Table of contents

The electrocatalytic conversion of CO₂ into value-added fuels has received increasing attention as a promising way to mitigate the atmospheric CO₂ concentration and close the broken carbon-cycle. Early studies, focused on polycrystalline metal electrodes, outlined in detail the overall trends in the catalytic activity and product selectivity of pure metals; however, several inherent limitations were found, such as low current density and high overpotential, which hindered electrocatalytic CO₂ reduction from practical application. Fortunately, the recent development of precisely synthesized nanocatalysts has led to several breakthroughs in catalytic CO₂ conversion. By carefully controlling the thermodynamic adsorption energies and flow dynamics of reaction intermediates, nanosized electrocatalysts afford more versatile and energetically efficient routes to convert CO₂ into desired chemicals. In this article, we review the state-of-the-art nanocatalysts applied for CO₂ conversion and discuss newly found phenomena at the local environment near the catalyst surface. The mechanistic understanding of these findings can provide insight into the future design of catalysts for the efficient and selective reduction of CO₂.

A globular protein's folded structure in its physiological environment is largely determined by its amino acid sequence. Recently, newly discovered transformer proteins as well as intrinsically disordered proteins may adopt the folding-upon-binding mechanism where their secondary structures are highly dependent on their binding partners. Due to the various applications of nanomaterials in biological sensors and potential wearable devices, it is important to discover possible conformational changes of proteins on nanomaterials. Here, through molecular dynamics simulations, we show that the first 17 residues of the huntingtin protein (HTT-N17) exhibit appreciable differences during its folding on 2D-nanomaterials, such as graphene and MoS₂ nanosheets. Namely, the protein is disordered on the graphene surface but is helical on the MoS₂ surface. Despite that the amphiphilic environment at the nanosheet-water interface promotes the folding of the amphipathic proteins (such as HTT-N17), competitions between protein-nanosheet and intra-protein interactions yield very different protein conformations. Therefore, as engineered binding partners, nanomaterials might significantly affect the structures of adsorbed proteins.

The search for low-dimensional materials with unique electronic properties is important for the development of electronic devices in the nanoscale. Through systematic first-principles calculations, we found that the band gaps of the two-dimensional honeycomb monolayers (HMs) and one-dimensional single-walled nanotubes (SWNTs) of IIB–VI semiconductors (ZnO, CdO, ZnS and CdS) are nearly chirality-independent and weakly diameter-dependent. Based on analysis of the electronic structures, it was found that the conduction band minimum is contributed to by the spherically symmetric s orbitals of cations and the valence band maximum is dominated by the in-plane $({d}_{{xy}}-{p}_{y})$ and $({d}_{{x}^{2}-{y}^{2}}-{p}_{x})$ hybridizations. These electronic states are robust against radius curvature, resulting in the invariant feature of the band gaps for the structures changing from HM to SWNTs. The band gaps of these materials range from 2.3 to 4.7 eV, which is of potential application in electronic devices and optoelectronic devices. Our studies show that searching for and designing specific electronic structures can facilitate the process of exploring novel nanomaterials for future applications.

We have performed Li and Na ion charge–discharge experiments of 9,10-phenanthrene quinone (PhQ) molecules encapsulated in single-walled carbon nanotubes (SWCNTs) with mean tube diameters of 1.5 and 2.5 nm at room temperature and also at low temperatures. The Na ion reversible capacity of PhQ encapsulated in the larger diameter SWCNTs, measured at a low temperature of 0 °C, remained as high as that measured at room temperature (RT), while the capacity of PhQ in the smaller diameter SWCNTs at 0 °C was about a half of that at RT. The diameter dependence of the capacity should be attributed to the difference in the interactions between the encapsulated PhQ molecules and the host SWCNTs, which was revealed by Raman peak profile analysis. Charge-transfer reaction from metallic tubes to PhQ molecules encapsulated in the smaller diameter SWCNTs was detected by Raman measurements. The electrostatic interaction between charged SWCNTs and PhQ molecules, induced by the charge-transfer reaction, would partly contribute to the stabilization of PhQ molecules in the smaller diameter SWCNTs, while only van der Waals interaction stabilizes PhQ molecules in the larger diameter SWCNTs. The difference in stability was confirmed by thermogravimetric, x-ray photoelectron spectroscopy, and Raman measurements. Charge–discharge curves of PhQ encapsulated in SWCNTs were also discussed based on the stability difference.

