The 100 most-cited articles in The Journal of Physical Chemistry C, 2016

In recent years, there has been a great deal of interest in the preparation and application of nanoparticles for cancer therapy. Gold nanoparticles are especially suited to thermal destruction of cancer due to their ease of surface functionalization and photothermal heating ability. Here, we review recent progress in gold nanoparticle-mediated thermal cancer therapies. We begin with an introduction to the properties of gold nanoparticles and heat-generating mechanisms which have been established. The pioneering work in photothermal therapy is discussed along with the effects of photothermal heating on cells in vitro . Additionally, radiofrequency-mediated thermal therapy is reviewed. We focus our discussion on the developments and progress in nanoparticle design for photothermal cancer therapy since 2010. This includes in vitro and in vivo studies and the recent progression of gold nanoparticle photothermal therapy toward clinical cancer treatment.

Nitrogen-doped carbon dots synthesized from citric acid as a carbon precursor have recently been considered to contain fluorescent derivatives of citrazinic acid, which contribute to their emission in the blue spectral range. To study the impact of such molecular fluorescent species on the optical properties of carbon dots, we synthesized three samples employing citric acid and three different nitrogen sources: ethylenediamine, hexamethylenetetramine, and triethanolamine. On the basis of the analysis of the nitrogen content and its coordination by X-ray photoelectron spectroscopy, FTIR spectra, and systematically comparing absorption, steady-state emission, and photoluminescence decays of each kind of carbon dot, we derive the influence of the molecular precursors and gain further understanding of the complex structure of carbon dots highlighting the strong impact of molecular fluorescence in the samples produced with ethylenediamine and hexamethylenetetramine.

The recent rapid growth in graphene-based supercapacitors has reached the point where there is a need for solid-state devices with physical flexibility, which will be a crucial advantage in modern electronic devices. Herein, we summarize recent developments toward an all solid-state graphene-based flexible supercapacitor. The routes to produce graphene-based electrode materials, along with the typical fabrication techniques for flexible devices, are thoroughly discussed. Furthermore, the structural morphology of the electrode materials is closely related to the electrochemical performance, and the influence of the electrode components on the mechanical flexibility of the fabricated devices is examined. Lastly, a summary of the overall electrochemical properties and current development of the reported devices is presented progressively to predict the future trends toward the realization of an ultimate-performance graphene-based flexible supercapacitor.

With the aim of obtaining a highly stable and active catalyst for oxygen evolution reaction (OER), a core–shell-like IrO 2 @RuO 2 material was synthesized by using a surface modification/precipitation method in ethanol medium. The comparison of this catalyst with pure RuO 2 and pure IrO 2 showed that the obtained mixed oxide catalyst displayed the highest amount of active sites as well as a good accessibility for water. Moreover, this catalyst was shown to be highly stable toward repetitive redox cycling. Polarization curves of the three catalysts showed that the IrO 2 @RuO 2 was the most active for the OER due to the large number and high accessibility of active sites. These catalytic benefic effects are attributed to an intimate contact between the two oxides in the IrO 2 -covered RuO 2 nanocatalyst that combines the RuO 2 intrinsic activity and the IrO 2 stability. The present study contributes therefore to the rational design of efficient and stable electrocatalysts for water splitting in acidic media.

Abstract Force fields developed with machine learning methods in tandem with quantum mechanics are beginning to find merit, given their (i) low cost, (ii) accuracy, and (iii) versatility. Recently, we proposed one such approach, wherein, the vectorial force on an atom is computed directly from its environment. Here, we discuss the multistep workflow required for their construction, which begins with generating diverse reference atomic environments and force data, choosing a numerical representation for the atomic environments, down selecting a representative training set, and lastly the learning method itself, for the case of Al. The constructed force field is then validated by simulating complex materials phenomena such as surface melting and stress–strain behavior, that truly go beyond the realm of ab initio methods, both in length and time scales. To make such force fields truly versatile an attempt to estimate the uncertainty in force predictions is put forth, allowing one to identify areas of poor performance and paving the way for their continual improvement.

Effects of electron donating (−CH 3 and −OH) and electron withdrawing (−NO 2 ) substituents on the corrosion inhibition efficiency of four glucosamine-based, substituted, pyrimidine-fused heterocycles (CARBs) on mild steel corrosion in 1 M HCl have been investigated using gravimetric, electrochemical, surface morphology (SEM, AFM, and EDX), and computational techniques. Gravimetric studies showed that protection performances of the compounds increase with increase in concentration. Both electron withdrawing (−NO 2 ) and electron donating (−CH 3 and −OH) groups were found to enhance the inhibition efficiency, but the effect is more pronounced with electron-donating substituents. The compounds were found to be cathodic-type inhibitors as inferred from the results of potentiodynamic polarization studies. EIS studies suggested that the studied compounds inhibit metallic corrosion by adsorbing on metallic surface. The adsorption of the inhibitor molecules on steel surface was further supported by SEM, AFM, and EDX analyses. Adsorption of CARBs on a mild steel surface obeyed the Langmuir…

Abstract Impedance spectroscopy offers access to all the different electronic and ionic processes taking place simultaneously in an operating solar cell. To date, its use on perovskite solar cells has been challenging because of the richness of the physical processes occurring within similar time domains. The aim of this work is to understand the general impedance response and propose a general equivalent circuit model that accounts for the different processes and gives access to quantitative analysis. When the electron-selective contacts and the thickness of the perovskite film are systematically modified, it is possible to distinguish between the characteristic impedance signals of the perovskite layer and those arising from the contacts. The study is carried out using mixed organic lead halogen perovskite (FA0.85MA0.15Pb(I0.85Br0.15)3) solar cells with three different electron-selective contacts: SnO2, TiO2, and Nb2O5. The contacts have been deposited by atomic layer deposition (ALD), which provides pinhole-free films and excellent thickness control in the absence of a mesoporous…

