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 Pd2+ with a small quantity (18%) of reduced PdO (Pd0) 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.