PdO Thin Films Research Articles

A Review on Preparation of Palladium Oxide Films

Fabrication aspects of PdO thin films and coatings are reviewed here. The work provides and organizes the up-to-date information on the methods to obtain the films. In recent years, the interest in Pd oxide for different applications has increased. Since Pd can be converted into PdO, it is instructive to pay attention to the preparation of the pure and the alloyed Pd films, heterostructures, and nanoparticles synthesized on different substrates. The development of PdO films is presented from the early reports on coatings’ formation by oxidation of Pd foils and wires to present technologies. Modern synthesis/growth routes are gathered into chemical and physical categories. Chemical methods include hydrothermal, electrochemical, electroless deposition, and coating methods, such as impregnation, precipitation, screen printing, ink jet printing, spin or dip coating, chemical vapor deposition (CVD), and atomic layer deposition (ALD), while the physical ones include sputtering and cathodic arc deposition, laser ablation, ion or electron beam-induced deposition, evaporation, and supersonic cluster beam deposition. Analysis of publications indicates that many as-deposited Pd or Pd-oxide films are granular, with a high variety of morphologies and properties targeting very different applications, and they are grown on different substrates. We note that a comparative assessment of the challenges and quality among different films for a specific application is generally missing and, in some cases, it is difficult to make a distinction between a film and a randomly oriented, powder-like (granular), thin compact material. Textured or epitaxial films of Pd or PdO are rare and, if orientation is observed, in most cases, it is obtained accidentally. Some practical details and challenges of Pd oxidation toward PdO and some specific issues concerning application of films are also presented.

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.

Nanostructured Pdo Thin Films by Self Assembly Method for No2 Gas Detection at Room Temperature

Nanostructured Pdo Thin Films by Self Assembly Method for No2 Gas Detection at Room Temperature

Molecular beam epitaxy of PdO on MgO (001)

PdO, widely used in catalysis in powder form for decades, has been predicted recently to be a Dirac semimetal. Synthesis of high-quality single crystals of this material is thus of great interest. Here, by using ozone-assisted molecular beam epitaxy, PdO thin films were grown on a MgO (001) substrate. X-ray diffraction and transmission electron microscopy indicate the film is $a/b$ axis oriented, with the $c$ axis lying in plane. Fully oxygenated PdO films have a low density of holelike carriers, and are insulating at the lowest temperatures. Our density functional theory calculations using the Heyd-Scuseria-Ernzerhof exchange-correlation functional suggest a \ensuremath{\sim}1.0 eV band gap at the $M$ point, where the gap can be reduced by tensile strain along the $c$ axis and the bands begin to invert when the tensile strain is as high as 18%. Although tensile strain of this magnitude is experimentally not viable using epitaxy, electrons can be doped by oxygen reduction. Our results emphasize the need for careful consideration of electron correlation effects and stoichiometry in ab initio modeling of topological semimetals involving transition metal oxides.

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.

Large third-order optical nonlinearity in PdO thin films

We report on a study of the absorption spectra and third-order nonlinear optical properties of crystalline PdO thin films, grown by oxidation of thin palladium films deposited on a substrate. The spectra reveal an absorption peak at [Formula: see text][Formula: see text]nm, which grows and narrows significantly as the temperature is lowered to [Formula: see text][Formula: see text]K. This behavior suggests an exciton nature of the peak. According to the position of the long wavelength absorption edge in the spectra, the bandgap of the material is [Formula: see text][Formula: see text]eV. Utilizing the techniques of degenerate four-wave mixing and Z-scan, using a pulsed laser radiation with a wavelength [Formula: see text][Formula: see text]nm and a pulse duration of [Formula: see text][Formula: see text]ns, we determined the real part of the third-order nonlinear susceptibility of Re[Formula: see text][Formula: see text]esu. Possible nature of this high refractive nonlinearity is discussed.