Chemical analysis of solid–liquid interfaces under electrochemical conditions has recently become feasible due to the development of new synchrotron radiation techniques. Here we report the use of “tender” X-ray ambient-pressure X-ray photoelectron spectroscopy (APXPS) to characterize a thin film of Ni–Fe oxyhydroxide electrodeposited on Au as the working electrode at different applied potentials in 0.1 M KOH as the electrolyte. Our results show that the as-prepared 7 nm thick Ni–Fe (50% Fe) film contains Fe and Ni in both their metallic as well as oxidized states, and undergoes further oxidation when the sample is subjected to electrochemical oxidation–reduction cycles. Metallic Fe is oxidized to Fe 3+ and metallic Ni to Ni 2+/3+ . This work shows that it is possible to monitor the chemical nature of the Ni–Fe catalyst as a function of potential when the corresponding current densities are small. This allows for operando measurements just above the onset of OER; however, current densities as they are desired in photoelectrochemical devices (∼1–10 mA cm –2 ) could not be achieved in…

Although Raman spectral fingerprints of Co 3 O 4 have been well established, the infrared spectrum of Co 3 O 4 is less understood due to its dependence on sample morphologies and experimental configurations. The same is true for both Raman and infrared spectra of CoO. In this study, we present a comprehensive optical characterization of Co 3 O 4 and CoO with Raman scattering and Fourier transform infrared spectroscopy (FTIR). Two of the transverse optical (TO) phonons and their corresponding longitudinal optical (LO) phonons of Co 3 O 4 above 500 cm –1 are observed in both transmission and diffuse reflectance with LO/TO intensity ratios depending on particle size and the incident angle of FTIR beam. CoO is featured by a broad infrared band around 510 cm –1 . In contrast to many previous reports, no Raman-active phonon line is observed, which is in agreement with the selection rule for rock-salt CoO. Nevertheless, CoO can still be characterized by Raman scattering from magnetic excitations in its antiferromagnetic phase at low temperature and a two-phonon Raman band at ∼1060 cm –1 .

The corrosion inhibition efficiencies of three novel naphthyridines namely, 5-amino-9-hydroxy-2-phenylchromeno[4,3,2- de ][1,6]naphthyridine-4-carbonitrile (N-1), 5-amino-9-hydroxy-2-( p -tolyl)chromeno[4,3,2- de ][1,6]naphthyridine-4-carbonitrile (N-2), and 5-amino-9-hydroxy-2-(4-methoxyphenyl)chromeno[4,3,2- de ][1,6]naphthyridine-4-carbonitrile (N-3) have been investigated for mild steel in 1 M HCl solution by using weight loss, electrochemical impedance spectroscopy, and potentiodynamic polarization methods. All three compounds show high inhibition activities at 6.54 × 10 –5 M: N-1, 94.28%; N-2, 96.66%; and N-3, 98.09%. Electrochemical impedance spectroscopy analysis reveals an increase in polarization resistance due to the adsorbed inhibitor molecules on metal surface. Potentiodynamic polarization analysis reveals that all three compounds act as mixed-type inhibitors but of predominantly cathodic type. The adsorption of the studied compounds on mild steel surface follows the Langmuir adsorption isotherm. Surface morphology examined by using scanning electron microscopy and…

Methanol is a promising fuel for direct methanol fuel cells in portable devices. A deeper understanding of its electro-oxidation is needed for evaluating electrocatalytic performance and catalyst design. Here we provide an in-depth investigation of the cyclic voltammetry (CV) of methanol electro-oxidation. The oxidation peak in backward scan is shown to be unrelated to residual intermediate oxidation. The origin of the second oxidation peak (I f2 ) is expected to the methanol oxidation on Pt–O x . Electrochemical impedance spectroscopy coupled with CV reveals the origin of CV hysteresis to be a shift in the rate-determining step, from methanol dehydration to OH adsorption by water dissociation, induced by a change in Pt surface coverage with oxygenated species. The peak ratio between forward oxidation peak current (I f ) and backward oxidation peak current (I b ), which is I f /I b, is not related to the degree of CO tolerance but to the degree of oxophilicity indeed.

Sodium storage capacity, mobility, and volume change during sodiation on the surfaces of interlayer-expanded Ti 3 C 2 MXenes are investigated using ab initio density functional theory. The theoretical results reveal that the interlayer-expanded bare, F-, O-, and OH-functionalized Ti 3 C 2 MXenes exhibit low barriers for sodium diffusion and small changes of lattice constant during sodiation. In addition, enlarged interlayer distance enables the stable multilayer adsorption on the bare and O-functionalized Ti 3 C 2 MXenes and therefore significantly enhances their theoretical capacities. Both bare and O-functionalized Ti 3 C 2 MXenes are predicted to be prospective anode materials for sodium-ion batteries with high theoretical capacities, fast discharge/charge rates, and good cycling performances. The present results provide a new route to improve the battery performances of anode materials based on MXene intercalation hosts.