An ultra rapid growth method for vertically aligned ZnO nanorod (NR) thin films on metal meshes was developed using a direct heating synthesis technique. A typical NR growth rate of 10 μm h⁻¹ was achieved. The effects of the applied heating power and growth duration on the morphologies of ZnO nanostructures were examined. High density surface defects were formed on the ZnO NRs, which is responsible for slow charge recombination and high efficiency in the photoelectrochemical (PEC) water splitting process. The light absorption for a photoanode was significantly improved by light trapping using a 3D stacked metal mesh photoanode structure. With the internal reflection between the stacked photoanodes, the final light leakage is minimised. The light absorption in the stacked photoanode is improved without restricting the charge transportation. In comparison with a single mesh photoanode and a chemical bath deposition grown flat photoanode, the PEC water splitting efficiency from the stacked photoanode was increased by a factor of 2.6 and 6.1 respectively.

The response of a graphene-based humidity sensor is considered as a function of film structures. Analysis of the resistance changes due to water molecule adsorption on the graphene or multi-layer graphene (MLG) surface is performed for films with different structures and resistivities from hundreds of ohms/sq to hundreds of kilo-ohms/sq. The results revealed possible increase, decrease and non-monotonous behavior of resistance with changes in film structure. Adsorption of water molecules at grain boundary defects is assumed to lead to an increase in film resistivity due to the donor property of water and the p-type conductivity of graphene. Another type of conductive center with a higher capture cross-section is realized in the case of water molecule adsorption at edge defects in MLG films (the formation of conductive chains with ionic conductivity). If these chains form a continuous network the film resistivity decreases. The result of the competition between the opposite effects of the conductivity compensation and formation of the water-based conductive chains depends on the film structure and determines the response of humidity sensors. Sensor sensitivity is found to increase when only one type of defect determines water adsorption (edge defects or grain boundary defects).

Functionalized carbon nanotubes and graphene nanoribbons nanostructures, serving as the basis for the creation of physical pressure and temperature nanosensors, are considered as tools for ecological monitoring and medical applications. Fragments of nanocarbon inclusions with different morphologies, presenting a disordered system, are regarded as models for nanocomposite materials based on carbon nanoсluster suspension in dielectric polymer environments (e.g., epoxy resins). We have formulated the approach of conductivity calculations for carbon-based polymer nanocomposites using the effective media cluster approach, disordered systems theory and conductivity mechanisms analysis, and obtained the calibration dependences. Providing a proper description of electric responses in nanosensoring systems, we demonstrate the implementation of advanced simulation models suitable for real time control nanosystems. We also consider the prospects and prototypes of the proposed physical nanosensor models providing the comparisons with experimental calibration dependences.

A facile approach is presented for the detection of bovine serum albumin (BSA), based on fiber optic surface plasmon resonance (FOSPR) combined with molecular imprinting (MI). The probe is fabricated by exploiting the plasmonic property of silver thin film and vinyl-functionalised carbon nanotube-based MIP platform. BSA template molecules are imprinted on the MIP layer coated over multi-walled carbon nanotubes to ensure high specificity of the probe in the interfering environments. In addition, FOSPR endorses the sensor capability of real-time and remote sensing along with very high sensitivity due to the use of nanostructured MI platform. The response of the probe is considered in terms of the absorbance spectrum recorded for various concentrations of BSA. The sensor shows a wide dynamic range of 0–350 ng l⁻¹ with a considerably linear response up to 100 ng l⁻¹ in the peak absorbance wavelength with BSA concentration. A highest sensitivity of 0.862 nm per ng l⁻¹ is achieved for the lowest concentration of BSA and it decreases with the increase in BSA concentration. The performance of the present sensor is compared with those reported in the literature in terms of the limit of detection. It is found that the probe possesses a lowest LOD of 0.386 ng l⁻¹ in addition to other advantages such as real-time online monitoring, high sensitivity, high specificity, and remote sensing.

