Controlling the surface chemistry of graphene oxide: Key towards efficient ZnO-GO photocatalysts
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.
A tool box to ascertain the nature of doping and photoresponse in single-walled carbon nanotubes
Physical Chemistry Chemistry Physics 2019, 21(7), pp 4063-4071
The effect of doping on the electronic properties in bulk single-walled carbon nanotube (SWCNT) samples is studied for the first time using a new in situ Raman spectroelectrochemical method, and further verified by DFT calculations and photoresponse. We use p-/n-doped SWCNTs prepared by diazonium reactions as a versatile chemical strategy to control the SWCNT behavior. The measured and calculated data testify an acceptor effect of 4-aminobenzenesulfonic acid (p-doping), and a donor effect (n-doping) in the case of benzyl alcohol. In addition, pristine and covalently functionalized SWCNTs were used for the preparation of photoactive film electrodes. The photocathodic current in the photoelectrochemical cell is consistently modulated by the doping group. These results validate the in situ Raman spectroelectrochemistry as a unique tool box for predicting the electronic properties of functionalized SWCNTs …
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 …
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.
Unique Properties and Behavior of Nonmercerized Type-II Cellulose Nanocrystals as Carbon Nanotube Biocompatible Dispersants
Biomacromolecules 2019, 20, 8, 3147-3160
Nanocellulose is increasingly being investigated as a paradigm of a sustainable nanomaterial because of its extraordinary physical and chemical properties, together with its renewable nature and worldwide abundance. The rich structural diversity of cellulose materials is represented by different crystalline allomorphs, from which types I and II stand out. While type I is naturally and ubiquitously present, type II is man-made and requires harsh and caustic synthesis conditions such as the so-called mercerization process. Here, we provide an optimal scenario to obtain either type-I or II nanocrystalline cellulose (NCC) by a mercerization-free method consisting only of the acid hydrolysis commonly used to produce nanocellulose from microcellulose. The possibility of having nonmercerized type-II NCC acquires a great relevance since this nanostructure shows particularly appealing properties. Moreover, an entangled and wrapped system arises when used as a dispersing agent for single-walled carbon nanotubes (SWCNTs), significantly different from that of type I. The biological testing of each NCC type and their respective SWCNT–NCC dispersions in human intestinal (Caco-2) cells reveals a general innocuous behavior in both cancer and normal stages of differentiation; however, the type-II-based SWCNT–NCC dispersions display cytotoxicity for cancer cells while enhancing mitochondrial metabolism of normal cells.