Vincent O. Nyamori

Layered double hydroxide- and graphene-based hierarchical nanocomposites: Synthetic strategies and promising applications in energy conversion and conservation

The persistent need for a sustainable energy economy has led researchers to focus on novel energy conversion and storage technologies, inspiring the discovery of smart material designs such as hierarchical nanocomposites. These nanocomposites have proven effective in the advancement of energy-based technologies. The synergistic properties of hierarchical nanocomposites composed of two types of two-dimensional layered materials, layered double hydroxides and graphene, have resulted in improved electrochemical as well as photocatalytic performance. Synthetic strategies and their effect on the electrochemical and photocatalytic performance of these nanocomposites as high-performance supercapacitors and water oxidation catalysts are discussed in detail in this review.

Effectiveness of carbon nanotube–cobalt ferrite nanocomposites for the adsorption of rhodamine B from aqueous solutions

The efficiency of adsorption of rhodamine B (RhB) from aqueous solution was investigated through a series of batch experiments by using cobalt ferrite nanoparticles (CoFe2O4), acid-functionalized multiwalled carbon nanotubes (MWCNT–COOH) and carbon nanotube–cobalt ferrite nanocomposites. The adsorption capacity was evaluated as a function of pH, contact time, adsorbent dose, dye concentration and temperature. The effect of increasing the percentage of MWCNT–COOH in the nanocomposites was also studied. The adsorption capacity was lowest in CoFe2O4 (5.165 mg g−1) and highest with MWCNT–COOH (42.68 mg g−1). For the nanocomposites, the adsorption capacity was enhanced with an increase in the amount of MWCNT–COOH. The optimum pH for adsorption was observed at 7 at which equilibrium was reached after 360 min. The kinetics of adsorption was fitted to the pseudo-first order, pseudo-second order, Elovich and intraparticle diffusion models. The results showed that the pseudo-second order model best described the data as reflected in the lowest value for the sum of squared residuals. Among the various adsorption isotherms tested, the Langmuir isotherm provided the best fit to the equilibrium data. The thermodynamic parameters, ΔH°, ΔS° and ΔG°, were obtained over a temperature range of 20–45 °C. Adsorption was spontaneous, endothermic and entropy-driven, except for one of the doped nanocomposites for which adsorption was exothermic. A good desorption of RhB from the loaded adsorbents was obtained by using either acetone or ethanol with a desorption efficiency in the range of 62–95%.

Pyrrolic nitrogen-doped carbon nanotubes: physicochemical properties, interactions with Pd and their role in the selective hydrogenation of nitrobenzophenone

Nitrogen-doped carbon nanotubes (N-CNTs) containing 63%, 73% and 80% pyrrolic-N were synthesized and used to evaluate the influence of pyrrolic nitrogen on the physicochemical properties and catalytic activity of Pd supported on N-CNTs (Pd/N-CNTs). Micrographs of Pd/N-CNTs showed that Pd was located along the defect sites of N-CNTs indicating strong Pd-support interactions. X-ray photoelectron spectroscopy revealed that the abundance of Pd0 decreased while that of Pd2+ increased as the quantity of pyrrolic nitrogen increased. The Pd2+ species were formed as Pd–N coordination complexes, which stabilized Pd2+ nanoparticles. Selective hydrogenation of nitrobenzophenone to aminobenzophenone or p-benzylaniline was used to evaluate the catalytic performance of catalysts. Pd/N-CNTs exhibited a higher selectivity towards aminobenzophenone than Pd on carbon nanotubes and Pd on activated carbon. The enhanced selectivity towards nitro-reduction alone, observed with Pd/N-CNTs was attributed to the promoting effect of pyrrolic-N. Hence, Pd/N-CNTs are promising catalysts for the selective reduction of nitro arenes.

Determination of Selected Heavy Metals Using Amperometric Horseradish Peroxidase (HRP) Inhibition Biosensor

Fabrication of an amperometric biosensor for determination of heavy metals in tap water based on the inhibition of horseradish peroxidase activity is reported. The inhibition of the biosensors response to 0.95 mM H2O2 at −200 mV (vs. Ag/AgCl) followed the order, Cd2+ < Cu2+ < Pb2+. The detection limits were 0.09 ppb, 0.03 ppb, and 0.10 ppb for Cd2+, Pb2+, and Cu2+, respectively. Kinetic parameters (apparent Michaelis-Menten constant) and Imax (maximum current) obtained in the absence ( = 1.16 mM, Imax = 0.51 µA) and the presence ( = 1.13 mM, Imax = 0.36) of 3.13 ppm Cd2+ showed that the enzyme inhibition process was reversible and non-competitive, and the corresponding KI (inhibition constant) value was 0.026 mM. After two weeks, the biosensor retained >95% of its initial response and gave a reproducibility of 3.8% and precision of 2.1%.

