Assessing the evolution of oxygenated functional groups on the graphene oxide surface upon mild thermal annealing in water.

Graphene oxide (GO) is known to be a 2D metastable nanomaterial that can be reconstructed under thermal annealing into distinct oxidized and graphitic phases. Up to now, such phase transformation, mainly related to epoxide and hydroxyl functional groups, has been usually achieved by thermally treating layers of GO in the solid state. Here, we present the mild annealing of GO dispersed in an aqueous medium, performed at two temperatures, 50 °C and 80 °C, for different intervals of time. We show experimental evidences of the epoxide instability in the presence of water by means of XPS, cyclic voltammetry and Raman spectroscopy, demonstrating the reorganization of epoxide and hydroxyl moieties initiated by water molecules. In fact, at 50 °C an increase in oxygen content is detected in all annealed samples compared to untreated GO, with a transformation of epoxide groups into vicinal diols. On the other hand, at 80 °C the oxygen content decreases towards the initial value since the vicinal diols, previously formed, transform into single hydroxyls and C[double bond, length as m-dash]C bonds. Moreover, the higher temperature annealing likely favours oxygenated functional groups rearrangements and clustering, in accordance with the literature, leading to a higher electron affinity and conductivity of the graphenic network.

One-Step Hydrothermal Synthesis of Cu2ZnSnS4 Nanoparticles as an Efficient Visible Light Photocatalyst for the Degradation of Congo Red Azo Dye.

A hydrothermal method was successfully employed to synthesize kesterite Cu2ZnSnS4 (CZTS) nanoparticles. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and optical ultraviolet-visible (UV-vis) spectroscopy were used for characterization of structural, chemical, morphological, and optical properties. XRD results confirmed that a nanocrystalline CZTS phase corresponding to the kesterite structure was formed. Raman analysis confirmed the existence of single pure phase CZTS. XPS results revealed the oxidation states as Cu+, Zn2+, Sn4+, and S2-. FESEM and TEM micrograph images revealed the presence of nanoparticles with average sizes between 7 nm to 60 nm. The synthesized CZTS nanoparticles bandgap was found to be 1.5 eV which is optimal for solar photocatalytic degradation applications. The properties as a semiconductor material were evaluated through the Mott-Schottky analysis. The photocatalytic activity of CZTS has been investigated through photodegradation of Congo red azo dye solution under solar simulation light irradiation, proving to be an excellent photo-catalyst for CR where 90.2% degradation could be achieved in just 60 min. Furthermore, the prepared CZTS was reusable and can be repeatedly used to remove Congo red dye from aqueous solutions.

High-performance asymmetric supercapacitor based on a CdCO3/CdO/Co3O4 composite supported on Ni foam - part II: a three-electrode electrochemical study.

A binder-free CdCO3/CdO/Co3O4 compound with a micro-cube-like morphology on a nickel foam (NF) made via a facile two-step hydrothermal + annealing procedure has been developed. The morphological, structural and electrochemical behavior of both the single compounds constituting this final product and the final product itself has been studied. The synergistic contribution effect of the single compounds in the final compounded resulting specific capacitance values are presented and discussed. The CdCO3/CdO/Co3O4@NF electrode exhibits excellent supercapacitive performance with a high specific capacitance (C S) of 1.759 × 103 F g-1 at a current density of 1 mA cm-2 and a C S value of 792.3 F g-1 at a current density of 50 mA cm-2 with a very good rate capability. The CdCO3/CdO/Co3O4@NF electrode also demonstrates a high coulombic efficiency of 96% at a current density as high as 50 mA cm-2 and also exhibits a good cycle stability with capacitance retention of ca. 100% after 1000 cycles at a current density of 10 mA cm-2 along with a potential window of 0.4 V. The obtained results suggest that the facilely synthesized CdCO3/CdO/Co3O4 compound has great potential in high-performance electrochemical supercapacitor devices.

Photophysical and Photoelectrochemical Properties of CsPbBr3 Films Grown by Electrochemically Assisted Deposition.