We study biological sensing using the hybridization phase of localized surface plasmon resonances (LSPRs) with diffraction modes (photonic lattice modes) in arrays of gold nanoantennas. We map the degree of the hybridization process using an embedding dielectric material (Si), identifying the critical thicknesses wherein the optical responses of the arrays are mainly governed by pure LSPRs (insignificant hybridization), Fano-type coupling of LSPRs with diffraction orders (hybridization state), and their intermediate state (hybridization phase). The results show that hybridization phase can occur with slight change in the refractive index (RI), leading to sudden reduction of the linewidth of the main spectral feature of the arrays by about one order of magnitude while it is shifted nearly 140 nm. These processes, which offer significant improvement in RI sensitivity and figure of merit, are utilized to detect monolayers of biological molecules and streptavidin-conjugated semiconductor quantum dots with sensitivities far higher than pure LSPRs. We further explore how these sensors can be used based on the uncoupled LSPRs by changing the polarization of the incident light.

We confirmed the occurrence of phase transformations in an atomic force microscopy silicon tip during loading and unloading experiments performed on a polycrystalline Ti sample. The influence of the phase transformations on the effective mechanical and electrical properties of the tip was observed with the help of load–unload curves measured simultaneously for the tip-sample contact stiffness k^* and the effective electrical resistance of the system R_eff. We used the atomic force acoustic microscopy (AFAM) method to determine the values of k^*. To measure the changes in R_eff, we combined a high voltage source/measure unit with the existing AFAM system. The data obtained showed that the phase transformation from Si-I to Si-II is preceded by other structural changes such as formation of distorted diamond structures and formation of Si-III. This conclusion was reached after observing a small hysteretic behavior in the load–unload stiffness curve accompanied by only very small changes in the resistance of the tip-sample system occurring on the unloading. The coinciding of a sudden increase in the values of the contact stiffness with a decrease in the resistance of the system indicated that the formation of metallic Si-II occurred in the subsequent measurements. The interpretation of our results found confirmation in the results of molecular dynamics and atomistic simulations performed for silicon under nanoindentation experiments.

We report the simultaneous investigation of both the plasmonic resonance and electrical conductance evolution in stretchable metal–polymer nanocomposite films. The films are produced by the implantation of neutral gold nanoparticles in a polydimethylsiloxane substrate by aerodynamic acceleration in a supersonic expansion. A redshift of the gold nanoparticle plasmon peak is found upon stretching as well as a strong correlation between the plasmonic peak wavelength and the nanocomposite electrical resistance. Optical simulations attribute the optical response to the compression of the polymer perpendicular to the stretching direction, which brings the gold particles closer to each other, increasing the plasmonic coupling. Mechanical stretching can induce a simultaneous modulation of the optical and electrical properties of the nanocomposite.

The application of the electrospun nanomaterials to surface-enhanced Raman spectroscopy (SERS) is a rapidly evolving field which holds potential for future developments in the generation of portable plasmonic-based detection platforms. In this study, a simple approach to fabricate electrospun poly(N-vinylpyrrolidone) (PVP) mats decorated with gold nanoparticles (AuNPs) by combining electrospinning and calcination was presented. AuNPs were decorated on the fiber mat surface through electrostatic interactions between positively charged aminosilane groups and negatively charged AuNPs. The size and coverage density of AuNPs on the fiber mats could be tuned by varying the calcination temperature. Calcination of AuNPs-decorated PVP fibers at 500 °C–700 °C resulted in the uniform decoration of high density AuNPs with very narrow gaps on every single fiber, which in turn contribute to strong electromagnetic SERS enhancement. The robust free-standing AuNPs-decorated mat which calcined at 500 °C (500/AuNPs-F) exhibited high SERS activity toward cationic (methylene blue, MB) and anionic (methyl orange, MO) dyes in single and binary systems with a detection range from tens of nM to a few hundred μM. The fabricated SERS substrate demonstrated high reproducibility with the spot-to-spot variation in SERS signal intensities was ±10% and ±12% for single and binary dye systems, respectively. The determination of MB and MO in spiked river water and tap water with 500/AuNPs-F substrate gave satisfactory results in terms of the percent spike recoveries (ranging from 92.6%–96.6%) and reproducibility (%RSD values less than 15 for all samples).

