Carbon xerogels (CXs) are materials obtained via the pyrolysis of resins prepared via the sol–gel polycondensation of resorcinol and formaldehyde. These materials attract great attention as ...adsorbents, catalyst supports, and energy storage materials. One of the most interesting features of CXs is the possibility of fine-tuning their structures and textures by changing the synthesis conditions in the sol–gel stage. Thus, the first part of this review is devoted to the processes taking place in the polycondensation stage of organic precursors. The formation of hydroxymethyl derivatives of resorcinol and their polycondensation take place at this stage. Both of these processes are catalyzed by acids or bases. It is revealed that the sol–gel synthesis conditions, such as pH, the formaldehyde/resorcinol ratio, concentration, and the type of basic modifier, all affect the texture of the materials being prepared. The variation in these parameters allows one to obtain CXs with pore sizes ranging from 2–3 nm to 100–200 nm. The possibility of using other precursors for the preparation of organic aerogels is examined as well. For instance, if phenol is used instead of resorcinol, the capabilities of the sol–gel method become rather limited. At the same time, other phenolic compounds can be applied with great efficiency. The methods of gel drying and the pyrolysis conditions are also reviewed. Another important aspect analyzed within this review is the surface modification of CXs by introducing various functional groups and heteroatoms. It is shown that compounds containing nitrogen, sulfur, boron, or phosphorus can be introduced at the polycondensation stage to incorporate these elements into the gel structure. Thus, the highest surface amount of nitrogen (6–11 at%) was achieved in the case of the polycondensation of formaldehyde with melamine and hydroxyaniline. Finally, the methods of preparing metal-doped CXs are overviewed. Special attention is paid to the introduction of a metal precursor in the gelation step. The elements of the iron subgroup (Fe, Ni, Co) were found to catalyze carbon graphitization. Therefore, their introduction can be useful for enhancing the electrochemical properties of CXs. However, since the metal surface is often covered by carbon, such materials are poorly applicable to conventional catalytic processes. In summary, the applications of CXs and metal-doped CXs are briefly mentioned. Among the promising application areas, Li-ion batteries, supercapacitors, fuel cells, and adsorbents are of special interest.
Nanoporous anatase thin films (see Figure) have been prepared via particulate sol–gel processes. The proton conductivity shows a sigmoidal dependence on relative humidity (RH). The highest value of ...proton conductivity was 3.78 × 10–2 S cm–1 at 80 °C and 81% RH. The comparable proton conductivity, lower cost, and higher hydrophilicity of these thin films make them potential substitutes for Nafion membranes in proton‐exchange membrane fuel cells.
•First time synthesis of pure phase La1-xLnxCr0.5Co0.5O3 (x = 0 and 0.2 &Ln = Pr, Sm and Gd).•Transition from rhombohedral R-3c for x = 0 to orthorhombic Pbnm for x = 0.2.•Highly crystalline faceted ...particles and chemical homogeneity.•Canted antiferromagnetic transition from TN ~ 124 K to 133 K for all the samples.•Possible 3d-4f interaction in La0.8Pr0.2Cr0.5Co0.5O3 and La0.8Sm0.2Cr0.5Co0.5O3.•The intermediate-spin state of Co3+ ions for all the samples.
We report the sol–gel synthesis of La1-xLnxCr0.5Co0.5O3 (x = 0 and 0.2 &Ln = Pr, Sm and Gd) perovskites. The samples were characterized using powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) and magnetization measurements. It is worth mentioning that all the samples in this study are found to be monophasic as attested from the PXRD patterns in sharp comparison with the previously reported work on LaCr0.5Co0.5O3 Solanki et al. J. Magn. Magn. Mater. 469 (2019) 95–99.. This could be related to the slight modifications in synthesis condition like the metal to citric acid ratio, drying as well decomposition temperature of gel, final calcination temperature and time. The parent phase (x = 0) crystallizes in rhombohedral, R-3c, structure whereas the Ln-doped samples are orthorhombic, Pbnm. FE-SEM imaging revealed the irregular shape and large variation in particle size. The particles are highly agglomerated but show faceted surface irrespective of shape and size. The EDX analysis confirms the nominal cationic composition of the samples. The magnetic measurements suggest the onset of antiferromagnetic ordering in all the samples with TN ranging from 130 K to 124 K depending on the size of the Ln-dopant ion. The high temperature effective paramagnetic moments obtained from Curie-Weiss law for all the samples suggest that the most likely spin-state of cobalt is intermediate-spin state. The low temperature magnetic behaviour of the substituted samples could be related to the rare earth-transition metal interaction. The magnetic ground states can be attributed to the Cr3+–O–Co3+, Cr3+–O–Cr3+ and Co3+–O–Co3+ antiferromagnetic superexchange interactions between the randomly distributed Cr3+ and Co3+ ions.
