Advances in Microwave Synthesis of Nanoporous Materials

Usually, porous materials are synthesized by using conventional electric heating, which can be energy- and time-consuming. Microwave heating is commonly used in many households to quickly heat food. Microwave ovens can also be used as powerful devices in the synthesis of various porous materials. The microwave-assisted synthesis offers a simple, fast, efficient, and economic way to obtain many of the advanced nanomaterials. This review summarizes the recent achievements in the microwave-assisted synthesis of diverse groups of nanoporous materials including silicas, carbons, metal-organic frameworks, and metal oxides. Microwave-assisted methods afford highly porous materials with high specific surface areas (SSAs), e.g., activated carbons with SSA ≈3100 m² g^-1 , metal-organic frameworks with SSA ≈4200 m² g^-1 , covalent organic frameworks with SSA ≈2900 m² g^-1 , and metal oxides with relatively small SSA ≈300 m² g^-1 . These methods are also successfully implemented for the preparation of ordered mesoporous silicas and carbons as well as spherically shaped nanomaterials. Most of the nanoporous materials obtained under microwave irradiation show potential applications in gas adsorption, water treatment, catalysis, energy storage, and drug delivery, among others.

MOF-derived Co2+-doped TiO2 nanoparticles as photoanodes for dye-sensitized solar cells

Facile synthesis and application of nano-sized semiconductor metal oxides for optoelectronic devices have always affected fabrication challenges since it involves multi-step synthesis processes. In this regard, semiconductor oxides derived directly from metal-organic frameworks (MOFs) routes have gained a great deal of scientific interest owing to their high specific surface area, regular and tunable pore structures. Exploring the application potential of these MOF-derived semiconductor oxides systems for clean energy conversion and storage devices is currently a hot topic of research. In this study, titanium-based MIL-125(Ti) MOFs were used as a precursor to synthesize cobalt-doped TiO2-based dye-sensitized solar cells (DSSCs) for the first time. The thermal decomposition of the MOF precursor under an air atmosphere at 400 °C resulted in mesoporous anatase-type TiO2 nanoparticles (NPs) of uniform morphology, large surface area with narrow pore distribution. The Co2+ doping in TiO2 leads to enhanced light absorption in the visible region. When used as photoanode in DSSCs, a good power conversion efficiency (PCE) of 6.86% with good photocurrent density (Jsc) of 13.96 mA cm-2 was obtained with the lowest recombination resistance and the longest electron lifetime, which is better than the performance of the pristine TiO2-based photoanode.

Self-Assembly Synthesis of the MoS2/PtCo Alloy Counter Electrodes for High-Efficiency and Stable Low-Cost Dye-Sensitized Solar Cells

In this work, MoS₂ microspheres/PtCo-alloy nanoparticles (MoS₂/PtCo-alloy NPs) were composited via a novel and facile process which MoS₂ is functionalized by poly (N-vinyl-2-pyrrolidone) (PVP) and self-assembled with PtCo-alloy NPs. This new composite shows excellent electrocatalytic activity and great potential for dye-sensitized solar cells (DSSCs) as a counter electrode (CE) material. Benefiting from heterostructure and synergistic effects, the MoS₂/PtCo-alloy NPs exhibit high electrocatalytic activity, low charge-transfer resistance and stability in the cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) test. Meanwhile, a high power-conversion efficiency (PCE) of 8.46% is achieved in DSSCs with MoS₂/PtCo-alloy NP CEs, which are comparable to traditional Pt CEs (8.45%). This novel composite provides a new high-performance, stable and cheap choice for CEs in DSSCs.

