Vibha R. Satsangi

A study on the photoelectrochemical properties of copper oxide thin films

Abstract The photoelectrochemical properties of Copper Oxide thin film, prepared by spray pyrolysis on conducting glass (SnO 2 : F-coated glass) substrate were investigated as a function of film deposition temperature and spray time. The variation in the deposition temperature affected the film morphology. The film deposited at substrate temperature 350°C exhibited better photoresponse than the films prepared at 300°C and 400°C. Creation of large number of dislocations and kink sites at 300°C and 400°C, which act as a recombination center for photogenerated electron has been held as a possible cause for poor photoresponse observed. The rise in photocurrent density with increase in spray time has been attributed to the increase in film thickness, which is probably allowing the film to absorb photons more efficiently. X-ray diffraction patterns of films confirm the presence of cupric oxide (CuO) phase. The films of CuO were found of n-type, apparently suggesting the existence of oxygen vacancies in the structure, on account of incomplete oxidation taking place at a relatively low temperatures (300–400°C).

Improved photoelectrochemical response of haematite by high energy Ag9+ ions irradiation

Haematite (?-Fe2O3) thin films deposited on a conducting glass substrate were irradiated with 120?MeV Ag9+ ions with increasing ion fluences. Their structural evolution was determined by x-ray diffraction and Raman spectroscopy. The irradiation of the samples of ?-Fe2O3 was found to be effective in improving its photoelectrochemical response. The film irradiated at a fluence of 1 ? 1013?ions?cm?2 showed a significantly higher photocurrent density than the unirradiated ?-Fe2O3. This effect is correlated with the transition from the ?-Fe2O3 to the Fe3O4 phase and the reduction in grain size that were observed on irradiation with Ag9+ ions. The measured flatband potential and donor density of the sample were also maximum at a fluence of 1 ? 1013?ions?cm?2.

Enhanced Photoelectrochemical Response of Zn-Dotted Hematite

Photoelectrochemical response of thin films of α-Fe2O3, Zn doped α-Fe2O3, and Zn dots deposited on doped α-Fe2O3 prepared by spray pyrolysis has been studied. Samples of Zn dots were prepared using thermal evaporation method by evaporating Zn through a mesh having pore diameter of 0.7 mm. The presence of Zn-dotted islands on doped α-Fe2O3 surface exhibited significantly large photocurrent density as compared to other samples. An optimum thickness of Zn dots ∼230 A is found to give enhanced photoresponse. The observed results are analyzed with the help of estimated values of resistivity, band gap, flatband potential, and donor density.

Morphological, optical and photoelectrochemical properties of Fe2O3–GNP composite thin films

The ever increasing demand for energy and increased environmental pollution has led to the search for new and renewable energy sources. Harnessing solar energy for the production of clean hydrogen is a very attractive method. This paper focuses on the synthesis of iron oxide photoanodes modified with graphene nanoplates (Fe2O3–GNP) as conducting scaffolds for the efficient generation of hydrogen in a photoelectrochemical (PEC) cell using solar energy. These GNP modified α-Fe2O3 samples were found to exhibit an enhanced photoresponse, which has been mainly attributed to: (i) efficient charge transfer at the semiconductor/electrolyte junction, and (ii) a red shift in the UV-vis spectra of the composite Fe2O3–GNP thin films with respect to the pristine α-Fe2O3 sample. The highest photocurrent density of 2.5 mA cm−2 at 0.75 V/SCE was observed for the 0.2 wt% GNP modified α-Fe2O3 sample, with a solar to hydrogen conversion efficiency of 1.8%. The flat band potential (−0.83 V/SCE) and donor density (1.09 × 1021 cm−3) were found to be maximized for the same sample. In virtue of their superior photoelectrochemical performance, GNP modified α-Fe2O3 thin films have substantial potential for use in PEC water splitting reactions.

Photoelectrochemical performance of bilayered Fe–TiO2/Zn–Fe2O3 thin films for solar generation of hydrogen

A visible-light sensitive bilayered photoanode of Fe–TiO2/Zn–Fe2O3 has been developed by spray pyrolytically depositing Zn–Fe2O3 layers onto predeposited Fe–TiO2 thin film on ITO substrate. Fe–TiO2/Zn–Fe2O3 photoelectrodes were characterized by XRD, Raman, AFM, UV-vis absorption spectroscopy. Photoelectrochemical properties of bilayered Fe–TiO2/Zn–Fe2O3 photoelectrode were studied by Mott–Schottky curves and I–V characteristics. Bilayered Fe–TiO2/Zn–Fe2O3 photoelectrode was observed to possess much higher separation efficiency of photogenerated charge carriers and could generate nine times better photocurrent density than pure Fe–TiO2. Solar to hydrogen conversion efficiency exhibited by this electrode was 0.77%.