Multiwalled Carbon Nanotube-titania Nanocomposites: Understanding Nano-Structural Parameters and Functionality in Dye-sensitized Solar Cells

Nanocomposites consisting of multiwalled carbon nanotubes and titania were synthesized by two methods, namely, sol-gel and chemical vapour deposition (CVD) methods. The work takes advantage of the bridging ability of nanotechnology between macromolecules and the solid state process in engineering alternative nanomaterials for various applications including solar cell fabrication. Physical and chemical characterization of the mesoporous nanocomposites from the two synthetic methods were investigated using Raman spectroscopy, thermogravimetric analysis, Fourier transformation infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, surface characterization and X-ray diffraction analysis. Physicochemical properties in the nanocomposites such as thermal stability, pore volume, crystallinity and surface area were observed to be a subject of MWCNT: titania ratios and synthetic methods. From the CVD synthetic method, observed attributes include more uniform and smoother coating; better crystallinity and larger pore width than sol-gel method. On the other hand, nanocomposites from sol-gel synthetic method had larger surface areas, were more defective and less thermally stable than those from CVD. Nanocomposites by the CVD method performed 39.2 % more efficient than those from sol-gel in light-harvesting experiments. The study shows that the nanocomposites synthesized were more effective than titania alone when the cheaper natural dye, Eosin B, was used. This highlights the great potential of typical nanomaterials in improving the performances of titania in DSSCs as well as lowering the cost of the ultimate devices.

Review: Multimetallic silver(I)–pyridinyl complexes: coordination of silver(I) and luminescence

This review highlights some structural features and luminescent properties of homo- and hetero-multinuclear silver(I)–pyridinyl complexes. It focuses on the coordination and geometry of the silver(I) ions to the pyridinyl-nitrogen. For this reason, we have considered only pyridinyl-N–Ag(I) complexes whose crystal data are available. In addition, this review does not consider mononuclear silver(I)–pyridinyl complexes as these have been reviewed elsewhere. This is motivated by the fact that multinuclear silver(I)–pyridinyl complexes have been shown to be more stable in solution, possess enhanced properties, and have fascinating structures compared to their mononuclear counterparts. The introduction highlights pyridinyl ligands used in complexation of silver(I) ions. The main body highlights complexation of silver(I) through pyridinyl nitrogen and the interactions found in the multinuclear silver(I)–pyridinyl complexes as well as the coordination number and geometry of silver(I) centers. Though silver(I) has been flaunted to prefer linear twofold coordination geometry, from this review, it is clear that higher coordination numbers in varied geometries are possible. These include distorted trigonal planar, T-shaped, distorted tetrahedral, trigonal bipyramidal, and octahedral geometries. Coordination of silver(I) to pyridinyl ligands and their metalloligands has been observed to impart or enhance luminescent properties in the ensuing complexes. Graphical abstract Possible coordination modes of silver(I) by pyridinyl ligands and Ag⋯Ag interactions.

Nitrogen-functionalised carbon nanotubes as a novel adsorbent for the removal of Cu(II) from aqueous solution

This study investigated the introduction of 4′-(4-hydroxyphenyl)-2,2′:6′,2′′-terpyridine (HO-Phttpy) onto the surface of multiwalled carbon nanotubes (MWCNTs) to obtain nitrogen-functionalized MWCNTs (MWCNT-ttpy). This novel material was characterised and tested for its possible use in the remediation of wastewater contaminated with heavy metal ions. Its efficacy was compared with that of acid-functionalized MWCNTs (MWCNT-COOH) for the removal of the heavy metal ion Cu2+ through adsorption. HO-Phttpy was first synthesized, followed by the functionalization of MWCNT-COOH to afford MWCNT-ttpy. MWCNT-ttpy showed significant textural enhancement due to an increase in the extent of functionalization. This was demonstrated by an increase in the surface area and pore volume of MWCNT-ttpy, from 126.8 to 189.2 m2 g−1 and 0.692 to 1.252 cm3 g−1, respectively, relative to MWCNT-COOH. Its application for Cu2+ removal showed a marked increase in uptake (qe), i.e. 19.44 to 31.65 mg g−1, compared with MWCNT-COOH. This is attributed to the introduction of more active/chelating sites for adsorption. Adsorption experiments were conducted at pH 5 at which equilibrium was reached after 360 min. The results showed that the adsorption process was best described by the pseudo-second order model. Among the isotherms tested, the Langmuir isotherm provided the best fit for the equilibrium data. Thermodynamic studies revealed that the adsorption process was spontaneous and endothermic. Desorption studies demonstrated a better removal efficiency of Cu2+ from MWCNT-ttpy, indicating its possible regeneration and the recovery of the Cu2+ adsorbate for reuse. Thus, MWCNT-ttpy shows superior properties for wastewater remediation compared to MWCNT-COOH.

