σ-complex on PdO(1 0 1). In the preferred bonding geometry, the n-butane molecule aligns parallel to a cus-Pd row and adopts a so-called η 1(2H) configuration with two coordinate H–Pd bonds per molecule. Our DFT calculations also show that σ-complex formation weakens C–H bonds, causing bond elongation and vibrational mode softening. For methane, we predict that coordination with a cus-Pd atom lowers the barrier for C–H bond cleavage on PdO(1 0 1) by more than 100 kJ/mol. These results demonstrate that dative bonding between alkane molecules and cus-Pd atoms serves to electronically activate C–H bonds on PdO(1 0 1) and suggest that adsorbed σ-complexes play a general role as precursors in alkane activation on transition metal oxide surfaces.

Strong Kinetic Isotope Effect in the Dissociative Chemisorption of H2 on a PdO(101) Thin Film

We investigated the dissociative chemisorption and oxidation of H2 and D2 on a PdO(101) thin film using temperature-programmed desorption (TPD) experiments and density functional theory (DFT) calculations. We find that the dissociation of H2 is highly facile on PdO(101), with more than 90% of a saturated H2 layer dissociating below 100 K. Most of the dissociated hydrogen reacts with the surface to produce H2O that desorbs above 350 K during TPD. Our experimental data demonstrate that H2 dissociation on PdO(101) occurs by a precursor-mediated mechanism in which a molecularly chemisorbed H2 species acts as a necessary precursor to dissociation. The experimental data also reveal that a kinetic isotope effect strongly suppresses the dissociation of D2 on PdO(101) terraces and causes the kinetic branching to shift toward desorption of the molecular D2 precursor. DFT calculations predict that H2 binds relatively strongly on PdO(101) by forming a σ complex on a coordinatively unsaturated (cus) Pd site. Using DFT, we identified only a single pathway for H2 dissociation that generates stable products on PdO(101). In this pathway, the adsorbed H2 σ complex dissociates by transferring an H atom to a neighboring cus-O site, thereby producing an OH species and an H atom bound to a cus-Pd site. Zero-point-corrected barriers determined for this pathway fail to explain our experimental observations of facile dissociation of H2 on PdO(101) and a strong kinetic isotope effect that suppresses D2 dissociation. We present evidence that quantum mechanical tunneling dominates the dissociation of H2 on PdO(101) at low temperatures and that differences in tunneling rates are responsible for the large kinetic isotope effect that we observe experimentally.

Molecular adsorption of small alkanes on a PdO(101) thin film: Evidence of σ-complex formation

We investigated the molecular adsorption of methane, ethane, and propane on a PdO(101) thin film using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The TPD data reveal that each of the alkanes adsorbs into a low-coverage molecular state on PdO(101) in which the binding is stronger than that for alkanes physically adsorbed on Pd(111). Analysis of the TPD data using limiting values of the desorption prefactors predicts that the alkane binding energies on PdO(101) increase linearly with increasing chain length, but that the resulting line extrapolates to a nonzero value between about 22 and 26 kJ/mol at zero chain length. This constant offset implies that a roughly molecule-independent interaction contributes to the alkane binding energies for the molecules studied. DFT calculations predict that the small alkanes bind on PdO(101) by forming dative bonds with coordinatively unsaturated Pd atoms. The resulting adsorbed species are analogous to alkane sigma-complexes in that the bonding involves electron donation from C-H sigma bonds to the Pd center as well as backdonation from the metal, which weakens the C-H bonds. The binding energies predicted by DFT lie in a range from 16 to 24 kJ/mol, in good agreement with the constant offsets estimated from the TPD data. We conclude that both the dispersion interaction and the formation of sigma-complexes contribute to the binding of small alkanes on PdO(101), and estimate that sigma-complex formation accounts for between 30% and 50% of the total binding energy for the molecules studied. The predicted weakening of C-H bonds resulting from sigma-complex formation may help to explain the high activity of PdO surfaces toward alkane activation.

Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)

We investigated the mechanism by which a surface oxide layer on Pd(1 1 1) transforms to a PdO(1 0 1) thin film during oxidation with gaseous oxygen atoms in ultrahigh vacuum. Our results provide evidence that the precursor to bulk PdO formation is a distinct oxide phase that forms as small particles, referred to as PdO seeds, after the surface oxide saturates. With increasing oxygen coverage, the PdO seeds grow in size and eventually transform to more stable particles that agglomerate to yield a PdO film. Oxidation effectively ceases when the surface oxide layer is completely replaced by the bulk PdO film, demonstrating that the surface oxide is needed for PdO formation at the conditions studied. Both the kinetics of PdO formation and the final thickness of the PdO thin film depend strongly on the thermal stability of the PdO seeds. Below the decomposition temperature of the seeds (∼600 K), oxidation follows kinetics similar to Langmuirian adsorption and appears to be limited only by the rate of oxygen adsorption onto the surface oxide. In contrast, PdO formation above 600 K initially exhibits acceleratory kinetics, with the rates starting low but increasing steadily during the initial growth of PdO. We also observe a significant decrease in PdO(1 0 1) film thickness and improved crystallinity when oxidation is conducted below 600 K. We show that the trends observed in the oxidation kinetics and film thickness can be qualitatively explained within the context of a model in which the thermodynamic stability of PdO particles increases with increasing particle size and PdO seeds/particles coexist with a two-dimensional (2D) gas of oxygen atoms adsorbed on the surface oxide layer. This model suggests that the PdO particle-2D gas coexistence relation gives rise to three distinct growth regimes, namely, stable seed nucleation, metastable seed nucleation and oxygen dissolution into the subsurface where the latter is established at 2D gas coverages below the stability limit of a flat PdO surface.