Study of charge separation and interface formation in a single nanorod CdS–CuxS heterojunction solar cell using Kelvin probe force microscopy

In the present investigation, Kelvin probe force microscopy (KPFM) is used to study the charge separation, shift in Fermi level position and interfacial depletion region formation in a single cadmium sulfide (CdS)-copper sulfide (CuxS) nanorod heterojunction fabricated using hydrothermal synthesis and a topotaxial conversion reaction. A detailed analysis of KPFM images in the dark shows work function (or Fermi energy) values of CdS and CuxS regions consistent with the energy band diagram of the CdS-CuxS junction. Under illumination, Fermi energy levels of both the CuxS and CdS shift away from the vacuum level by 0.2 and 0.4 eV, respectively, which is very different from the behaviour expected in the case of a bulk p-n junction. The existence of interfacial regions topographically placed between ITO-CdS and CdS-CuxS with intermediate work function values as well as the observed narrowing of the work function spread under illumination are important for understanding the fundamental process of charge separation and junction formation in semiconductor nanorod solar cells.

Enhanced Photoelectrochemical Activity of 120 MeV Ag9+ Irradiated Nanostructured Thin Films of ZnO for Solar-Hydrogen Generation via Splitting of Water

This paper deals with a study on 120 MeV Ag9+ irradiated thin films of zinc oxide (ZnO), obtained by sol-gel – spin coating onto TCO glass plates. Films irradiated at fluence 5×1011, 3×1012, 5×1012 and 2×1013 ions cm-2, were optically characterized for band gap determination. XRD analysis revealed polytypism as both wurtzite and zincblend phases co-evolved. Scherrer’s calculations indicated grain size in nanodimensions, while SEM analysis indicated smooth surface morphology of films. Flat band potentials and donor densities were evaluated by Mott-schottky calculations. For PEC studies, thin films of ZnO were employed as working electrode in conjunction with Platinum Counter electrode, Saturated Calomel Reference electrode, 13 pH aqueous solution of NaOH as electrolyte and 150W Xenon Arc light source for illumination. A significant gain in photoelectrochemical current was recorded on SHI irradiation. The films irradiated at fluence 3×1012 ions cm-2 yielded maximum increase in photocurrent that was nearly five times compared to unirradiated samples.

Size and oxygen passivation induced reversal of photoconducting behaviour in CdS nanorods

The effect of oxygen adsorption and desorption on the photoconducting gain, spectral dependence of quantum efficiency and optical switching was studied in CdS nanorods with diameters of 20, 50 and 100 nm. These were found to have an increasing degree of crystallinity and consequently a decreasing overall density of defects leading to better stoichiometry being maintained. These properties, along with the complete depletion of electrons from the nanorod volume and oxygen induced passivation of defects, resulted in: (i) a large difference in photoconducting gain, (ii) reversal of the photoconducting behaviour on annealing in oxygen and a vacuum, and (iii) onset of an absorption edge in the spectral dependence of quantum efficiency on oxygen annealing in 20 nm diameter nanorods in comparison to the normal photoconducting behaviour expected from an n-type semiconductor observed in 50 and 100 nm diameter nanorods. Single CdS nanorod devices show stable I-V characteristics in dark and light conditions under a wide temperature range and the effect of oxygen and vacuum annealing can be clearly observed. The oxygen induced defect passivation observed in this study is important for the application of compound semiconductor nanorods in optoelectronic devices.

Gradient doping – a case study with Ti-Fe2O3 towards an improved photoelectrochemical response

The present study investigates the effect of gradient doping on modifying the photoelectrochemical response of Ti-doped Fe2O3 photoanodes for their use in sunlight based water splitting for hydrogen evolution. The deposition of a thin film over the ITO (tin doped indium oxide) substrate was carried out using a spray pyrolysis method. The concentration of dopant was varied from 0.5-8.0 at% and two sets of samples were also prepared with low to high (0.5-8%) and high to low (8-0.5%) dopant concentrations in the direction towards the substrate. The prepared thin films were characterized using X-ray Diffractometry (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) Spectroscopy, Secondary Ion Mass Spectroscopy (SIMS), X-ray Photoelectron Spectroscopy (XPS) and UV-visible Spectroscopy. The photoelectrochemical studies revealed that the deposition of dopant layers with a low to high concentration towards the substrate exhibited a highly improved photoresponse (200 times) in comparison to the pristine sample and a two fold enhancement in comparison to 2% Ti-doped Fe2O3. The improvement in the photoresponse has been attributed to the values of a high flat band potential, low resistance, high open circuit voltage, carrier separation efficiency, applied bias photon-to-current conversion efficiency (ABPE), and incident photon-to-current conversion efficiency (IPCE). A reduced charge transfer resistance has been demonstrated with Nyquist plots.

Nanostructured ZnO for photoelectrochemical splitting of water to produce hydrogen: swift heavy ion irradiation vis-à-vis dye-sensitisation

Nanostructured ZnO is a promising material for solar light driven photoelectrochemical splitting of water to produce hydrogen. With a band gap around 3.3 eV, it can easily generate required photopotential for electrolysis of water. However, its high band gap does not permit efficient absorption of solar light. To overcome this limitation, several approaches are being tried. A popular approach is its sensitisation with a dye having λ max in the visible region. In an innovative approach, authors irradiated sol-gel derived nanostructured ZnO films by 120 MeV Ag 9+ ions to induce structural defects that might shift absorption threshold in the visible region. This report presents, with regard to PEC splitting of water, a comparison of SHI irradiated vis-a-vis dye-sensitised films of ZnO. Films were subjected to XRD, SEM and Mott-Schottky analysis and optical characterisation. For PEC studies, these were used in conjunction with Pt counter electrode, saturated calomel reference electrode and 150 W Xenon arc light source.