Advances in carbon nanotubes as efficacious supports for palladium-catalysed carbon-carbon cross-coupling reactions

Since the 1970s, palladium-catalysed carbon–carbon (C–C) bond formation has made a critical impact in organic synthesis. In early studies, homogeneous palladium catalysts were extensively used for this reaction with limitations such as difficulty in separation and recycling ability. Lately, heterogeneous palladium-based catalysts have shown promise as surrogates for conventional homogeneous catalysts in C–C coupling reactions, since the product is easy to isolate, while the catalyst is reusable and hence sustainable. Recently, a better part of these heterogeneous palladium catalysts are supported on carbon nanotubes (Pd/CNTs), that have shown superior catalytic performance and better recyclability since the CNT support imparts stability to the palladium catalyst. This review discusses the wide variety of surface functionalization techniques for CNTs that improve their properties as catalyst supports, as well as the methods available for loading the catalyst nanoparticles onto the CNTs. It will survey the literature where Pd/CNTs catalysts have been utilized for C–C coupling reactions, with particular emphasis on Suzuki–Miyaura and Mizoroki–Heck coupling reactions. It will also highlight some of the important parameters that affect these reactions.

Zn(II) and Cu(II) formamidine complexes: structural, kinetics and polymer tacticity studies in the ring-opening polymerization of ε-caprolactone and lactides

Treatment of N,N′-bis(2,6-dimethylphenyl)formamidine (L1), N,N′-bis(2,6-diisopropylphenyl)formamidine (L2), and N,N′-dimesitylformamidine (L3) with Zn(OAc)2·2H2O or Cu(OAc)2·H2O produced the corresponding Zn(II) and Cu(II) N,N′-diarylformamidine complexes [Zn3(L1)2(OAc)6] (1), [Zn2(L2)2(OAc)4] (2), [Zn2(L3)2(OAc)4] (3) and [Cu2(L2)2(OAc)4] (4), respectively. While complex 1 is trinuclear, compounds 2–4 are dimeric in the solid state. The X-band EPR spectra of complex 4 in solid and solution states are consistent with perfect axial symmetry and confirm retention of the dinuclear paddle-wheel core in the solution state. Complexes 1–4 formed active catalysts in the ring opening polymerization (ROP) of e-caprolactone (e-CL) and lactides (LA). Complexes 1 and 3 exhibited higher rate constants of 0.1009 h−1 and 0.0963 h−1 compared to the rate constants of 0.0479 h−1 and 0.0477 h−1 observed for 2 and 4, respectively, in the ROP of e-CL at 110 °C. Higher rate constants of 0.5963 h−1 and 1.2962 h−1 were obtained for complexes 1 and 3 in the ROP of LAs compared to those reported in the ROP of e-CL at 110 °C. Activation parameters were determined as ΔH‡ = 25.08 kJ mol−1 and ΔS‡ = −201.7 J K−1 mol−1 for the ROP of e-CL using 3. Investigation of the kinetics of polymerization of e-CL and LAs revealed first order dependence of the polymerization reactions on monomer concentration. Moderate molecular weight polymers of up to 21 286 g mol−1 exhibiting relatively moderate molecular weight distributions and moderately heterotactic PLAs with Pr up to 0.65 were obtained.

A dual-purpose silver nanoparticles biosynthesized using aqueous leaf extract of Detarium microcarpum: An under-utilized species

The need for green synthesis of emerging industrial materials has led to the biosynthesis of nanoparticles from plants to circumvent the adverse by-products of chemical synthesis. In this study, the leaf extract of Detarium mirocarpum Guill & Perr, a small tree belonging to the family Fabaceae (Legume), was used to synthesize silver nanoparticles (DAgNPs). DAgNPs were characterized using spectroscopic techniques (Ultraviolet-Visible spectroscopy and Fourier Transform Infrared spectroscopy) which showed hydroxyl and carbonyl functional groups to be responsible for their synthesis. DAgNPs were observed to be crystalline and spherical. The average size, determined by transmission electron microscopy (TEM) was 17.05nm. The antioxidant activity of DAgNPs ranked from moderate to good. The ability of DAgNPs to sense Hg(2+) and Fe(3+) ions in aqueous medium was also investigated. The quenching of the SPR peak at 430nm was used to monitor the toxic and heavy metal ions with linear ranges of 20-70µgmL(-1) and 10-40µgmL(-1) for Hg(2+) and Fe(3+), respectively. The limit of detection (LOD) and limit of quantification (LOQ) obtained for Hg(2+) was 2.05µgmL(-1) and 6.21µgmL(-1), respectively and for Fe(3+) was 5.01µgmL(-1) and 15.21µgmL(-1), respectively. The intra- and inter-day assessments of accuracy and repeatability gave relative errors less than 1% in all instances. DAgNPs can therefore provide a convenient method of sensing the toxic metals easily.