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Metal–organic frameworks (MOFs), as a new generation of intrinsically porous extended crystalline materials formed by coordination bonding between the organic ligands and metal ions or clusters, have attracted considerable interest in many applications owing to their high porosity, diverse structures, and controllable chemical structure. Recently, 2D transition-metal- (TM-) based MOFs have become a hot topic in this field because of their high aspect ratio derived from their large lateral size and small thickness, as well as the advantages of MOFs. Moreover, 2D TM-based MOFs can act as good precursors to construct heterostructures with high electrical conductivity and abundant active sites for a range of applications. This review comprehensively introduces the widely adopted synthesis strategies of 2D TM-based MOFs and their composites/derivatives. In addition, this paper summarizes and highlights the recent advances in energy conversion and storage, including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO2 reduction reaction, urea oxidation reaction, batteries, and supercapacitors. Finally, the challenges in developing these intriguing 2D layered materials and their composites/derivatives are examined, and the possible proposals for future directions to enhance the energy conversion and storage performance are reviewed.

Strategies to decrease the dehydrogenation enthalpy (ΔHd) of dibenzyl toluene (DBT) were examined by density functional theory (DFT) modeling. The stronger electron-donating substituent showed higher hydrogen-releasing properties. The sequences of the dehydrogenation process of perhydro-dibenzyl toluene (18H-DBT) and perhydro-lithium 3,5-dibenzyl phenolate (18H-DBT-OLi), which is the compound of modified DBT with the highest potential, were the same. The energy required to release hydrogen from 18H-DBT-OLi (11.514 kcal/mol) was smaller than that from 18H-DBT (12.574 kcal/mol). In the hydrogen-releasing process, the rate-determining steps for the dehydrogenation of 18H-DBT and 18H-DBT-OLi were the 12H-DBT → 10H-DBT + H2 and 12H-DBT-OLi → 10H-DBT-OLi + H2 steps, respectively. Furthermore, the charge distribution of 18H-DBT and 18H-DBT-OLi was also explored.

A transition metal (Zn) and alkaline earth metal (Ca)-hybridized metal-organic framework (MOF) based electrocatalyst was synthesized using a one-pot process. The as-prepared ZnCa-MOF74 (ZC) catalyst exhibited a low overpotential, an excellent CO selectivity, and high stability in the electrochemical reduction of CO2 (ERC). The Faradaic efficiency of ZC towards CO was as high as 93%, which is two times higher than that of the Zn-MOF74 catalyst. The computational simulation also showed that Ca lowered the activation barrier of the rate-determining step during the ERC.

Functionalization of aniline molecules on zinc oxide (ZnO) nanoparticles is reported using a simple impregnation technique. Functionalized ZnO samples were systematically characterized based on morphology, surface and optical properties, and photocatalytic performance towards methyl orange (MO). Aniline functionalization increased the surface charge of the modified ZnO. Compared to pristine ZnO, the aniline-functionalized ZnO yielded faster photodegradation of MO, degrading 98.29 % of MO in 60 min and superoxide radicals were the major species during the MO photodegradation reaction. These results indicate that the improvement of photocatalytic degradation could be attributed to opposite charge-induced surface adsorption. Hence, protonated amine as a positively charged molecule was expected to increase the surface adsorption of MO (as an anionic dye) on ZnO nanoparticles surfaces, thereby increasing their photocatalytic degradation performance.

Nitrogen-doped carbon dots (CDts) with tunable fluorescence properties in aqueous media were synthesized hydrothermally. The excitation wavelength variation to obtain the maximum emission produced a blue shift in the emission peaks upon dilution in an aqueous solution. The shift can be explained by a re-absorption phenomenon in a concentrated solution. The interparticle interaction within was responsible to show dilution-dependent optical behavior. The as-synthesized solution of CDts did not show any prominent absorption peak over a wide range. However, upon dilution, two peaks became predominant. The concentration-dependent behavior was observed during the interaction with metal cations. Cationic salts of Co(II) and Hg(II) caused quenching at different dilutions of CDts. This might be explained by the exposure of different surface functional groups during dilution and metal-ion–CDts charge transfer. The quenched fluorescence of CDts was rescued using ascorbic acid. Therefore, the one-pot detection of Co(II)/Hg(II) and ascorbic acid was designed through a ‘Turn Off/On’ phenomenon.

