Artículos > Grupo de Nanoestructuras de Carbono y Nanotecnología (G-CNN)

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Controlling the surface chemistry of graphene oxide: Key towards efficient ZnO-GO photocatalysts

Catalysis Today 2020

Graphene oxide (GO) and related materials are widely reported to enhance the photocatalytic activity of zinc oxide. However, the origin of the observed performance improvements remains elusive and studies contributing to a deeper understanding of this critical issue are largely missing. In this work, we have prepared a set of benchmark ZnO-GO hybrid materials in order to systematically put under closer scrutiny the influence of the surface chemistry of GO on the photocatalytic degradation of methylene blue. The set of ZnO-GO hybrids has been synthesized in an ultrasonication process involving ZnO nanoparticles obtained in a microwave synthesis process and GO with three distinct oxidation degrees, employed in three different loading fractions. Structural and physical-chemical characterization by XRD, FTIR, Raman, UV–vis, photoluminescence and spectroscopy and XPS, consistently demonstrate the importance of the surface chemistry of GO for establishing photo-induced charge-transfer interface interactions with ZnO, facilitating the enhancement of the catalytic activity of the ZnO-GO catalyst. Optimized interface interactions thus enabled the design of a ZnO-GO catalyst exhibiting a conversion rate of 80% obtained in a time of 70 min and at a catalyst concentration of only 0.045 mg/mL.

Towards high-efficient microsupercapacitors based on reduced graphene oxide with optimized reduction degree

Energy Storage Materials 2020, 25, 740-749

Reduced graphene oxide aerogels synthetized using different times of hydrothermal treatment have been tested as raw material to prepare electrochemical supercapacitors. The gravimetric electrochemical capacitance measured using 1M Na2SO4 as electrolyte was maximized for aerogels that underwent 45 min of hydrothermal treatment. The aerogels synthetized for longer durations of hydrothermal treatment exhibited higher electrical conductivity but the gravimetric capacitance drops dramatically due to an increasing resistance to diffusion of the electrolyte ions. The impeded diffusion is boosted by the intensified crosslinking between graphene sheets, which narrows the pores between them in the prepared electrode. The rGO aerogel attained for 45 min of hydrothermal treatment provided a high gravimetric capacitance of 400 F g-1 and 100 F g-1 at 50 A g-1 for three-electrode and two electrodes configuration, respectively, as well as good cyclic stability, competing with other similar carbon materials. Activation pretreatments or adding a second component (glucose, dopamine, Mn, Fe, CNT) did not provide significant change of capacitance respect to pristine rGO aerogel.

Ru supported on N-doped reduced graphene oxide aerogels with different N-type for alcohol selective oxidation

MOLECULAR CATALYSIS. 2020, 484, 110737

Reduced graphene oxide aerogels doped with nitrogen have been prepared via two different strategies, namely, direct doping of rGO nanosheets with NH3 (GANH3) and coating the rGO nanosheets with an N-containing amorphous carbon derived from polydopamine (GADA). Both methods lead to comparable N/C ratios (3.4–4.7 %) but the relative contribution of quaternary nitrogen is 30 % for GADA while only 15 % for GANH3. Moreover, the N is incorporated within graphene lattice for GANH3 while it is within an amorphous carbon layer on top of graphene nanosheets for GADA. The Ru catalyst supported on the aerogels exhibited very distinct performance in the selective oxidation of benzyl alcohol depending on the support material despite having similar Ru particle size. The best performance for 5 wt% Ru on GADA can be explained by the larger reduction extent of Ru and the more hydrophobic surface. The open macroporosity of the aerogel makes it an excellent platform for using as structured catalyst in a continuous flow process.

Differential properties and effects of fluorescent carbon nanoparticles towards intestinal theranostics.

COLLOIDS AND SURFACES B: Biointerfaces 2020, 185, 110612

Given the potential applications of fluorescent carbon nanoparticles in biomedicine, the relationship between their chemical structure, optical properties and biocompatibility has to be investigated in detail. In this work, different types of fluorescent carbon nanoparticles are synthesized by acid treatment, sonochemical treatment, electrochemical cleavage and polycondensation. The particle size ranges from 1 to 6 nm, depending on the synthesis method. Nanoparticles that were prepared by acid or sonochemical treatments from graphite keep a crystalline core and can be classified as graphene quantum dots. The electrochemically produced nanoparticles do not clearly show the graphene core, but it is made of heterogeneous aromatic structures with limited size. The polycondensation nanoparticles do not have CC double bonds. The type of functional groups on the carbon backbone and the optical properties, both absorbance and photoluminescence, strongly depend on the nanoparticle origin. The selected types of nanoparticles are compatible with human intestinal cells, while three of them also show activity against colon cancer cells. The widely different properties of the nanoparticle types need to be considered for their use as diagnosis markers and therapeutic vehicles, specifically in the digestive system.

In-situ growth and immobilization of CdS nanoparticles onto functionalized MoS2 for managing charge-transfer processes.

CHEMISTRY- An Asian Journal 2020.

A facile strategy for the controllable growth of CdS nanoparticles at the periphery of MoS2 en route the preparation of electron donor‐acceptor nanoensembles is developed. Precisely, the carboxylic group of α‐lipoic acid, as addend of the modified MoS2 obtained upon 1,2‐dithiolane functionalization, was employed as anchor site for the in situ preparation and immobilization of the CdS nanoparticles in an one‐pot two‐step process. The newly prepared MoS2/CdS hybrid material was characterized by complementary spectroscopic, thermal and microscopy imaging means. Absorption spectroscopy was employed to register the formation of MoS2/CdS, by observing a broad shoulder centered at 420 nm due to CdS nanoparticles, while the excitonic bands of MoS2 were also evident. Moreover, based on the efficient quenching of the characteristic fluorescence emission of CdS at 725 nm by the presence of MoS2, strong electronic interactions at the excited state between the two species within the ensemble were identified. Photoelectrochemical assays of MoS2/CdS thin‐film electrodes revealed a prompt, steady and reproducible anodic photoresponse during repeated on‐off cycles of illumination. A significant zero‐current photopotential of −540 mV and an anodic photocurrent of 1 μA were observed, underlining improved charge‐separation and electron transport from CdS to MoS2. The superior performance of the charge‐transfer processes in MoS2/CdS is of direct interest for the fabrication of photoelectrochemical and optoelectronic devices.