A flexible carbon fiber aerogel with a very high surface area for supercapacitor application is reported by carbonization and chemical activation of low-cost natural cotton with KOH. The carbon fibers in the aerogel present as a twisted and tubular structure. Depending on the amount of KOH used in the activation process, the specific surface area of aerogels ranges from 1536 to 2436 m 2 g –1, while their electrical conductivity remains ∼860 S m –1 . In spite of pore size in the range of 1.0–4.0 nm and pore volume mainly contributed by micropores, the carbon aerogel exhibits a high specific capacitance of 283 F g –1 (1 A g –1 ) in 6 M KOH aqueous electrolyte and retains a high capacitance retention of 224 F g –1 at current density up to 100 A g –1 . Importantly, a symmetric capacitor built with the aerogel electrodes exhibits a rather small time constant (0.56 s). The superior capacitive performance of a CF electrode is closely related to its distinct structural advantage. The tubular carbon fibers that are several millimeters in length offer ultralong electronic and ionic pathways,…

Abstract In this work, we introduce Cu2O thin films as a hole-transport layer in planar perovskite solar cells. Here, a Cu2O layer was formed through successive ionic layer adsorption and reaction (SILAR) method. With methylammonium lead triiodide (MAPbI3) we form a direct structure (p–i–n), where the perovskite layer is sandwiched between a layer of p-type Cu2O and another layer of n-type PCBM (phenyl-C61-butyric acid methyl ester), which acted as hole- and electron-transport materials, respectively. We locate band edges of the materials with respect to their Fermi energy by recording scanning tunneling spectroscopy that has correspondence to their density of states (DOS). We observe that the energy levels of the materials form type II band alignments at each of the two interfaces (p–i and i–n) for charge separation and uninterrupted carrier transport upon illumination. Such a band alignment enabled charge transfer from MAPbI3 as evidenced from quenching of its photoluminescence emission when the perovskite was in contact with either the hole- or the electron-transport layer. With…

We report the vibrational and optical properties of the ‘defect’ perovskites Cs 2 SnX 6 (X = Cl, Br, I) as well as their use as hole-transporting materials (HTMs) in solar cells. All three air-stable compounds were characterized using powder X-ray diffraction and Rietveld refinement. Far-IR reflectance, Raman, and UV–vis spectroscopy as well as electronic band structure calculations show that the compounds are direct band gap semiconductors with a pronounced effect of the halogen atom on the size of the energy gap and the vibrational frequencies. Scanning electron microscopy and atomic force microscopy confirmed that the morphology of the perovskite films deposited from N, N -dimethylformamide solutions on TiO 2 substrates also strongly depends on the chemical composition of the materials. The Cs 2 SnX 6 perovskites were introduced as hole-transporting materials in dye-sensitized solar cells, based on mesoporous titania electrodes sensitized with various organic and metal–organic dyes. The solar cells based on Cs 2 SnI 6 HTM and the Z907 dye performed best with a maximum power…

The interfacial region of aqueous systems also known as the electrical double layer can be characterized on the molecular level with second harmonic and sum-frequency generation (SHG/SFG). SHG and SFG are surface specific methods for isotropic liquids. Here, we model the SHG/SFG intensity in reflection, transmission, and scattering geometry taking into account the spatial variation of all fields. We show that, in the presence of a surface electrostatic field, interference effects, which originate from oriented water molecules on a length scale over which the potential decays, can strongly modify the probing depth as well as the expected intensity at ionic strengths <10 –3 M. For reflection experiments this interference phenomenon leads to a significant reduction of the SHG/SFG intensity. Transmission mode experiments from aqueous interfaces are hardly influenced. For SHG/SFG scattering experiments the same interference leads to an increase in intensity and to modified scattering patterns. The predicted scattering patterns are verified experimentally.

Yb 3+ /Er 3+ codoped Ba 5 Gd 8 Zn 4 O 21 up-conversion (UC) phosphors with tunable emission were synthesized using a facile sol–gel method. UC spectra are composed of green emission from 2 H 11/2 / 4 S 3/2 → 4 I 15/2 transitions and red emission from 4 F 9/2 → 4 I 15/2 transition of Er 3+ ion with the excitation of 980 nm laser diodes. Modulation of emitting color from green to red could be achieved by adjusting dopant concentrations or pulse width of 980 nm laser. The mechanism of the former strategy was figured out through analyzing visible and near-infrared (NIR) down-conversion emission spectra together with the corresponding green level ( 4 S 3/2 ) lifetimes under excitation of 490 nm light, and the latter method was explained by the non-steady-state up-converison process. Temperature detection range was expanded to low temperature region by utilizing red-emitting stark levels of Er 3+ ion as thermally coupled levels. Thermal sensing performances based on green-emitting levels ( 2 H 11/2 / 4 S 3/2 ) and red emitting stark levels ( 4 F 9/2(1) / 4 F 9/2(2) ) of Er 3+ ion were…

Ag and CuO nanoparticles (NPs) synthesized on the surface of commercial TiO 2 (P25) by radiolytic reduction were characterized by diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). In the case of modification with silver and copper, results from HAADF-STEM, EDS, XPS, and XAS show that Ag@CuO nanoparticles (large silver cores decorated with small clusters of CuO) were obtained on TiO 2 –P25. The photocatalytic properties of bare and modified TiO 2 –P25 were studied for phenol photodegradation and for acetic acid oxidation under UV and visible irradiation. The mechanisms involved in photocatalysis were studied by time-resolved microwave conductivity (TRMC) and action spectra (AS). The electronic properties of the surface-modified TiO 2 –P25 were studied by TRMC to follow the charge-carrier dynamics. The modification with Ag nanoparticles or CuO…

First-principles calculations are used to compare the binding energies of O, OH, and F on two-dimensional, metal carbide and nitride, or MXene, surfaces in order to predict the dependence of the thermodynamic stability of these compounds on their chemical composition. Solvation effects are implicitly included in the calculations to reproduce experimental conditions as closely as possible. The results indicate that all MXene surfaces are saturated with oxygen when exposed to H 2 O/HF solutions at low hydrogen chemical potential, μ H, and that Sc-based MXenes can also be fluorinated in solutions of higher μ H . After investigating the thermodynamic stability of all 54 MXene compounds M n +1 X n O 2 (M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta; X = C, N; n = 1, 2, 3), 38 are predicted to have formation energies below 200 meV/atom. Of these, six are predicted to have formation energies below 100 meV/atom, only one of which has been synthesized. Sc-based MXenes are found to be highly stable when their surfaces are terminated with F, which also results in the easiest exfoliation to produce…