Carbon dots (CDs) have unique optoelectronic properties and are considered efficient fluorescent probes as organic dyes, semiconducting quantum dots, metallic clusters, etc. A carbon dot-dye system has been established to study alternative photoelectric conversion processes. For this purpose, a CuO-associated CD was synthesized using a green chemical method. The energy conversion efficiency was monitored using a cationic, anionic, and neutral dye. The nature of the dye has an important effect on the energy transfer process. This limitation from its electrostatic nature was overcome by introducing a surfactant into the system. Both positively and negatively charged surfactants were used in this study. The nature of the surfactant was found to have a significant effect on the energy transfer process within positively charged CDs and organic dyes. The individual interactions within surfactant-CD and surfactant-dye system can cause modification. This CD-dye-surfactant system with further modifications can be used appropriately in dye-sensitized solar cells.

A multi-functional NiO/g-C3N4 (NC) hybrid nanostructure was synthesized by a hydrothermal process using melamine and Ni(OH)2 as precursors followed by thermal treatment. The optimal conditions were determined by studying the process conditions, such as the Ni(OH)2 to melamine ratio and thermal treatment temperature. The NC prepared in this study exhibited both excellent glucose sensing properties and supercapacitor properties. A very high glucose sensitivity, as high as 5,387.1 µA mM−1cm−2, and excellent energy density of 49.6 Wh kg−1 at a power density of 1,064.2 W kg−1 were obtained when NC was used as the electrode material for glucose sensing and symmetric supercapacitor, respectively. A flexible glucose sensing device using a flexible substrate and self-powered glucose sensor system that used the same material (NC) for the both power supply and sensing devices were also demonstrated.

Low-cost nitrogen and boron-doped carbon nanodots (CPAP-CDs) with a high quantum yield (64.07%) were synthesized through a facile hydrothermal treatment. The obtained CPAP-CDs exhibited wide absorption, strong fluorescence, and pH-dependent behavior. The high fluorescence of CPAP-CDs was quenching in the presence of the nitrite ion in a concentration-dependent manner. The detection limit was as low as 6.6 nM with a wide linear detection range of 2 μM - 1 mM. Diazotization between the NO

This paper reports a highly sensitive photoluminescence glucose sensor based on aminoboronic acid-functionalized carbon nitride quantum dots (g-CNQDs/3APBA) fabricated using melamine and 3-aminophenylboronic acid via a facile two-step synthesis process. By introducing the covalent bonds between g-CNQDs and boronic acid groups, it can be effectively used as “on-off-on” based multi-chemical sensor. The g-CNQDs/3APBA exhibited quantum yields (QYs) as high as 78.5%, which is the highest QYs among fluorescence sensors based on g-CNQDs reported thus far. The material showed a wide linear range of 0–10 mM and a detection limit as low as 42 nM with excellent selectivity. In addition, it exhibited comparable performance compared to those of a commercial glucometer in a real blood test. Owing to the excellent bio-imaging properties and low cytotoxicity, g-CNQDs/3APBA is a promising candidate as a sensing material for biomedical and clinical applications.

Highly fluorescent N-doped carbon dots (NCDs) were fabricated using pyromellitic acid (PA) and ethylene diamine (EDA) as novel precursors. The NCDs exhibited excellent fluorescent properties with a quantum yield as high as 75% due to the enhancement effect of the N dopant on the surface of the NCDs. Moreover, owing to its strong cyan fluorescence, the newly synthesized NCDs were applied effectively as a fluorescence platform for the sensitive and selective determination of 4-nitrophenol (4-NP) without further chemical modification. The NCDs fabricated in this study exhibited a wide linear range, low detection limit, and excellent selectivity towards other organic compounds.

Herein, we report a simple hydrothermal approach for the simultaneous reduction and functionalization of graphene oxide (GO) employing three different types of alkanolamine(s)/alkyl alkanolamine (monoethanolamine (MEA), triethanolamine (TEA) and N,N-diethylethanolamine (DEEA) without using any external reductant/surfactant. The simultaneous reduction and functionalization of GO was evidenced by Fourier-transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses. Interestingly, the simultaneous reduction and functionalization of MEA, TEA and DEEA molecules on GO resulted in stabilized reduced graphene oxide (rGO) structures with enhanced surface area for rGO-MEA (967 m2/g), rGO-TEA (881 m2/g) and rGO-DEEA (430 m2/g) compared to that of non-functionalized rGO (376 m2/g). Furthermore, the differences in the capacitive performances of functionalized rGO(s) have been investigated in terms of different surface chemistry, surface area, graphitic character and conductivity. Among all the as-synthesized functionalized rGO(s), rGO-TEA exhibited improved specific capacitance (827 F/g at 1 A/g), long cycling stability, high coulombic efficiency and high energy/power density, demonstrating their potential for supercapacitor applications.