Perovskite have had a great impact on the solid-state physics world in the last decade not only achieving great success in photovoltaics but, more recently, also in the implementation of other optoelectronic devices. One of the main obstacles for the adoption of Pb-based perovskite technologies are the high amounts of Pb needed in the conventional preparation methods. Here we present for the first time a detailed analysis of the photophysical and photoelectrochemical properties of CsPbBr3 films directly grown on fluorine-doped tin oxide (FTO) coated glass through a novel technique based in the electrodeposition of PbO2 as CsPbBr3 precursor. This technique allows to save up to 90 % of the Pb used compared to traditional methods and can be scalable compared with the commonly used spin-coating process. The low temperature analysis of their photoluminescence spectra, performed in both steady state and time dependence, revealed a strong interaction between electrons and longitudinal optical (LO) phonons dominant at high temperatures. On the other hand, the electrochemical and photoelectrochemical analysis proves that CsPbBr3 prepared using this new method has state-of-the-art features, showing a p-type behavior under depletion regime. This is also confirmed by photoelectrochemical measurements using p-benzoquinone as target molecule. These results prove that the proposed method can be used to produce excellent CsPbBr3 films, saving much of the lead waste.

Silver Nanoparticle Arrays onto Glass Substrates Obtained by Solid-State Thermal Dewetting: A Morphological, Structural and Surface Chemical Study.

Silver nanoparticles (NPs) on glass substrates were obtained by a solid-state thermal dewetting (SSD) process using vacuum-evaporated-silver precursor layers. An exhaustive investigation of the morphological, structural, and surface chemistry properties by systematically controlling the precursor film thickness, annealing temperature, and time was conducted. Thin silver films with thicknesses of 40 and 80 nm were deposited and annealed in air by applying a combined heat-up+constant temperature-time program. Temperatures from 300 to 500 °C and times from 0 to 50 min were assayed. SSD promoted the morphological modification of the films, leading to the Ag NPs having a discrete structure. The size, shape, surface density, and inter-nanoparticle distance of the nanoparticles depended on the initial film thickness, annealing temperature, and time, exhibiting a cubic silver structure with a (111) preferred crystallographic orientation. The prepared NPs were found to be highly enriched in the Ag{111}/Ag{110}/Ag{100} equilibrium facets. SSD not only promotes NP formation but also promotes the partial oxidation from Ag to AgO at the surface level. AgO was detected on the surface around the nanoparticles synthesized at 500 °C. Overall, a broad framework has been established that connects process factors to distinguish resultant Ag NP features in order to develop unique silver nanoparticles for specific applications.

Comparison of Different Synthetic Routes of Hybrid Hematite-TiO2 Nanotubes-Based Electrodes.

Nowadays, green hydrogen is an important niche of interest in which the search for a suitable composite material is indispensable. In this sense, titanium oxide nanotubes (TiO2 nanotube, TNTs) were prepared from double anodic oxidation of Ti foil in ethylene glycol electrolyte. The morphology of the nanotubes was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Once characterized, nanotubes were used as templates for the deposition of hematite. The use of three synthetic procedures was assayed: Chemical Vapor Deposition (CVD), Successive Ionic Layer Adsorption and Reaction (SILAR), and electrochemical synthesis. In the first case, CVD, the deposition of hematite onto TiO2 yielded an uncovered substrate with the oxide and a negative shift of the flat band potential. On the other hand, the SILAR method yielded a considerable amount of hematite on the surface of nanotubes, leading to an obstruction of the tubes in most cases. Finally, with the electrochemical synthesis, the composite material obtained showed great control of the deposition, including the inner surface of the TNT. In addition, the impedance characterization showed a negative shift, indicating the changes of the interface electrode-electrolyte due to the modification with hematite. Finally, the screening of the methods showed the electrochemical synthesis as the best protocol for the desired material.

High-performance asymmetric supercapacitor based on CdCO3/CdO/Co3O4 composite supported on Ni foam.

A CdCO3/CdO/Co3O4 composite has been prepared on nickel foam through a combined hydrothermal-annealing method. An asymmetric hybrid supercapacitor (SC) device was assembled with this composite as the positive electrode and activated carbon was the negative electrode. The SC exhibited a high specific capacitance of 84 F g-1 @ 1 mA cm-2, a maximum energy density of 26.3 W h kg-1, and a power density of 2290 W kg-1, along with a wide potential window of 1.5 V and long cycle life (92% after 6000 cycles). SCs assembled in series powered various light-emitting diodes and moved an electrical mini-motor.

