Scheme illustrating the lower density of deep energy states in NTs grown in 12wt.% H2O. As a result, such NTs exhibit efficient electron transport properties. In contrast electron transport is ...difficult in the case of 2wt.% H2O NTs as the density of deep states, acting as recombination center, is larger.
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The microstructural and photoelectrochemical properties of anodized TiO2 Nanotubes (NTs) grown in ethylene glycol containing 0.1M NH4F and a variable amount of H2O, either 2wt.% (low water content) or 12wt.% (high water content) have been investigated. The study has been conducted at a fixed anodizing potential, as it is known that also the applied potential can affect the electron transport properties of the NTs. The amount of water also has an impact on the microstructure of the NTs; those prepared in 12wt.% H2O have a rough wall, whereas those made at 2wt.% are mostly smooth. In addition the water content affects the dissolution of the forming oxide and consequently has an impact also on the diameter and the length of the NTs. For a fixed potential (30V), when working at 12wt.% H2O content, it is necessary to extend the anodizing time to 60min to obtain ∼1.4μm long NTs. A similar length is achieved at 2wt.% H2O content after 20min.
The photocurrent density measurements reveal an improved photoactivity for NTs grown in high water content electrolytes (1.1mA/cm2 in 12wt.% H2O vs 0.75mA/cm2 in 2wt.% H2O at 1V vs RHE). Similarly the Incident Photon to Current Efficiency (IPCE) of NTs obtained in 12wt.% H2O is consistently higher than for 2wt.% H2O over the range 350-300nm (with values > 40% at 320-300nm). Electrochemical Impedance Spectroscopy (EIS) analysis shows similar electron transport and charge transfer properties for NTs grown in low and high water contents. EIS is mainly sensitive to the bottom of the NTs, whereas the NTs wall is not active at potentials more positive than the flat band potential. EIS also reveals that the poorer photoelectrochemical properties of NTs grown in 2wt.% H2O are due to a larger density of deep intra band-gap energy states in comparison with 12wt.% H2O.
The polymer-to-ceramic transformation of a polysilazane/divinylbenzene aerogel leading to SiCN aerogel was studied. The pre-ceramic samples were obtained by crosslinking a commercially available ...polysilazane with divinylbenzene in a highly diluted solution. Wet gels were supercritically dried using CO2 to get the pre-ceramic aerogel. The weight change during pyrolysis in flowing argon was studied by thermogravimetric analysis (in-situ measurements) and by measuring the weight change on samples pyrolyzed in a tubular furnace after cooling down to room temperature (ex-situ measurements). The structural transformation was followed by infrared spectroscopy while the microstructural changes were studied by nitrogen adsorption analysis. Results point out that samples pyrolyzed at intermediate temperature, i.e., around 600–800°C, react with the laboratory atmosphere forming SiO bonds and SiOH moieties. This out-of-furnace oxidation leads to an uncontrolled increase of the oxygen content of the pyrolyzed ceramics and eventually reduces the microporosity of the samples and its stability.
Polymer derived silicon oxycarbide (SiOC) ceramics are investigated as potential anodes for lithium ion batteries. Different SiOC ceramics are prepared by pyrolysis (1000°C and 1400°C under ...controlled argon atmosphere) of polysiloxanes ceramic precursors. Preceramic polymers are synthesized using the sol–gel method. Phenyltriethoxysilane (PhTES) and methyltriethoxysilane (MTES) have been used as starting precursors and mixed with different ratios in order to tailor the chemical composition and the structure of the final product. The obtained SiOC ceramics are amorphous with various content of free carbon phase (from approx. 25 to 40wt.%). The presence of disordered carbons in the ceramic structure is confirmed by the appearance of a well pronounced D band at 1330cm−1 in the Raman spectra. Additionally, 29Si MAS-NMR spectra show the presence, in the structure of the materials pyrolysed at 1000°C, of mixed bond tetrahedra such as: SiO3C, SiO2C2, SiOC3 and SiO4 units. Pyrolysis at an elevated temperature (1400°C) promotes the phase separation into oxygen rich (SiO4) and carbon rich (SiC4) units with consumption of mixed bonds. Carbon rich SiOC samples exhibit significant reversible capacity and enhanced cycling stability (up to 600mAhg−1 measured at a slow current rate of C/20 after 140cycles of continuous charging–discharging with increasing current density). However, the high irreversible capacity of the first few cycles remains an issue to be solved.