The interaction between Pt and CeO 2 under reducing and oxidizing conditions as well as its effect on the thermal stability of Pt/CeO 2 were extensively investigated by means of N 2 adsorption/desorption, Raman spectroscopy, CO chemisorption, H 2 TPR, XRD, and XPS techniques. In situ and ex situ Raman spectroscopy showed that Pt is anchored with the surface oxygen of CeO 2 by forming Pt–O–Ce bond during the oxidative treatment of Pt/CeO 2 . Under the reducing condition, the static CO chemisorption presented that the amount of CO adsorbed on CeO 2 is almost equal to that on Pt/CeO 2, implying that Pt atom is located on the oxygen vacancy generated on reduced CeO 2 surface. Strong Pt–O–Ce bond maintained the textural properties of Pt/CeO 2 from oxidative treatment at temperature as high as 800 °C, as evidenced by the XRD patterns and BJH curves of the samples. Selective removal of the surface oxygen of Pt/CeO 2 resulted in the decreased thermal stability of Pt/CeO 2 due to the loss of Pt–O–Ce bond. Stronger interaction between Pt and CeO 2 is observed when the oxidation temperature…

Despite magnetic hyperthermia being considered one of the most promising techniques for cancer treatment, until now spherical magnetite (Fe 3 O 4 ) or maghemite (γ-Fe 2 O 3 ) nanoparticles, which are the most commonly employed and only FDA approved materials, yield the limited heating capacity. Therefore, there is an increasing need for new strategies to improve the heating efficiency or the specific absorption rate (SAR) of these nanosystems. Recently, a large improvement in SAR has been reported for nanocubes of Fe 3 O 4 relative to their spherical counterpart, as a result of their enhanced surface anisotropy and chainlike particle formation. Considering the proven advantages of high aspect ratio one-dimensional (1D) Fe 3 O 4 nanostructures over their spherical and cubic counterparts, such as larger surface area, multisegmented capabilities, enhanced blood circulation time, and prolonged retention in tumors, we propose a novel approach that utilizes this 1D nanostructure for enhanced hyperthermia. Here, we demonstrate that the SAR of iron oxide nanostructures can be enhanced and…

Zeolitic imidazolate frameworks (ZIFs) have been foregrounded as structures with exceptional, intrinsic chemical and thermal stability. However, there has yet to be a systematic study of the isothermal stability of ZIFs, specifically the well-studied ZIF-8. In this work, ZIF-8 isothermal TGA decomposition kinetics were studied in air, argon, H 2 /CO 2, and nitrogen environments by exposing ZIF-8 to each gas for 20 h at temperatures of 200, 250, and 300 °C, respectively. ZIF-8 crystallinity was preserved under the experimental isothermal conditions at 200 °C in each atmosphere, but crystallinity was increasingly eliminated at higher temperatures. Decomposition kinetics data show that the rate of ZIF-8 carbonization significantly increases at temperatures above 200 °C irrespective of environment. ZIF-8 decomposition in the H 2 /CO 2 reducing mixture exhibits the slowest decomposition kinetics at all temperatures and the greatest morphological change. At 300 °C, oxidative effects enhance ZIF-8 decomposition in air. At lower temperatures the decomposition rate in air behaves more…

To develop efficient visible light driven photocatalysts for air purification, we constructed a novel semimetal–semiconductor Bi–Bi 2 MoO 6 (Bi–Mo) nanohybrid via the in situ deposition of Bi nanoparticles on the surface of Bi 2 MoO 6 microspheres. In this strategy, the Bi 3+ ions were in situ reduced to metallic Bi particles by glucose during in hydrothermal process. The XRD, XPS, SEM, TEM, UV–vis, DRS, PL spectra, and surface photovoltage were employed to explore the structural and optical properties. The as-synthesized Bi–Bi 2 MoO 6 nanohybrid was applied in photocatalytic removal of NO in air. The results indicated that the amount of reductive glucose not only exerted a pivotal effect on the morphological structure but also affected the photocatalytic capability of the Bi–Bi 2 MoO 6 nanohybrid. The optimized Bi–Mo-50 hybrids exhibited exceptionally high visible-light photocatalytic performance with a NO removal ratio up to 68.1%, far outperforming other decent photocatalysts, like BiOBr (21.3%), C-doped TiO 2 (21.8%), N-doped TiO 2 (36.5%), N-doped (BiO) 2 CO 3 (43.5%), and g-C…

Ceria is an interesting component for a variety of catalytic and fuel cell applications. In the study described here, ten different commercial ceria samples as well as synthesized ceria samples were investigated in detail regarding their (defect) structure and characteristic properties using XRD, N 2 adsorption–desorption, and optical spectroscopy (Raman, DRIFTS, UV–vis). The investigations revealed correlations of surface defect features (Raman, DRIFTS) as well as those of bulk defects (Raman, UV–vis). The Raman feature at around 250 cm –1 was demonstrated to be related to surface defects rather than a 2TA vibration as described in the literature. A correlation between UV–vis band gap values and the presence of Raman bulk defects was established based on the observed decrease of the band gap energy with increasing number of defects. Detailed Raman analysis revealed that the frequently mentioned linear equation for the determination of the crystal size from the half-width of the F 2g Raman feature is erroneous, since the F 2g half-width depends on ceria bulk defects. Apart from…