Microstructure and electrical properties of palladium oxide thin films for oxidizing gases detection

Palladium oxide thin and ultrathin films have been presented as promising materials for detection of oxidizing gases. PdO films were prepared by thermal oxidation of previously formed Pd layers on different substrates: SiO2/Si (100), Si (100), optical quality quartz, and KCl (100) with a buffer layer of amorphous carbon. By X-ray analysis, reflection high-energy electron diffraction, and high resolution transmission electron microscopy investigations it has been established that homogenous polycrystalline PdO films (thickness was about 5–40nm) were formed by oxidation at dry oxygen at temperatures T=770–870K. The experimental results of X-ray analysis and transmission electron microscopy have shown that the increase in oxidation temperature led to uniformity and enlargement of crystalline grains in homogeneous PdO films. It has been found that the energy band gap values increased monotonically from ΔEg=2.15eV to 2.3eV for PdO films with p-type conductivity oxidized at Тan=670K and 1070K, respectively. At ozone detection PdO films fabricated on polished poly-Al2O3 have shown high values of sensitivity, signal stability, and reproducibility of sensor response. For fabrication of gas sensors the application of PdO thin and ultrathin films has some advantages in comparison with other materials because PdO films have shown good values of sensor functional parameters, and their synthesis procedure is rather simple and is compatible with planar processes of the microelectronic industry also.

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.

A mechanistic study of hydrogen gas sensing by PdO nanoflake thin films at temperatures below 250 °C.

We prepared PdO nanoflake thin films on the SiO2 substrate by reactive sputter deposition, and studied their sensing response to H2 at temperatures between 25 and 250 °C. In addition to the oxygen ionosorption model, which is used to describe the early H2 sensing response over the temperature range studied, the H2 sensing kinetics of the PdO thin films can be separated into three temperature regimes: temperatures below 100 °C, around 150 °C and above 200 °C. At temperatures below 100 °C, PdO reduction is the dominant reaction affecting the H2 sensing behavior. At temperatures around 150 °C, Pd reoxidation kinetically competes with PdO reduction leading to a complicated sensing characteristic. Active PdO reduction by H2 promotes the continuing growth of Pd nanoislands, facilitating dissociative oxygen adsorption and thus the subsequent Pd reoxidation in the H2-dry air gas mixture. The kinetic competition between the PdO reduction and reoxidation at 150 °C leads to the observation of an inverse of the increase in the sensor conductivity. At temperatures above 200 °C, the PdO sensor exhibits a sensor signal monotonically increasing with the H2 concentration, and the H2 sensing behavior is consistent with the Mars-van-Krevelen redox mechanism.

Surface reactivity of oxide phases generated on Pd(111) during the growth vs. reduction of PdO(101) films

We used temperature programmed reaction spectroscopy (TPRS) to investigate the adsorption and complete oxidation of propane on various surface oxygen phases which evolve during the growth and reduction of PdO(101) films on Pd(111). Desorption spectra reveal clear differences in the binding of molecular propane on the different oxygen phases investigated, including the p(2×2)-O chemisorbed phase on Pd(111), two-dimensional (2D) oxide(s) and multilayer PdO(101) thin films. We find that propane reacts to an immeasurable extent on the p(2×2)-O and 2D oxide phases during TPRS, while reaction is facile on the PdO(101) surface. For the partially oxidized and reduced surfaces, we find that the CO2 product yield obtained from propane-saturated surfaces increases continuously with increasing oxygen coverage above about 1 ML (monolayer), and observe a hysteresis in the surface reactivity such that more CO2 is produced when a given oxygen coverage is generated during the growth vs. reduction of a thin PdO(101) film. We show that the fractional areas of coexisting oxygen phases can be estimated by fitting propane TPD spectra obtained from partially oxidized surfaces with propane TPD spectra obtained from well-defined reference surfaces. Based on the estimated surface area fractions, we find that the CO2 product yields scale linearly with the amount of PdO(101) at the surface, irrespective of the types and amounts of coexisting phases, and show that the observed hysteresis arises from differences in the PdO(101) surface fractions obtained at a given coverage during growth and reduction of PdO(101) films. The estimated surface fractions also reveal that 2D oxide domains develop during the first stage of PdO(101) reduction, while metallic surface domains become prevalent only during the latter half of the oxide reduction process. A key finding is that the activation of propane on partially oxidized Pd(111) is relatively insensitive to the complexity of the surface phase distribution, and depends almost exclusively on the amount of PdO(101) domains that are present.

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.