•New silicon oxycarbide SiOC materials were studied as anodes for Li-ion batteries.•The chemical composition and the structure influence the electrochemical activity.•The SiOC samples exhibit a high recovered capacity and long-life stability.•The best electrochemical performance was achieved for carbon-rich samples.
We report the first synthesis of silicon nanocrystals embedded in a silicon nitride matrix through a direct pyrolysis of a preceramic polymer (perhydropolysilazane). Structural analysis carried out ...by XRD, XPS, Raman and TEM reveals the formation of silicon quantum dots and correlates the microstructures with the annealing temperature. The photoluminescence of the nanocomposites was investigated by both linear and nonlinear measurements. Furthermore we demonstrate an enhanced chemical resistance of the nitride matrix, compared to the typical oxide one, in both strongly acidic and basic environments. The proposed synthesis via polymer pyrolysis is a striking innovation potentially allowing a mass-scale production nitride embedded Si nanocrystals.
Silicon was selectively removed from a silicon carbonitride (SiCN) aerogel by hot chlorine gas treatment, leading to a N-doped carbon aerogel (N-CDC aerogel). The combined effects of pyrolysis and ...etching temperature were studied with regard to the change in the composition of the material after etching as well as the microstructure of the produced hierarchically porous material. Upon removal of Si from amorphous SiCN, carbon and nitrogen, which are not bonded together in the starting material, react, creating new C-N bonds. The removal of silicon also gives rise to a high amount of micropores and hence a high specific surface area, which can be beneficial for the functionality of the carbonaceous material produced. The mesoporous structure of the aerogel allows us to complete the etching at low temperature, which was found to be a crucial parameter to maintain a high amount of nitrogen in the material. The combination of a high amount of micropores and the mesopore transport system is beneficial for adsorption processes due to the combination of a high amount of adsorption sites and effective transport properties of the material. The N-CDC aerogels were characterized by nitrogen physisorption, X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG/DTA), and infrared spectroscopy (DRIFT) and they were evaluated as CO
2
absorbers and as electrodes for electric double-layer capacitors (EDLCs).
Silicon was selectively removed from a silicon carbonitride (SiCN) aerogel by hot chlorine gas treatment, leading to a N-doped carbon aerogel (N-CDC aerogel) possessing high specific surface area and hierarchical pore structure.
Three silicon oxycarbide glasses (SiCO) with increasing C content were obtained through pyrolysis in inert atmosphere at 1000 °C of sol−gel derived siloxane networks containing Si−CH3 and Si−H bonds. ...The glasses were further annealed at 1200, 1400, and 1500 °C to follow their evolution at high temperature. Quantitative information concerning the structure of glasses before and after annealing at high temperature was collected with a wide range of techniques (some of them used for the first time in this field) with the aim of probing the following: (i) the short-range order and chemical composition (29Si and 1H MAS NMR, RDF derived from X-ray and neutron scattering, inelastic neutron scattering, FT-IR, and elemental analysis), and (ii) the long-range order (X-ray and neutron diffraction) and microstructural features (HR-TEM combined with electron diffraction, Raman, porosity, and surface area measurements). This extensive collection of data, carried out on the same set of specimens, provided detailed and sound structural information on nearly-stoichiometric SiCO glasses and their high-temperature behavior.
•Polymer-derived SiOC ceramics are studied as anode in Li-ion batteries.•Li insertion capacity and cycling stability are correlated to the structure of SiOC.•First insertion capacity of SiCxO2(1-x) ...network goes up to 1300 mAhg−1.•The specific capacity of the C-poor SiOC degrades faster than the C-rich SiOC.