Ambient temperature sodium batteries hold the promise of a new generation of high energy density, low-cost energy storage technologies. Particularly challenging in sodium electrochemistry is achieving high stability at high charge/discharge rates. We report here mixtures of inorganic/organic cation fluorosulfonamide (FSI) ionic liquids that exhibit unexpectedly high Na + transference numbers due to a structural diffusion mechanism not previously observed in this type of electrolyte. The electrolyte can therefore support high current density cycling of sodium. We investigate the effect of NaFSI salt concentration in methylpropylpyrrolidinium (C 3 mpyr) FSI ionic liquid (IL) on the reversible plating and dissolution of sodium metal, both on a copper electrode and in a symmetric Na/Na metal cell. NaFSI is highly soluble in the IL allowing the preparation of mixtures that contain very high Na contents, greater than 3.2 mol/kg (50 mol %) at room temperature. Despite the fact that overall ion diffusivity decreases substantially with increasing alkali salt concentration, we have found that…

The facet-dependent catalytic performance of Fe 2 O 3 /CeO 2 catalysts for the selective catalytic reduction of NO with NH 3 (NH 3 –SCR) has been investigated using combined experimental and density functional theory (DFT) methods. The structure and surface characteristics of the synthesized samples were characterized by XRD, XPS, TEM, ICP–AES, N 2 sorption isotherms, Raman spectra, photoluminescence spectra, H 2 –TPR, NH 3 –TPD and NO + O 2 –TPD. It is found that the CeO 2 nanorods and Fe 2 O 3 /CeO 2 nanorods predominately exposed {110} and {100} facets rather than the stable {111} facets on CeO 2 nanopolyhedra and Fe 2 O 3 /CeO 2 nanopolyhedra. The influence of the micromorphologies and surface properties of CeO 2 supports on the NO conversion and N 2 selectivity has been compared. The Fe 2 O 3 /CeO 2 nanorods achieve higher catalytic activity than the Fe 2 O 3 /CeO 2 nanopolyhedra for NH 3 –SCR of NO. The synergetic effect between CeO 2 supports and Fe 2 O 3 species has been demonstrated. The insight into molecular facet dependence by the DFT method clearly showed that the Fe 2…

We theoretically propose a surface-plasmon-based fiber optic biosensor in metal/graphene/MoS 2 configuration with molybdenum disulfide (MoS 2 ) as a bio recognition layer. The proposed configuration works in the visible region of the electromagnetic spectrum with a very high sensitivity. A comparative theoretical study of the sensors with different metallic layers of gold (Au), copper (Cu), and aluminum (Al) has been performed. The sensor has been found to be the most sensitive in both Cu/graphene/MoS 2 and Al/graphene/MoS 2 configurations with sensitivity of 6.2 μm/RIU. In both of the configurations the thicknesses of Cu and Al layers is 50 nm and the number of layers of graphene is 16 and 27, respectively, while only a single layer of MoS 2 has been used. The sensitivity of the sensor in the Au/graphene/MoS 2 configuration is 5.0 μm/RIU with comparatively high depth of resonance.

A small amount of carbon nanotubes (CNTs) was added into poly(vinylidene fluoride) (PVDF)/boron nitride (BN) composites through melt blending processing. The thermal conductivity, microstructure changes including the crystallization behavior of PVDF matrix and the dispersion states of fillers in the composites, and the electrical conductivity of the composites were comparatively investigated. The results demonstrated that compared with the PVDF/BN composites at the same BN content, the ternary PVDF/BN/CNT composites exhibited largely enhanced thermal conductivity. In the PVDF/BN/CNT composites, the crystallinity of the PVDF matrix was slightly increased while the crystal form remained invariant. BN particles exhibited homogeneous dispersion in the PVDF/BN composites, and they did not affect the rheological properties of the PVDF/BN composites when the BN content was lower than 10 wt %. The presence of CNTs did not affect the interfacial adhesion between BN and PVDF, but they facilitated the formation of denser BN/CNT network structure in the composites. The mechanisms were then…

Mo 2 C, the newly synthesized MXene with a large lateral size and superconductivity property, has attracted increasing interest in material science. Employing first-principles density functional calculations, its intrinsic structural, electrical, thermal, and mechanical properties are investigated in this work. It is found that this MXene is nonmagnetic with a small molar volume. The electrical conductivity is predicted in the order of 10 6 Ω –1 m –1, and its value is significantly influenced by doping. For thermal conductivity, both of the electron and phonon contributions are studied. At room temperature, the Mo 2 C’s thermal conductivity is determined to be 48.4 Wm –1 K –1, which can be further enhanced by increasing temperature and introducing n-type dopants. The specific heat and thermal expansion coefficient are also assessed, and their values at room temperature are calculated as 290 Jkg –1 K –1 and 2.26 × 10 –6 K –1, respectively. Moreover, the thermal contraction of the MXene is found at low temperatures. Under biaxial strains, the elastic modulus is predicted as 312 ± 10…

The use of plasmonic metal nanoparticles as photosensitizers has undergone a strong development in the past few years given their ability to increase the activity of semiconductors into the visible and near-infrared regions. The present work reports an experimental and theoretical study on the critical influence that shape anisotropy of gold nanoparticles exerts on the photocatalytic performance of Au–TiO 2 nanoarchitectures. The obtained results show that for a given amount of metallic material, Au nanostars endow titania with a strongly enhanced catalytic efficiency compared to that found in the presence of Au nanospheres or nanorods. This is ascribed to the ability of nanostars to locally create extremely large electromagnetic field enhancements around their spikes, which ensures an increased population of hot electrons close to the interface between the metal and the semiconductor. Therefore, these nanostructures exhibit a novel regime of photocatalytic activity that could be described as plasmonic hot-spot photocatalysis. Numerical simulations confirm that the hot electron…