Effect of Electrolytic Medium on the Electrochemical Reduction of Graphene Oxide on Si(111) as Probed by XPS.

The wafer-scale integration of graphene is of great importance in view of its numerous applications proposed or underway. A good graphene-silicon interface requires the fine control of several parameters and may turn into a high-cost material, suitable for the most advanced applications. Procedures that can be of great use for a wide range of applications are already available, but others are to be found, in order to modulate the offer of different types of materials, at different levels of sophistication and use. We have been exploring different electrochemical approaches over the last 5 years, starting from graphene oxide and resulting in graphene deposited on silicon-oriented surfaces, with the aim of understanding the reactions leading to the re-establishment of the graphene network. Here, we report how a proper choice of both the chemical environment and electrochemical conditions can lead to a more controlled and tunable graphene-Si(111) interface. This can also lead to a deeper understanding of the electrochemical reactions involved in the evolution of graphene oxide to graphene under electrochemical reduction. Results from XPS, the most suitable tool to follow the presence and fate of functional groups at the graphene surface, are reported, together with electrochemical and Raman findings.

Biocompatible N-acetyl cysteine reduces graphene oxide and persists at the surface as a green radical scavenger.

We demonstrate that N-acetyl cysteine (NAC) reduces graphene oxide (GO) at room temperature. This represents a new green method to produce reduced GO (rGO). NAC adheres to the rGO surface as demonstrated by several spectroscopy techniques and avoids GO-mediated oxidation of glutathione. This method offers new opportunities for the production of green biocompatible rGO and NAC-based therapies.

Vertically aligned ZnO@CuS@PEDOT core@shell nanorod arrays decorated with MnO2 nanoparticles for a high-performance and semi-transparent supercapacitor electrode.

Hybrid nano-architectures with high electrochemical performance for supercapacitors have been designed by growing hierarchical ZnO NRs@CuS@PEDOT@MnO2 core@shell heterostructured nanorod arrays on ITO/glass substrates. This hybrid nano-structured electrode exhibits excellent electrochemical performance, with a high specific areal capacitance of 19.85 mF cm(-2), good rate capability, cycling stability and diffused coloured transparency.

ZnO nanorod/CdS nanocrystal core/shell-type heterostructures for solar cell applications.

ZnO/CdS core/shell nanorod arrays were fabricated by a two-step method. Single-crystalline ZnO nanorod arrays were first electrochemically grown on SnO(2):F (FTO) glass substrates. Then, CdS nanocrystals were deposited onto the ZnO nanorods, using the successive ion layer adsorption and reaction (SILAR) technique, to form core/shell nanocable architectures. Structural, morphological and optical properties of the nanorod heterojunctions were investigated. The results indicate that CdS single-crystalline domains with a mean diameter of about 7 nm are uniformly and conformally covered on the surface of the single-crystalline ZnO nanorods. ZnO absorption with a bandgap energy value of 3.30 ± 0.02 eV is present in all optical transmittance spectra. Another absorption edge close to 500 nm corresponding to CdS with bandgap energy values between 2.43 and 2.59 eV is observed. The dispersion in this value may originate in quantum confinement inside the nanocrystalline material. The appearance of both edges corresponds with the separation of ZnO and CdS phases and reveals the absorption increase due to CdS sensitizer. The photovoltaic performance of the resulting ZnO/CdS core/shell nanorod arrays has been investigated as solar cell photoanodes in a photoelectrochemical cell under white illumination. In comparison with bare ZnO nanorod arrays, a 13-fold enhancement in photoactivity was observed using the ZnO/CdS coaxial heterostructures.

Template assisted electrochemical growth of cobalt nanowires: influence of deposition conditions on structural, optical and magnetic properties.