Polymer derived silicon oxycarbide (SiOC) materials are prepared by the pyrolysis of preceramic polymers obtained from polyhydridomethylsiloxane using 1,3,5,7-tetramethyl1,3,5,7-tetravinyl cyclotetrasiloxane or divinyl benzene as a cross-linking agent. The pyrolysis is carried out in an inert atmosphere at 1000 and 1300°C. The carbon content of SiOC is varied by changing the amount of starting precursors maintaining the same O/Si atomic ratio of about 1. Electrochemical measurements are performed in order to evaluate the materials in terms of their application as anodes in Li-ion batteries. Detailed structural characterization study is performed using complementary techniques with the aim of correlating the electrochemical behavior with the structure of the SiOC anodes. Results suggest that SiOC anodes behave as a composite material consisting of a disordered silicon oxycarbide phase having a very high first insertion capacity of ca 1300 mAh g−1 and a free C phase. However, the charge irreversible trapped into the amorphous silicon oxycarbide network is also high and therefore the maximum reversible lithium storage capacity of 650mAh g−1 is measured on high-C content SiOCs for which the balance between the two phases, namely the amorphous silicon oxycarbide and the free C phase, is optimal. The high carbon content SiOC show also an excellent cycling stability and performance at high charging/discharging rate: the reversible capacity at 2C rate being around 200 mAh g−1. Increasing the pyrolysis temperature has an opposite effect on the low-C and high-C materials: for the latter one the reversible capacity decreases following a known trend while the former shows an increase of the reversible capacity which has never been observed before for similar materials.
A new approach to obtain visible luminescence from sol–gel‐derived SiOC films is proposed. In order to investigate the influence of Si and C content, Si‐ and C‐rich SiOC films were prepared as well ...as stoichiometric SiOC films. High intense white luminescence is obtained where the emission color can be controlled by the experimental parameters such as the starting sol–gel‐composition and the pyrolysis temperature. In stoichiometric SiOC films, low pyrolysis temperature yields UV‐blue luminescence, whereas high temperatures favor green‐yellow luminescence. On the contrary, in Si‐rich SiOC films, intense white luminescence is obtained with a broad emission from 430 to 900 nm and an external quantum efficiency (EQE) of 11.5%. Relatively, stoichiometric SiOC films showed an EQE of 5%. Finally, C‐rich SiOC films did not show noticeable luminescence due to absorption by carbon clusters.
•SiOC ceramic aerogels are synthesized by pyrolysis of low-density preceramic aerogels.•SiOC ceramic aerogels are studied as anode material for faster charging and discharging.•The porous nature ...allows fast ionic transport.•At a rate of 10C (3600mAg−1) the specific capacities as high as 200mAhg−1 are recovered.
Porous carbon-rich SiOC ceramic aerogels have been synthesized from a linear polysiloxane cross-linked with divinylbenzene (DVB) via hydrosilylation reaction in presence of a Pt catalyst and acetone as a solvent. The obtained wet gels are aged in solvent followed by drying under supercritical conditions using liquid carbon dioxide. The resulting pre-ceramic aerogels are subjected to pyrolysis at 1000°C under controlled argon atmosphere to form the desired SiOC aerogel. The synthesized SiOC ceramics contain 43wt% of free carbon, which is segregated within amorphous SiOC matrix. The high BET surface area up to 230m2g−1 of preceramic aerogels is only slightly diminished to 180m2g−1 after pyrolysis at 1000°C. The electrochemical characterization reveals a high specific capacity of more than 600mAhg−1 at a charging rate of C (360mAg−1) along with a good cycling stability. At a rate of 10C (3600mAg−1) the specific capacities as high as 200mAhg−1 are recovered. The excellent properties of the materials are discussed with respect to their structural features. The porous nature of the carbon rich ceramics allows for fast ionic transport and helps to accommodate the structural changes which in turn allow a stable performance during repeated lithiation/delithiation.
This manuscript reports on the photoelectrochemical properties of anodized 0.1 and 0.15 at% Nb-doped NTs prepared using electrolytes containing different amount of water (2 to 12 wt%). It is shown ...that Nb-doped NTs perform better than undoped NTs, achieving a photocurrent density up to 1.35 mA/cm2 at 1 V vs RHE when the water content is sufficiently high (12 wt%). The IPCE also confirms the superiority of Nb-doped NTs, again the larger values (55% IPCE at 300 nm) are obtained when using 12 wt% H2O in the electrolyte. EIS analysis and capacitance measurements highlight the peculiarity of the Nb-doped TiO2 NTs which behave differently than undoped NTs. The electron transport resistance in Nb-doped NTs is similar (103 Ω cm2) to the one of undoped NTs. However, Nb-doped NTs exhibit larger electron mobility and require lower activation energy for electron transport (i.e. a less negative bias) than undoped NTs. Furthermore, Nb-doping, combined with an optimal water content of 12 wt%, is found to be an effective strategy to fully suppress the formation of deep intra band-gap energy states. The presence of Nb in the TiO2 lattice dictates the fate and the energy of defects introduced during anodization and annealing. These defects are shallower than those observed in undoped NTs.
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