The effect of graphene oxide reduction temperature on the kinetics of low-temperature sorption of hydrogen
The effect of thermal reduction of graphene oxide on the hydrogen sorption and desorption kinetics was studied by temperature-programmed desorption in the 7–120 K temperature range. The heat treatment of graphene oxide samples resulted in a decrease in the activation energy for hydrogen diffusion by more than an order of magnitude (by a factor of 12–13) compared with the initial graphite oxide. This change in the activation energy is, most likely, caused by exfoliation (loosening) of the graphite oxide carbon sheets upon the thermal removal of intercalated water, which changes the sorption character by decreasing the influence of the opposite walls in the interlayer spaces.
Reduced Graphene Oxide Aerogels with Controlled Continuous Microchannels for Environmental Remediation

ACS Appl. Nano Mater., 2019, 2 (3), pp 1210–1222

3D porous graphene microarchitectures with aligned and continuous channels are of paramount interest for several applications such as pollutant removal, energy storage, or biomedical engineering. For these applications, an accurate control over the pore microstructure is of capital importance. Freeze casting is a well-stablished technique to prepare graphene aerogels with unidirectional channels. This technique is typically applied to plain GO colloids, leading to discontinuous microchannels. Herein we have carried out the freeze process starting directly with partially reduced graphene (rGO) hydrogels prepared by a prior hydrothermal treatment in autoclave. This approach leads to the formation of aerogels with aligned and continuous microchannels, enabled by an intermediate cross-linking degree of the rGO nanosheets, carefully controlled by keeping the time of the prior hydrothermal process between the thresholds of 45–75 min. To the best of our knowledge, the effect of the degree of cross-linking in the freeze casting process is not yet reported. The resulting rGO aerogels with highly aligned microchannel structure reveal superior properties over its isotropic counterpart of randomly oriented pores for the absorption of nonpolar solvents and the selective adsorption of an aromatic compound dissolved in an alkane. Our combined hydrothermal freeze casting approach thus affords aligned microchannel rGO aerogels of enormous potential for environmental remediation.

Photoactivity improvement of TiO2 electrodes by thin hole transport layers of reduced graphene oxide

Electrochimica Acta,  2019, 298,  Pages 279-287

Nanostructured TiO2 and graphene-based materials constitute components of actual interest in devices related to solar energy conversion and storage. In this work, we show that a thin layer of electrochemically reduced graphene oxide (ECrGO), covering nanostructured TiO2 photoelectrodes, can significantly improve the photoactivity. In order to understand the working principle, ECrGO/TiO2 photoelectrodes with different ECrGO thicknesses were prepared and studied by a set of photoelectrochemical measurements. Methanol in alkaline conditions was employed as effective hole acceptor probe to elucidate the electronic phenomena in the electrode layers and interfaces. These studies underline the hole accepting properties of ECrGO and reveal the formation of a p-n junction at the interface between ECrGO and TiO2. It is shown for the first time that the resulting space charge region of about 10 nm defines the …

Chemical Postdeposition Treatments To Improve the Adhesion of Carbon Nanotube Films on Plastic Substrates

ACS OMEGA, 2019, 4(2), pp 2804-2811

The robust adhesion of single-walled carbon nanotubes (SWCNTs) to plastic substrates is a key issue toward their use in flexible electronic devices. In this work, semitransparent SWCNT films were prepared by spray-coating on two different plastic substrates, specifically poly(ethylene terephthalate) and poly(vinylidene fluoride). The deposited SWCNT films were treated by dipping in suitable solvents separately, namely, 53% nitric acid (HNO3) and N-methyl pyrrolidone. Direct evidence of SWCNT adhesion to the substrate was obtained by a peel-off test carried out with an adhesive tape. Moreover, these treatments caused enhanced film transparency and electrical conductivity. Electron microscopy images suggested that SWCNTs were embedded in the plastic substrates, forming a thin layer of conductive composite materials. Raman spectroscopy detected a certain level of doping in the SWCNTs after the chemical …

Capacitive and charge transfer effects of single‐walled carbon nanotubes in TiO2 electrodes

ChemPhysChem 2019, 20, 838-847

The transfer of nanoscale properties from single‐walled carbon nanotubes (SWCNTs) to macroscopic systems is a topic of intense research. In particular, inorganic composites of SWCNTs and metal oxide semiconductors are being investigated for applications in electronics, energy devices, photocatalysis, and electroanalysis. In this work, a commercial SWCNT material is separated into fractions containing different conformations. The liquid fractions show clear variations in their optical absorbance spectra, indicating differences in the metallic/semiconducting character and the diameter of the SWCNTs. Also, changes in the surface chemistry and the electrical resistance are evidenced in SWCNT solid films. The starting SWCNT sample and the fractions as well are used to prepare hybrid electrodes with titanium dioxide (SWCNT/TiO2). Raman spectroscopy reflects the optoelectronic properties of SWCNTs in the SWCNT/TiO2electrodes, while the electrochemical behavior is studied by cyclic voltammetry. A selective development of charge transfer characteristics and double‐layer behavior is achieved through the suitable choice of SWCNT fractions.