This paper presents the first report of the excellent bi-functional electro-catalytic activity and durability of a structure that consists of a highly active inner core of a ruthenium metal organic framework with a mesoporous silica based shell covered with templated CdS nanoparticle electro-catalysts and reduced graphene oxide nanosheets. Excellent electro-catalytic activity toward both the oxygen reduction and oxygen evolution reactions is achieved by selecting a highly active core-shell of silica with CdS and rGO nanosheets as co-catalysts. In particular, the excellent catalytic activity of the 2 wt. % rGO/CdS/ silica nanocapsule catalyst for both reactions can be attributed to the large surface area and mesoporous nature, because they facilitate facile electrochemical activity, and lower the overpotentials and smaller Tafel values than other catalysts. In addition, it is more tolerant to methanol fuel crossover than commercial Pt/C, and exhibits long-term durability.

Highly biocompatible phenylboronic acid-functionalized graphitic carbon nitride quantum dots (g-CNQDs/PBA) were prepared by a simple hydrothermal process and used as a glucose sensing fluorescence material. Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy were used to characterize chemical and physical structures of g-CNQDs/PBA. The quantum yield (QY) of g-CNQDs/PBA was as high as 67% when measured using quinine sulfate as a standard, which is the highest QY value ever reported for fluorescent g-CNQDs. The g-CNQDs/PBA exhibited two wide linear regions in the range of 25 nM – 1 μM and 1 μM – 1 mM with a detection limit as low as 16 nM. In addition, it showed excellent selectivity in the presence of various interfering reagents as well as very low cellular toxicity and excellent bio-imaging properties. Overall, the g-CNQDs/PBA fabricated in this study can be a promising candidate for a range of clinical diagnostics and biomedical applications.

This comprehensive review focuses on recent advances in energy harvesting of micro-scale and nano-scale generators based on piezoelectric and triboelectric effects. The development of flexible and hybrid devices for a variety of energy harvesting applications are systematically reviewed. A fundamental understanding of the important parameters that determine the performance of piezoelectric, triboelectric and hybrid devices are summarized. Current research directions being explored and the emerging factors to improve harvester functionality and advance progress in achieving high performance and durable energy conversion are provided. Investigations with regard to integrating flexible matrices and optimizing the composition of the piezoelectric and triboelectric materials are examined to enhance device performance and improve cost-effectiveness for the commercial arena. Finally, future research trends, emerging device structures and novel materials in view of imminent developments and harvesting applications are presented.

Graphene, a two-dimensional (2D) carbon nanomaterial, has received great attention owing to its advantageous characteristics, including superior electrical, chemical and physical properties. In particular, exfoliation of graphite with strong oxidants has been explored to prepare a suspension of individual 2D graphene oxide (GO) nanosheets dispersed in various types of solvents. This colloidal suspension can readily be employed in a wide range of promising potential applications. However, one major challenge is to assemble such individual nanosheets on defined areas of surfaces forming specific structures. Here, we developed a simple and robust one-step strategy to create highly regular ordered nanostructures composed of chemically reduced graphene oxide (rGO) nanosheet building blocks on SiO2/Si or glass substrates over large areas. An aqueous suspension of rGO nanosheets dispersed in a volatile solvent was subjected to a confined geometry to induce a spontaneous formation of rGO patterned arrays with unprecedented regularity by utilizing both a controlled coffee ring effect and consecutive stick-slip motions of rGO solutions during the drying process. As a result, the generation of ultrathin nanotextured films in the form of micropatterns (i.e., concentric gradient rGO coffee rings) was effectively tailored. As a biological approach, to fully utilize patterned rGO associated with the nanoscale surface topography, we used these patterned rGO arrays as a biomimetic in vitro architecture to study interfacial interactions of living cells such as fibroblasts, myoblasts, and neuronal cells. The observed results show that the patterned rGO arrays can be used as manipulated cellular responsive templates to form living cell assemblies and related patterns that are useful for regenerative tissue engineering.

