Search

Nickel (II) oxide, NiO, is a wide band gap Mott insulator characterized by strong Coulomb repulsion between d-electrons and displays antiferromagnetic order at room temperature. NiO has gained attention in recent years as a very promising candidate for applications in a broad set of areas, including chemistry and metallurgy to spintronics and energy harvesting. Here, we report on the fabrication of polycrystalline NiO using spray-pyrolysis technique, which is a deposition technique able to produce quite uniform films of pure and crystalline materials without the need of high vacuum or inert atmospheres. The composition and structure of the NiO thin films were then studied using x-ray diffraction, and atomic force and scanning electron microscopies (SEM). The phononic and magnonic properties of the NiO thin films were also studied via Raman spectroscopy, and the ultrafast electron dynamics by using optical pump probe spectroscopy. We found that the NiO samples display the same phonon and magnon excitations expected for single crystal NiO at room temperature, and that electron dynamics in our system is like those of previously reported NiO mono- and polycrystalline systems synthesized using different techniques. These results prove that spray-pyrolysis can be used as affordable and large-scale fabrication technique to synthesize strongly correlated materials for a large set of applications.

In this work, the morphology, surface composition, and electronic properties of porous GaN films containing hexagonal V-shaped pits were studied. The V-pits are orientated along the [0001] direction of GaN, and we observed a clear relation between the growth time with the surface composition, film thickness, and pit morphology, which in turn had a significant impact on the band gap, valence band maximum, and the work function. The effect on the position of the valence band maximum and work function is explained by the formation of superficial oxygen-rich phases such as Ga2O3 and nonstoichiometric GaNxOy as supported by X-ray photoelectron spectroscopy and density functional theory (DFT). We further show a change in the optical band gap with the thickness of the porous films explained by a change in the tensile strain caused by open-core screw dislocations that gives rise to the formation of V-pits. The correlation between strain and the band gap is supported by DFT calculations. Our study provides insights into the intricate relation between surface states and electronic properties of semiconducting materials and offers directions for designing GaN heterojunctions with specific optical and electronic properties.

Electrochemistry is a key technology to synthesize, study and scale-up materials and processes for applications in solar energy conversion systems. Mexico has had a tradition of excellence in electrochemistry research and methodology development, and this paper intends to honor some of the key contributors in the subjects of solar energy conversion to useful heat or electricity. We summarize the use of electrochemical techniques as a tool for the deposition and characterization, including the analysis of electrodeposition solutions and deposition mechanisms. In addition, we describe the use of electrodeposited and hybrid ZnO films for application in dye-sensitized solar cells, which are photoelectrochemical systems, and discuss the mechanisms that govern solar cell performance., Special Issue. Tribute to the electrochemical emeritus researchers of SNI

Foam-like NiMo coatings were produced from an inexpensive mixture of Ni, Al, and Mo powders via atmospheric plasma spraying. The coatings were deposited onto stainless-steel meshes forming a highly porous network mainly composed of nanostructured Ni and highly active Ni4Mo. High material loading (200 mg cm−2) with large surface area (1769 cm2 per cm2) was achieved without compromising the foam-like characteristics. The coatings exhibited excellent activity towards both hydrogen evolution (HER) and oxygen evolution (OER) reactions in alkaline media. The HER active coating required an overpotential of 42 mV to reach a current density of −50 mA cm−2 with minimum degradation after a 24 h chronoamperometry test at −10 mA cm−2. Theoretical simulations showed that several crystal surfaces of Ni4Mo exhibit near optimum hydrogen adsorption energies and improved water dissociation that benefit the HER activity. The OER active coating also consisting of nanostructured Ni and Ni4Mo required only 310 mV to achieve a current density of 50 mA cm−2. The OER activity was maintained even after 48 h of continuous operation. We envisage that the development of scalable production techniques for Ni4Mo alloys will greatly benefit its usage in commercial alkaline water electrolysers.

In this work, we study the influence of the distance between electrodes on the performance of dye-sensitized solar cells based on TiO2 using the organic dye LEG4 and a Cu(dmp)(2) redox couple (dmp = dimethyl phenantroline). The solar cells are characterized by a large open circuit voltage of up to 1.03 V, and an efficiency of 8.2% has been achieved for a 5.3 mm thick TiO2 film using an epoxy resin-based sealed cell configuration with a minimal separation between electrodes. Transient short-circuit photocurrent measurements up to an intensity of 3 Suns show a significant decay in photocurrent after an initial peak current upon switching on the light for larger distance, resulting in a lower steady state photocurrent. For the smaller distance cells, the steady state photocurrent is linear with light intensity up to 2 Suns. Charge extraction measurements under short-circuit conditions show that reducing the distance between electrodes increases the electron collection efficiency and thus, the attainable photocurrent. Recombination losses increase with larger electrode separation distance and higher light intensity due to mass transport limitation of the redox mediator. Electrochemical impedance measurements confirm the effect of electrode distance on the redox couple transport, showing an additional loop with increasing distance. For the configuration where the TiO2 film is in very close proximity to the PEDOT-covered counter electrode, inductive behavior is observed at low frequencies. The inductive behavior disappears with the incorporation of an insulating porous ZrO2 layer. The equivalent circuit for the solar cell has been expanded to include this effect.