Solvatochromic shifts in nitrogen-doped and nitrogen–sulfur-co-doped carbon dots are studied by analyzing absorption, photoluminescence excitation and photoluminescence emission spectra, and their emission lifetimes in two different solvents, protic water (H 2 O) and aprotic dimethyl sulfoxide (DMSO). We identify three emission bands belonging to the sp 2 -hybridized core, the edge, and the functional surface groups of carbon dots, as well as surface-attached fluorophores that emit within the edge band energy range. Edge and surface bands show opposite solvatochromic shifts solely depending on the doping heteroatoms. We are able to reproduce emission shifts observed in DMSO by heating CDs in H 2 O from 7 to 87 °C, when the polarity and hydrogen-bonding strength of the solvent are reduced. Intrinsic edge band transitions are found to be strongly influenced by the solvent polarity, as charge transfer processes dominate. Surface band transitions are found to be influenced especially by hydrogen bonding between the carbon dots and the solvent. Together, these processes lead to…

This study reports Raman and infrared spectra of hybrid organic–inorganic MAPbX 3 perovskites (MA = CH 3 NH 3, X = Cl, Br, I) and their mixed-halide derivatives. Raman spectra were recorded at three laser wavelengths (514, 785, and 1064 nm) under on- and off-resonance conditions, as well as at room temperature and 100 K. The use of different excitation wavelengths allowed the unambiguous acquisition of “true” Raman spectra from the perovskites, without degradation or photoinduced structural changes. Low-frequency PbX vibrational modes were thoroughly identified by comparison of Raman and far-IR results. Red Raman frequency shifts for almost all MA vibrations from 200 to 3200 cm –1, and particularly intense for the torsional mode, were observed toward heavy halide derivatives, indicative of strengthening the interaction between halides and the organic cation inside the inorganic cage. Different MA–X bonding schemes are evidenced by torsional mode pairs emerging in the orthorhombic phase. MAPbBr 3 was further characterized by variable temperature Raman measurements (100–295 K).…

The role of the interface between a metal and oxide (CeO x –Cu and ZnO–Cu) is critical to the production of methanol through the hydrogenation of CO 2 (CO 2 + 3H 2 → CH 3 OH + H 2 O). The deposition of nanoparticles of CeO x or ZnO on Cu(111), θ oxi < 0.3 monolayer, produces highly active catalysts for methanol synthesis. The catalytic activity of these systems increases in the sequence: Cu(111) < ZnO/Cu(111) < CeO x /Cu(111). The apparent activation energy for the CO 2 → CH 3 OH conversion decreases from 25 kcal/mol on Cu(111) to 16 kcal/mol on ZnO/Cu(111) and 13 kcal/mol on CeO x /Cu(111). The surface chemistry of the highly active CeO x –Cu(111) interface was investigated using ambient pressure X-ray photoemission spectroscopy (AP-XPS) and infrared reflection absorption spectroscopy (AP-IRRAS). Both techniques point to the formation of formates (HCOO – ) and carboxylates (CO 2 δ− ) during the reaction. Our results show an active state of the catalyst rich in Ce 3+ sites which stabilize a CO 2 δ− species that is an essential intermediate for the production of methanol. The inverse…

In situ surface-enhanced spectroscopic and reactivity investigations of the electrochemical reduction of CO 2 at low overpotentials (<0.7 V) was conducted on Cu surfaces. Vibrational bands corresponding to adsorbed hydrogen (H ad ) and carbon monoxide (CO ad ) on Cu have been identified at 2090 and 2060 cm –1, respectively. Spectroscopic investigations show that H ad is capable of partially displacing CO ad; however, CO ad is unable to displace H ad to any detectable level. The preferential adsorption of H over CO on Cu is consistent with the high selectivity toward the hydrogen evolution reaction at potentials >−0.8 V versus reversible hydrogen electrode.

Using picosecond excitation at 1064 nm, surface-enhanced hyper-Raman scattering (SEHRS) spectra of the nucleobases adenine, guanine, cytosine, thymine, and uracil with two different types of silver nanoparticles were obtained. Comparing the SEHRS spectra with SERS data from the identical samples excited at 532 nm and with known infrared spectra, the major bands in the spectra are assigned. Due to the different selection rules for the one- and two-photon excited Raman scattering, we observe strong variation in relative signal strengths of many molecular vibrations obtained in SEHRS and SERS spectra. The two-photon excited spectra of the nucleobases are found to be very sensitive with respect to molecule-nanoparticle interactions. Using both the SEHRS and SERS data, a comprehensive vibrational characterization of the interaction of nucleobases with silver nanostructures can be achieved.

This paper reports the successful synthesis of various copper sulfide nanostructures via coprecipitation and hydrothermal routes using new starting reagents such as Na 2 SO 3 as a reducing agent for converting Cu 2+ to Cu + and PMP dye–Cu(II) and carminic acid–Cu(II) complexes as new copper precursors for synthesizing quantum dots in aqueous medium. The as-synthesized products were extensively characterized by techniques including XRD, EDS, SEM, TEM, AFM, and DRS. Effects of different parameters such as temperature, surfactant, solvent, concentration, copper precursor, sulfide source, etc., on morphology and particle size of as-synthesized nanostructures were investigated. Moreover, the efficiency of various as-synthesized nanostructures in thin layer solar cells was evaluated. The results showed that particle size and morphology have a salient effect on solar cell efficiency. Also, utilizing prepared Cu 2 S quantum dots as a barrier layer in dye-sensitized solar cells (DSSCs) presented a remarkable increase in the efficiency of solar cells from 6.08% to 8.34% (∼37% improvement).