The influence of electrodeposition potential, pH, composition and temperature of the electrolytic bath on the structure of cobalt nanowires arrays electrodeposited into anodic aluminum oxide (AAO) porous membranes is reported. XRD, SEM, and TEM analysis were employed to characterize structural (crystal phase, crystallographic texture, and grain size), and morphological nanowire properties. It was confirmed that at pH 2 the electrodeposition potential has not influence on the preferred crystallographic orientation of the electrochemically grown Co nanowires. At pH 4 the electrodeposition potential controls the growth of cobalt nanowires along some preferential crystallographic planes. The electrolytic pH bath modulates the fcc or hcp phase exhibited by the cobalt nanowires. Single crystalline nanowires with a hcp phase strongly oriented along the (2021) crystallographic plane were obtained at pH 4 and at -1.1 V (vs. Ag/AgCl), a result not previously reported. High electrolytic bath temperatures contributed to improve the single crystalline character of the cobalt nanowires. The presence of chloride anion in the electrolytic bath also influenced on the structural properties of the resulting cobalt nanowires, improving their crystallinity. The optical reflectance of the samples shows a structure in the UV-blue region that can be assigned to the two-dimensional morphology arising in the shape of the almost parallel nanowires. Magnetic measurements showed that different electrodeposition potentials and electrolytic bath pH lead to different magnetic anisotropies on the nanowire array samples.

Single-crystal growth of nickel nanowires: influence of deposition conditions on structural and magnetic properties.

This paper examines the influence of electrodeposition potential, pore size, pH, composition, and temperature of the electrolytic bath on the structure of nickel nanowires arrays electrodeposited into anodic alumina oxide porous membranes. Scanning electron microscopy, X-ray diffraction, and transmission electron microscopy analysis were employed to characterize the structural and morphological properties of the nanowires. Results show that the electrodeposition potential controls the growth of nickel nanowires along some preferential crystallographic planes. At -0.90 V (vs. Ag/AgCl) single crystalline nanowires with a strong (111) orientation were obtained. High temperatures and a moderately acid pH solution contributed to improve the single crystalline character of nanowires. The presence of chloride ions produced polycrystalline nanowires at low temperature and single crystalline nanowires at high temperature. The influence of the electrodeposition potential in their magnetic anisotropies is also reported.

The influence of poly(ethylene oxide) and illumination on the copper electrodeposition process onto n-Si(100).

In this study, we examined the influence of illumination and the presence of poly(ethylene oxide) (PEO) as an additive for the copper electrodeposition process onto n-Si(100). The study was carried out by means of cyclic voltammetry (CV) and the potential steps method, from which the corresponding nucleation and growth mechanism (NGM) were determined. Likewise, a morphologic analysis of the deposits obtained at different potential values by means of atomic force microscopy (AFM) was carried out. In a first stage, Mott-Schottky measurements so as to characterize the energetics of the semiconductor/electrolyte interface were made. Also, parallel capacity measurements were carried out in order to determine the surface state density of the substrate. It was found that when PEO concentration is increased, the number of these surface states decreases. The CV results indicated that the presence of PEO inhibits the photoelectrochemical reaction of oxide formation on the surface of the semiconductor. This allows a decrease in the overpotential associated with the electrodeposition process. The analysis of the j/t transients shows that the NGM corresponds to progressive three-dimensional (3D) diffusional controlled (PN3D(Diff)), which was confirmed by the AFM technique. Neither illumination nor the presence of PEO changes the mechanisms. Their influence is in that they diminish the size of the nuclei and the speed with which these are formed, which produces a more homogeneous electrodeposit.

Polymer-dispersed liquid-crystal voltage sensor.

We present a novel electric-field and voltage sensor based on the electro-optical properties of polymer-dispersed liquid-crystals (PDLCs). In principle, the transmittance of PDLCs is a nonlinear function of the applied electrical field. To measure an AC field we superposed to it a known DC field. This allowed us to achieve linearization of the PDLC response and to measure transmittance changes independently of the light-intensity level variations. Validation experiments are presented.

Electrochemical reversibility of vinylferrocene monolayers covalently attached on H-terminated p-Si(100).

A reversible electrochemical behavior is demonstrated on a specially prepared redox-functionalized H-Si(100) surface, obtained via an extra-mild grafting procedure from vinylferrocene. The results of a detailed XPS and electrochemical characterization of the resulting hybrid are reported and discussed to propose it as a reference system for high-quality electroactive monolayers on Si. The investigated ferrocene derivative bears a functional group suitable for a mild route to covalent anchoring on Si, which is based on a photoinduced reaction with visible light under an inert atmosphere. Electrochemical reversibility is shown by sharp symmetric voltammograms on freshly prepared p-Si electrodes. Anodic oxide growth is responsible for the progressive degradation of the electrochemical response. Still, fast electron transfer to the surface redox species is maintained during several thousands cycles.