On the mechanism of the transition from surface to bulk metal oxides (A perspective on the article “Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)”)

On the mechanism of the transition from surface to bulk metal oxides (A perspective on the article “Mechanism of PdO thin film formation during the oxidation of Pd(1 1 1)”)

Facile C−H Bond Cleavage and Deep Oxidation of Propane on a PdO(101) Thin Film

We investigated the adsorption and reactions of propane on a PdO(101) thin film under ultrahigh vacuum conditions and find that C−H bond cleavage of propane is highly facile on this PdO surface. At 85 K, propane adsorption on the PdO film produces two distinct states that desorb at temperatures of 120 and 190 K during temperature programmed measurements. The 190 K desorption peak is consistent with propane molecules that interact strongly with coordinatively unsaturated surface atoms, possibly through a donor−acceptor interaction, whereas the 120 K peak is attributed to physically adsorbed propane. Upon further heating, reaction of propane with the PdO(101) surface generates an H2O desorption peak at 345 K and then simultaneously produces H2O and CO2 desorption peaks at 463 K. The relative yields of H2O at 345 and 463 K provide strong evidence that the dissociation of molecularly adsorbed propane occurs by cleavage of a single C−H bond. The resulting propyl fragments selectively undergo complete oxidation...

Adsorption of Water on a PdO(101) Thin Film: Evidence of an Adsorbed HO−H2O Complex

We investigated the adsorption of water on a PdO(101) thin film using temperature-programmed desorption (TPD) measurements and density functional theory (DFT) calculations. TPD spectra obtained from high water coverages exhibit sharp peaks at 149 and 197 K that arise from water desorbing from a multilayer and a second layer, where the second layer appears to be stabilized by direct interactions with the PdO(101) surface. The TPD spectra also exhibit features at 318 and 350 K that originate from different forms of chemisorbed water. The feature at 350 K grows as a sharp peak with increasing water coverage, whereas the feature at 318 K develops as a broad peak only after the water coverage exceeds ∼50% of the density of coordinatively unsaturated (cus) Pd atoms (0.50 MLcus). Consistent with the experimental observations, DFT predicts that formation of an HO−H2O complex is highly favored at low water coverages on PdO(101), and that water chemisorbs into less stable molecular states at coverages above 0.50 ML...

Molecular Chemisorption of O2 on a PdO(101) Thin Film on Pd(111)

We used temperature programmed desorption (TPD) to investigate the molecular chemisorption of O2 on a (101)-oriented PdO thin film grown on Pd(111) in ultrahigh vacuum (UHV) using an oxygen atom beam. Our results show that the molecular chemisorption of O2 is facile on the PdO film at 85 K, producing a saturation O2 coverage of 0.27 ML (monolayers). Experiments with co-adsorbed 16O2 and 18O2 further reveal that molecularly chemisorbed O2 dissociates negligibly on the PdO(101) surface under the conditions examined. The O2 TPD spectrum from the PdO(101) surface at O2 saturation exhibits two main features centered at 117 K and 227 K, as well as smaller features at 275 K and 315 K, associated with the desorption of molecularly chemisorbed O2. Comparison with O2 TPD obtained from clean Pd(111) demonstrates that a large fraction of the O2 molecules on the PdO(101) surface are more strongly bound than O2 chemisorbed on the metallic surface at 85 K and saturation of the respective O2 layers. We find that O2 molecules chemisorb only in small quantities (< 0.03 ML) on the p(2 × 2) and two-dimensional Pd5O4 phases of atomic oxygen on Pd(111), indicating that these phases have much weaker binding affinities toward O2 than the PdO(101) surface generated in our experiments. Finally, temperature programmed reaction spectra with co-adsorbed 18O2 and CO demonstrate that both PdO and molecularly adsorbed O2 actively participate in the oxidation of CO, with the atomic and molecular species exhibiting similar activities for the conditions studied. The results of this study may have implications for understanding Pd oxidation catalysis at high pressures given that we find relatively strong binding states of O2 on PdO and observe that these molecularly adsorbed species are active in CO oxidation.

A PdO(1 0 1) thin film grown on Pd(1 1 1) in ultrahigh vacuum

We present experimental results demonstrating that a high quality PdO(1 0 1) thin film can be grown on Pd(1 1 1) in ultrahigh vacuum by oxidizing the metal at 500 K using an oxygen atom beam, followed by annealing to 675 K. Low energy electron diffraction (LEED) images show that the [0 1 0] direction of the PdO(1 0 1) thin film aligns with the [−1 1 0] direction of the Pd(1 1 1) substrate, and that the PdO film grows in three degenerate domains, rotated 120° relative to one another. Based on excellent agreement between the experimental and simulated LEED patterns, we conclude that the surface structure of the PdO thin film deviates minimally from bulk-terminated PdO(1 0 1). Recent temperature programmed desorption (TPD) experiments also provide evidence that the PdO(1 0 1) thin film on Pd(1 1 1) is terminated by the stoichiometric surface in which half of the Pd atoms are coordinatively unsaturated (cus), corresponding to a cus-Pd atom density equal to about 35% of the surface density of Pd(1 1 1). The ability to generate a well-defined PdO(1 0 1) surface in ultrahigh vacuum should provide new opportunities for conducting model surface science studies of PdO, particularly studies aimed at elucidating the reactivity of PdO(1 0 1) toward species important in commercial applications of Pd catalysis.