The electrochemical reduction of CO 2 is a promising way to store renewable energy in fuels or other chemicals. However, the high overpotential and low efficiency of the reaction hinder the development of the area. More work is needed on the investigation of the mechanism in order to obtain new insights into developing efficient catalysts. We report here a density functional theory (DFT) study of the electrochemical reduction of CO 2 on cobalt porphyrin. The CO 2 – anion adduct is demonstrated to be the key intermediate formed only when the cobalt center of the complex is in the Co I oxidation state. We find that formic acid can be produced as minor product through a [Co(P)–(OCHO)] intermediate, while CO is the main product through a decoupled proton–electron transfer. CH 4 is produced as minor product from subsequent CO reduction by concerted proton-coupled electron transfers assumed for each electrochemical step. Our theoretical interpretations are consistent with the experimental results presented in our recent experiments and give deeper insights into the mechanism of the CO 2…

A combination of N 1s X-ray photoelectron spectroscopy (XPS) and first principles calculations of nitrogen-containing model electrocatalysts was used to elucidate the nature of the nitrogen defects that contribute to the binding energy (BE) range of the N 1s XPS spectra of these materials above ∼400 eV. Experimental core level shifts were obtained for a set of model materials, namely N-doped carbon nanospheres, Fe–N–carbon nanospheres, polypyrrole, polypyridine, and pyridinium chloride, and were compared to the shifts calculated using density functional theory. The results confirm that the broad peak positioned at ∼400.7 eV in the N 1s XPS spectra of N-containing catalysts, which is typically assigned to pyrrolic nitrogen, contains contributions from other hydrogenated nitrogen species such as hydrogenated pyridinic functionalities. Namely, N 1s BEs of hydrogenated pyridinic-N and pyrrolic-N were calculated as 400.6 and 400.7 eV, respectively, using the Perdew–Burke–Ernzerhof exchange-correlation functional. A special emphasis was placed on the study of the differences in the XPS…

CdS is one of the most well-known and important visible-light photocatalytic materials for water splitting to produce hydrogen energy. Owing to its serious photocorrosion property (poor photoinduced stability), however, CdS photocatalyst can unavoidably be oxidized to form S 0 by its photogenerated holes, causing an obviously decreased photocatalytic performance. In this study, to improve the photoinduced stability of CdS photocatalyst, amorphous TiO 2 (referred to as Ti(IV)) as a hole cocatalyst was successfully loaded on the CdS surface to prepare Ti(IV)/CdS photocatalysts. It was found that the resultant Ti(IV)/CdS photocatalyst exhibited an obviously enhanced photocatalytic stability, namely, its deactivation rate clearly decreased from 37.9% to 13.5% after five cycles of photocatalytic reactions. However, its corresponding photocatalytic activity only showed a very limited increase (ca. 37.4%) compared with the naked CdS. To further improve its photocatalytic performance, the amorphous Ni(II) as an electron cocatalyst was subsequently modified on the Ti(IV)/CdS surface to…

Abstract The interactions between a Cu-based metal–organic framework (MOF), Cu-BTC, and an ionic liquid (IL), 1-ethyl-3-methylimidazolium ethyl sulfate, were studied by employing density functional theory (DFT) calculations and vibrational spectroscopy. The Fourier transform infrared (FTIR) and Raman spectra show that the confinement of the IL in the MOF has significant impact on the structure of the MOF as well as on the IL. Raman spectra and DFT calculations reveal a perturbation of the symmetry of the MOF structure due to the interaction of the IL anion with the Cu ions. FTIR and Raman spectra show that the molecular interactions in turn influence the structure of the ion pair. Inside the MOF, two different types of structure of IL ion pairs are formed. One ion-pair structure exhibits enhanced interionic interactions by strengthening the hydrogen bonding between cation and anion, whereas the other structure corresponds to weaker interactions between the IL cation and anion. Moreover, it is shown that the IL imidazolium ring can directly interact with either the MOF or the anion.…

In this work, we made a comprehensive investigation to unravel the underlying causes for the selectivity of CO 2 electroreduction toward ethylene on Cu 2 O-derived Cu catalysts. Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, chronopotentiometry, online gas chromatography, nuclear magnetic resonance spectroscopy, and numerical simulations of local pH were used toward this end. Ten Cu 2 O-derived Cu films of different thicknesses and morphologies were prepared and extensively characterized. Aqueous 0.1 M KHCO 3 was used as the electrolyte. We report here, for the first time, a remarkably strong correlation between the statistically relevant crystallite sizes of the Cu 2 O-derived Cu particles and selective CO 2 electroreduction to C 2 H 4 . Specifically, as the crystallite size of the particles decreased from 41 to 18 nm, the Faradaic efficiency (FE) of C 2 H 4 formation increased from 10 to 43%. Using cyclic voltammetry, samples with smaller particle crystallite sizes were found to possess more diverse adsorption sites for CO (a known reaction…

Magnetic hyperthermia treatment requires biocompatible magnetic nanoparticles with improved heating capacities to become a viable clinical method for cancer treatment. Although small superparamagnetic iron oxide nanoparticles under low fields have been favored (linear response theory regime), these nanoparticles present a series of limitations, including relatively low heating efficiency (specific absorption rate or SAR), that need to be overcome to make magnetic hyperthermia an efficient clinical application. We here show that by modifying the shape into deformed cubes (octopods) and tuning their size, their SAR can be greatly increased up to 70% (from 140 to 240 W/g). By using nonhydrolytic thermal decomposition, we have obtained highly crystalline monodisperse nano-octopods for different sizes (17–47 nm), and their heating response has been extensively studied in a wide range of AC fields (20–800 Oe) using combined calorimetric and AC magnetometry experiments. Our results consistently reveal that at AC fields (≤300–400 Oe), the nano-octopods with the smallest size (17 nm)…