A chemically controlled facile approach is developed for the in-situ growth of Co3O4 microspheres on N,S-co-doped rGO sheets (Co3O4/N,S-rGO) employing thiamine hydrochloride (THCl) (7.41 mM) as the N and S co-doping agent under mild experimental conditions. The THCl concentration plays a vital role in controlling the growth of cobalt crystals anchored on the surface of N,S-rGO. The change in concentration of THCl from 7.41 to 22.20 mM alters the deposition of cobalt crystal on the N,S-rGO surface from Co3O4 to Co9S8 (Co9S8/N,S-rGO). This change is understood due to the formation of different intermediate complex structure driven by supramolecular interactions. The altered cobalt crystal phase leads to tuned morphologies, surface areas and catalytically active centers which are investigated by various characterization techniques. Co3O4/N,S-rGO exhibits superior electrocatalytic activity for oxygen reduction reaction (ORR) compared to Co9S8/N,S-rGO. Moreover, Co3O4/N,S-rGO shows more positive onset potential (0.90 V vs. RHE), half-wave potential (0.74 V vs. RHE), relatively high limiting current density and smaller Tafel slope (52 mV per decade) compared to its separate components owing to synergistic effects. Co3O4/N,S-rGO hybrid also outperforms the electrocatalytic activity of commercial Pt/C in terms of durability and methanol tolerance under alkaline conditions.

Pd, Cu, and Zn trimetallic metal-organic framework electrocatalysts (PCZs) based on benzene-1,3,5-tricarboxylic were synthesized using a simple solvothermal synthesis. The as-synthesized PCZ catalysts exhibited as much as 95% faradaic efficiency towards CO, with a high current density, low onset potential, and excellent long-term stability during the electrocatalytic reduction of CO2.

Herein, a feasible and effective approach is developed to build an electrically conductive and double percolation network-like structure via the incorporation of highly reduced graphene oxide (HRGO) into a polymer blend of diglycidyl ether of bisphenol A/polyetherimide (DGEBA/PEI). With the assistance of the curing reaction-induced phase separation (CRIPS) technique, an interconnected network of HRGO is formed in the phase-separated structure of the DGEBA/PEI polymer blend due to selective localization behavior. In this study, HRGO was prepared from a unique chemical reduction technique. The DGEBA/PEI/HRGO nanocomposite was analyzed in terms of phase structure by content of PEI and low weight fractions of HRGO (0.5 wt.%). The HRGO delivered a high electrical conductivity in DGEBA/PEI polyblends, wherein the value increased from 5.03 &times, 10&minus, 16 S/m to 5.88 S/m at a low content of HRGO (0.5 wt.%). Furthermore, the HRGO accelerated the curing reaction process of CRIPS due to its amino group. Finally, dynamic mechanical analyses (DMA) were performed to understand the CRIPS phenomenon and selective localization of HRGO reinforcement. The storage modulus increased monotonically from 1536 MPa to 1660 MPa for the 25 phr (parts per hundred in the DGEBA) PEI polyblend and reached 1915 MPa with 0.5 wt.% HRGO reinforcement. These simultaneous improvements in electrical conductivity and dynamic mechanical properties clearly demonstrate the potential of this conductive polyblend for various engineering applications.

The development of robust nano- and microstructured catalysts on highly conductive substrates is an effective approach to produce highly active binder-free electrodes for energy conversion and storage applications. As a result, nanostructured electrodes with binder-free designs have abundant advantages that provide superior electrocatalytic performance; these include more exposed active sites, large surface area, strong adhesion to substrates, facile charge transfer, high conductivity, high intrinsic catalytic activity, and fine-tuning of its electronic nature through nanostructure modification. Notably, the interface chemistry of an electrocatalyst plays a significant role in their optimized electrocatalytic activity and stability. This review provides an overview of recent progress in nano- and microstructured catalysts, such as one, two, and 3D catalysts as binder-free electrodes for electrocatalytic water splitting via the hydrogen evolution reaction and oxygen evolution reaction, and beyond. Furthermore, this review focuses on the current challenges and synthesis strategies of binder-free electrodes, with a focus on the impact of nanostructure on their functional property relationships and enhanced bifunctional electrocatalytic performance. Finally, an outlook for their future advances in energy conversion and storage is provided.