We have characterised the Lewis acidity of unsaturated surface cations of MgF2 crystals using periodic calculations at the B3LYP level. The relative importance of low index surfaces was determined by ...calculating surface energies, and these surfaces were then probed by CO adsorption. We found that a MgF2 microcrystal should expose mainly (110), (100) and (101) surfaces in which the undercoordinated cations are fivefold coordinated. The adsorption energies and CO stretching frequencies are discussed with respect to the coordination number of surface cations and the stability of the surfaces and are compared to IR spectra from the literature.
► DFT calculations on the Lewis acidity of unsaturated surface cations of MgF2 crystals ► MgF2 exposes mainly (110), (100) and (101) surfaces with only 5-fold coordinated cations. ► Surfaces that contain also 4-fold coordinated cations – (001) and (111) – are less stable. ► Lewis acidity of 5- and 4-fold coordinated cations is probed by CO adsorption. ► Calculated frequencies give insights into the complex IR pattern of high-surface MgF2.
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The thin films of calcium hydroxyapatite were deposited on a stainless steel substrate modified with titanium nitride (TiN) using an aqueous sol–gel method and characterized by X-ray ...diffraction analysis. It was demonstrated for the first time that the formation of calcium hydroxyapatite during heat treatment inhibited the formation of Fe2O3 and promoted the formation of TiO2 on the surface.
By using TiO2 and Ta2O5 colloids, a stable and efficient visible-light driven photocatalyst, Er3+:Y3Al5O12/TiO2Ta2O5/MoO2 composite membrane, was successfully prepared via sol–gel dip coating method ...at room temperature. The XRD, FTIR, SEM, TEM and EDX results confirm that approximately spherical Er3+:Y3Al5O12 nanoparticles were embedded in TiO2Ta2O5 matrix. UV–vis absorption and PL spectra of Er3+:Y3Al5O12 were also determined to confirm the visible absorption and ultraviolet emission. The photocatalytic hydrogen generation was carried out by using methanol as sacrificial reagent in aqueous solution under visible-light irradiation. Furthermore, some main influence factors such as heat-treated temperature, heat-treated time and molar ratio of TiO2 and Ta2O5 on visible-light photocatalytic hydrogen generation activity of Er3+:Y3Al5O12/TiO2Ta2O5/MoO2 composite membrane were studied in detail. The experimental results showed that the photocatalytic hydrogen generation activity of Er3+:Y3Al5O12/TiO2Ta2O5/MoO2 composite membrane heat-treated at 550 °C for 3.0 h was highest when the molar ratio of TiO2 and Ta2O5 was adopted as 1.00:0.50. And that a high level photocatalytic activity can be still maintained after four cycles. In addition, a possible mechanism for the visible-light photocatalytic hydrogen generation of the Er3+:Y3Al5O12/TiO2Ta2O5/MoO2 membrane was proposed based on PL spectra.
•Er:YAG/TiO2Ta2O5/MoO2 membrane was prepared as visible-light driven photocatalyst.•Er:YAG/TiO2Ta2O5/MoO2 can effectively perform photocatalytic H2 evolution.•Presence of Er:YAG can obviously expand the light response range of TiO2Ta2O5.•Use of MoO2 can be conducive to separation of photo-generated electron and hole.
The synthesized TiN (s-TiN) with superior catalytic activity was fabricated through a simple, low-cost, and non-toxic process as a potential material of waste water treatment. The photocatalytic ...performance of s-TiN was estimated by chemical reduction of Cr(VI) and photo degradation of RhB with existence of visible light. For s-TiN, the removal rate of Cr(VI) was 98% within 1 minute and the photo degradation of RhB reached to 99.7% within 4 minutes. However, for the purchased TiN (p-TiN), the removal rate of Cr(VI) was 17% and the degradation rate of RhB was only 1%. The excellent photocatalytic activity is owing to the increased molecular adsorption, visible light absorption and effective charge separation. Furthermore, another interesting discovery is that the metal Zinc was produced along with TiN synthesis on the inside crucible cover simultaneously.
Highly sensitive to methane semiconductor sensors were obtained with the use of nanosized tin dioxide synthesized via sol-gel method. Sensitization of the sensors was achieved by introduction of Pt ...additives into semiconductor gas sensitive materials. Experimental data of dependences of conductivities of the created sensors on methane concentrations were used for approximation by a mathematical model of the sensor conductivity and response formation. The model is based on features of a methane oxidation reaction on the surface of the gas sensitive material and allows to connect conductivity of the sensor with surface processes occurred on its gas sensitive layer.
Nanosized initial semiconductor tin dioxide based material was synthesized by a sol-gel technique and characterized by XRD, IR-spectroscopy, DTA-DTG and TEM methods. Nanosized SnO
2
-based sensor ...materials were prepared using the pastes with different quantities of tin dioxide and carboxymethyl cellulose and formatted at various temperatures. It was found that the compositions of the pastes significantly affect the characteristics of the sensors obtained on their base. The characteristics of the sensors of different compositions were explained by necessity of the presence of sufficient number of contacts between the nanoparticles of the sensor material, which ensure the electrical conductivities of the sensors.