Microwave-assisted hydrometallurgical extraction of Li4Ti5O12 and LiFePO4 from ilmenite: effect of PPy-Br2 derived C-coating with N, Br, and Nb5+ Co-doping on electrodes for high-rate energy storage performance

Microwave-driven hydrometallurgical (MW-HM) method has been adopted to extract spinel-type Li4Ti5O12 (LTO) and orthorhombic-type LiFePO4 (LFP) from naturally occurring Ilmenite (FeTiO3) within 2 h unlike the conventional process that requires >30 h. We have successfully demonstrated aliovalent-Nb5+ doping and carbon coating with N, Br co-doping upon the pyrolysis of a polypyrrole-(PPy)-Br2 charge-transfer-(CT)-complex via MW-hydrothermal and MW-solid state heating within 30 min. Further, we also investigated the effect of carbon coating and co-doping of LTO and LFP electrodes at high C-rate performance of the lithium battery. XRD, XPS, FTIR, and Raman spectroscopy results confirmed the co-existence of dual-phase Li4Ti5O12/rutile-TiO2 (LTO-RTO) with substitution-induced transition of Ti4+ → Ti3+ in spinel-LTO due to Nb5+ and N, Br co-doping, which facilitates fast Li+ ion and electron transfer at the electrode-electrolyte interface. Conversely, in situ Nb5+ doped LiFePO4 combined with ex situ carbon-coating with N, Br co-doping improved the overall electronic conductive behavior. The UV-Visible absorption spectra and Tauc plots further support the decrease in the band gap upon co-doping, thus promoting n-type electronic behavior of the electrodes. A significant enhancement in the discharge-capacities of the carbon-coated N, Br co-doped Li4Ti4.97 Nb0.03O12/rutile-TiO2 (NBC-LTNO-RTO) and pristine anode in the range of 174-148 mA h g-1 and 167-125 mA h g-1 was exhibited at different rates in the range of 0.2 C-20 C with 97% and 94% capacity retention, respectively. Instead, the carbon-coated N, Br co-doped LiFe0.99 Nb0.01PO4 (NBC-LFNP) and pristine cathode exhibited discharge capacities in the range of 169-73 mA h g-1 and 136-65 mA h g-1 at different rates in the range of 0.2 C-20 C with 92% and 40% capacity retention for 500 cycles, respectively. Hence, this innovative, rapid, and sustainable chemical process for the fabrication of the modified cathode and anode from the earth abundant ilmenite ore as the single source with high-rate capability and ultra-stability can be used for high-power and safer lithium-ion energy storage.

Microwave-Assisted Synthesis of High-Energy Faceted TiO2 Nanocrystals Derived from Exfoliated Porous Metatitanic Acid Nanosheets with Improved Photocatalytic and Photovoltaic Performance

A facile one-pot microwave-assisted hydrothermal synthesis of rutile TiO₂ quadrangular prisms with dominant {110} facets, anatase TiO₂ nanorods and square nanoprisms with co-exposed {101}/[111] facets, anatase TiO₂ nanorhombuses with co-exposed {101}/{010} facets, and anatase TiO₂ nanospindles with dominant {010} facets were reported through the use of exfoliated porous metatitanic acid nanosheets as a precursor. The nanostructures and the formation reaction mechanism of the obtained rutile and anatase TiO₂ nanocrystals from the delaminated nanosheets were investigated. The transformation from the exfoliated metatitanic nanosheets with distorted hexagonal cavities to TiO₂ nanocrystals involved a dissolution reaction of the nanosheets, nucleation of the primary [TiO₆]⁸⁻ monomers, and the growth of rutile-type and anatase-type TiO₂ nuclei during the microwave-assisted hydrothermal reaction. In addition, the photocatalytic activities of the as-prepared anatase nanocrystals were evaluated through the photocatalytic degradation of typical carcinogenic and mutagenic methyl orange (MO) under UV-light irradiation at a normal temperature and pressure. Furthermore, the dye-sensitized solar cell (DSSC) performance of the synthesized anatase TiO₂ nanocrystals with various morphologies and crystal facets was also characterized. The {101}/[111]-faceted pH2.5-T175 nanocrystal showed the highest photocatalytic and photovoltaic performance compared to the other TiO₂ samples, which could be attributed mainly to its minimum particle size and maximum specific surface area.