Nano-friction of phosphorene plays a significant role in affecting the controllability and efficiency of applying strain engineering to tune its properties. So far, the friction behavior of phosphorene has not been studied. This work studies the friction of single-layer and bilayer phosphorene on an amorphous silicon substrate by sliding a rigid tip. For the single-layer phosphorene, it is found that its friction is highly anisotropic, i.e. the friction is larger along the armchair direction than that along the zigzag direction. Moreover, pre-strain of the phosphorene also exhibits anisotropic effects. The friction increases with the pre-strain along the zigzag direction, but decreases with that along the armchair direction. Furthermore, the strong adhesion between the phosphorene and its substrate increases the friction between the phosphorene and the tip. For bilayer phosphorene, its friction highly depends on its stacking mode, which determines the contact interface with a commensurate or incommensurate pattern. This friction behavior is quite unique and greatly differs from that of graphene and molybdenum disulfide. Detailed analysis reveals that this behavior results from the combination effect of the friction contact area, the potential-energy profile of phosphorene, and its unique elongation.

We calculate the tensile and shear moduli of a series of boron nitride nanotubes and their piezoelectric response to applied loads. We compare in detail results from a simple molecular mechanics (MM) potential, the universal force field, with those from the atomistic finite element method (AFEM) using both Euler–Bernoulli and Timoshenko beam formulations. The MM energy minimisations are much more successful than those using the AFEM, and we analyse the failure of the latter approach both qualitatively and quantitatively.

When fabricating nanowires (NWs) in a doubly-clamped beam configuration it is possible for a residual axial stress to be generated. Here, we show that material characterisation of metal and semiconductor NWs subjected to residual axial stress can be problematic. Benchmark measurements of the Young's modulus of NWs are performed by sectioning a doubly-clamped NW into two cantilevered wires, eliminating residual axial stress. Use of models for doubly-clamped beams that incorporate the effects of residual stress are found to lead to ambiguity in the extracted Young's modulus as a function of displacement fit range, even for NWs with no residual stress. This is due to coupling of bending and axial stress effects at small displacements, and the limited displacement range of force curves prior to fracture or plastic deformation. This study highlights the importance of fabricating metal and semiconductor NWs that exhibit little or no residual axial stress for materials characterisation.

Atomic force microscope infrared spectroscopy (AFM-IR) combines the spatial resolution of AFM with the chemical specificity of IR spectroscopy. In AFM-IR, sample absorption of pulsed IR light causes rapid thermomechanical expansion, which excites resonance in an AFM cantilever in contact with the sample. The cantilever resonant amplitude is proportional to the local sample IR absorption coefficient. It is difficult to detect thermomechanical expansion in the smallest samples such as 1D and 2D nanomaterials. In this work, we overcome this limitation and use AFM-IR to measure nanometer-scale IR absorption in individual single walled carbon nanotubes and monolayer graphene. By placing a thin layer of polymer beneath the sample, the AFM-IR signal may be increased by up to two orders of magnitude. The polymer beneath the sample thermally insulates the sample and amplifies thermomechanical expansion. Finite element simulations agree with the measurements and provide a general framework for applying this approach to arbitrary samples, including other 1D and 2D materials and thin biological samples.

Carbon nanofibers decorated with porous carbon nanotubes were prepared by electrospinning and annealing methods. The microwave reflection loss of the products was investigated in the frequency range of 2–18 GHz. The bandwidth with a reflection loss less than −10 dB covers a wide frequency, ranging from 7.0 to 14.1 GHz with thickness of 3.0–5.5 mm, and the minimum reflection loss is −44.5 dB at 10.7 GHz with thickness of 2.0 mm. The large reflection loss and wide reflection band reveal that the products could be a promising candidate for microwave absorption.