Growth and photoresponse study of PdO nanoflakes reactive-sputter deposited on SiO2

This study prepares PdO nanostructures on a SiO2 substrate by reactive-sputter deposition and examines photoresponse characteristics of the thin films. PdO thin films deposited at 25 °C is composed of bowed nanoflakes standing on the SiO2 substrate, which have a single-crystalline structure after thermal anneal at 400 °C. The 400 °C-annealed nanoflake thin film has a band gap energy in the red-light range (∼2.06 eV), and exhibits a very sensitive photoresponse upon the UV (365 nm) illumination. The high photoresponse sensitivity of the 400 °C-annealed nanoflake thin film is ascribed to a lower density of recombination centers and traps due to an excellent crystallinity and a high carrier extraction efficiency due to a low electrical resistivity. A slight decrease in the photocurrent density during the initial stage of the UV illumination is attributed to adsorption of O2− anions on the 400 °C-annealed nanoflakes.

Hydrogen gas sensing properties of PdO thin films with nano-sized cracks

We report on a novel method for the fabrication of highly sensitive hydrogen gas sensorsbased on palladium oxide thin films and have investigated their hydrogen sensing propertiesand nanostructures. To our knowledge, this is the first report on the use of palladium oxideand reduced palladium thin films as hydrogen sensors. The palladium oxide thin films weredeposited on thermally oxidized Si substrates using a reactive direct current (DC)magnetron sputtering system. Considerable changes in the resistance of the palladiumoxide thin films were observed when they were initially exposed to hydrogen gas, asa result of the reduction process. After the initial exposure to hydrogen gas ofPdO30%, its sensitivityincreased up to ∼ 4.5 × 103%. The morphology of the PdO surface was analyzed using a scanning electron microscope(SEM), in order to investigate the interactions between palladium oxide and hydrogen. TheSEM images showed a large number of nano-sized cracks on the surface of the palladiumoxide during the reduction process, which acted to increase the effective surface-to-volumeratio. The response behaviors of the reduced Pd films to hydrogen gas were reversible andhad an enhanced sensing property when compared with those of the pure Pd films. Inaddition, their sensitivities and response times were improved due to the nano-sizedcracks on the surfaces. The results demonstrate that palladium oxide and reducedpalladium thin films can be applied for use in highly sensitive hydrogen sensors.

Fabrication and characterization of surface‐conduction electron emitters for SED application

Abstract— A surface‐conduction electron emitter (SCE) for next‐generation flat‐panel displays has been developed. PdO thin films (approximately 10 nm thick) produced by an ink‐jet process were used to form the surface‐conduction electron emitter. The films were electroformed and activated while a voltage was applied, and an electron emitter with good characteristics was obtained. A current density of approximately 30 mA/cm2 was attained when an anode voltage of 10 kV was applied. Furthermore, a 36‐in. surface‐conduction electron‐emitter display (SED), consisting of SCEs and a phosphor screen similar to that of a CRT, was also developed.

71.2: Fabrication and Characterization of Surface Conduction Electron Emitters

We have developed a surface conduction electron emitter (SCE) for next-generation flat displays. PdO thin films (approximately 10 nm) produced by the ink-jet process were used for forming the surface conduction electron emitter. The films were electroformed and activated while a certain voltage was applied, and an electron emitter with good characteristics was obtained. A current density of approximately 30 mA/cm2 was attained when an anode voltage of 10 kV was applied. We confirmed operation for 60,000 hours by an acceleration test at a current density of 3 mA/cm2. This current density is required for “good brightness” on the surface conduction electron emitter display (SED).

Optical studies of PdO thin films

Palladium oxide thin films are studied by ellipsometry, optical transmission, and Raman scattering. The PdO films are made by completely oxidizing Pd films sputtered onto fused silica substrates, and their optical constants determined using a combination of ellipsometry and transmission measurements. Oxidation kinetics experiments are performed on thick Pd substrates at temperatures of 300–500 °C. The optical properties of the films show a thickness dependence suggesting an increased absorption at the PdO-Pd interface. Ellipsometry on oriented single crystals of PdO yields the anisotropic optical constants at 632.8 nm. These results are combined with an effective medium theory to show that the oxidized films have a void density of 20–25%. Raman scattering is demonstrated to be a very sensitive probe for detecting the presence of PdO on Pd. An oxide only a few Å thick is readily detectable, despite the absence of any surface enhanced Raman effect.

Raman investigation of palladium oxide, rhodium sesquioxide and palladium rhodium dioxide

AbstractRaman spectra were obtained from PdO, Rh2O3 and PdRhO2 thin films formed by oxidizing foil samples. Polarization measurements allow symmetry species assignments to be made for the lines from the corundum phase of Rh2O3 and from PdRhO2, which has a delafossite structure.