New and improved dielectric materials with high dielectric breakdown strength are required for both high energy density electric energy storage applications and continued miniaturization of electronic devices. Despite much practical significance, accurate ab initio predictions of dielectric breakdown strength for complex materials are beyond the current state-of-the art. Here we take an alternative data-enabled route to address this design problem. Our informatics-based approach employs a transferable machine learning model, trained and validated on a limited amount of accurate data generated through laborious first-principles computations, to predict intrinsic dielectric breakdown strength of several hundreds of chemical compositions in a highly efficient manner. While the adopted approach is quite general, here we take up a specific example of perovskite materials to demonstrate the efficacy of our method. Starting from several thousands of compounds, we systematically downselect 209 insultors which are dynamically stable in a perovskite crystal structure. After making predictions…

In this work, phase stability of Ni-rich LiNi 1–2x Co x Mn x O 2 (NCM) ( x < 0.20) is investigated by means of a bond model based on the effective interaction of transition metal (TM) ions (represented as TM–TM bond), fitted to results obtained within the DFT+U framework. The developed bond model reveals the intrinsic relationship between phase stability and TM–TM bonds, which explains the different roles of TM ions in the phase stability of Ni-rich NCM. A sequence of TM–TM bond strengths (Mn 4+ Mn 4+ > Ni 2+ Mn 4+ > Ni 3+ Mn 4+ > Co 3+ Mn 4+ > Co 2+ Mn 4+ > Ni 2+ Ni 4+ ) is then predicted by bond model and subsequently used to understand the intricate layered LiTMO 2 (TM = Ni, Co, Mn) phase triangle. Our results also show that Co and Mn ions segregate to form clusters within the Ni environment when x < 0.1 (Ni ≥ 80 at. %), and such segregation is responsible for the electrochemical degradation during cycling. The obtained results agree excellently with the validation experiment in the present work and also other experiments in the literature, and could help to clarify the existing…

Solid polymer electrolytes are important materials in realizing safe and flexible energy storage devices. The present study looks at correlation between solvation structure and the ion-conductive behavior of poly(ethylene carbonate) (PEC)/lithium bis(fluorosulfonyl)imide (LiFSI) electrolytes which have high Li transference number ( t + ) and show unusual salt-concentration dependence of conductivity. From FT-IR and Raman spectroscopy, we determined that Li ions interact with carbonyl (C═O) groups and also with FSI ions, which can be referred to as contact ion pair or aggregate. 7 Li magic-angle-spinning NMR spectroscopy and density functional theory calculations for model species suggest that a loose coordination structure, in which Li ions interact with C═O groups and FSI ions with appropriate strength, allows the electrolytes to have both reasonable conductivity and high t + with a flexible and transparent character. A high salt dissociation rate is generally considered essential in SPEs, but the presence of aggregated ions having the loose coordination structure gives rise to…

Using first-principles calculations combined with ab initio molecular dynamics and tight binding model, we predict the existence of a kinetically stable two-dimensional (2D) penta-CN 2 sheet, which is isostructural to the recently discovered penta-graphene. The concentration of N in this new carbon nitride sheet exceeds the maximum N content, namely 21.66%, that has been achieved experimentally in honeycomb geometry. It even exceeds the N content found recently in hole-doped carbon nitride C 0.5 N 0.5 as well as in porous graphitic C 3 N 4 . The penta-CN 2 sheet contains N–N single bonds with an energy density of 4.41 kJ/g, higher than that predicted recently in nitrogen-rich B–N compound. Remarkably, penta-CN 2 has an in-plane axial Young’s modulus of 319 N/m, even stiffer than the h -BN monolayer. The electronic band structure of penta-CN 2 exhibits an interesting double degeneracy at the first Brillouin zone edges which is protected by the nonsymmorphic symmetry and can be found in other isostructural chemical analogues. The band gap of penta-CN 2 calculated using HSE06…

Active Pd nanoclusters smaller than 1.2 nm encapsulated inside the cage of NH 2 –Uio-66(Zr) (Pd@NH 2 –Uio-66(Zr)) were prepared via a double-solvent approach combined with a photoreduction process. The resultant Pd@NH 2 –Uio-66(Zr) showed excellent performance for Suzuki coupling reaction under visible-light irradiations as a result of the efficient electron transfer from the light-excited NH 2 –Uio-66(Zr) to the confined Pd nanoclusters and the presence of large amounts of catalytic active Pd species. The successful coupling of MOF-based photocatalysis with metal-based catalysis over metal@MOFs is expected to bring about vast opportunities for developing active catalysts for a variety of light-induced organic transformations attributed to the diversified MOF structures and their tunable characteristics.

The amount of anthropogenic CO 2 emission keeps increasing worldwide, and it urges the development of efficient CO 2 capture technologies. Among various CO 2 capture methods, adsorption is receiving more interest, and carbonaceous materials are considered good CO 2 adsorbents. There have been many studies of N-containing carbon materials that have enhanced surface interaction with CO 2; however, various N-containing functional groups existing in the carbon surface have not been investigated in detail. In this study, first-principle calculations were conducted for carbon models having various N-functional groups to distinguish N-containing heterogeneity and understand carbon surface chemistry for CO 2 adsorption. Among N-functional groups tested, the highest adsorption energies of −0.224 and −0.218 eV were observed in pyridone and pyridine groups, respectively. Structural parameters including bond angle and length revealed an exceptional hydrogen-bonding interaction between CO 2 and pyridone group. Charge accumulation on CO 2 during interaction with pyridine-functionalized surface…

Blue Ti 3+ self-doped TiO 2 nanoparticles with mixed phases of anatase and rutile have been synthesized via a facile solvothermal method. The phase evolution of the blue TiO 2 varying from anatase to rutile has been studied by simply controlling the initial volume ratios of TiCl 3 to titanium isopropoxide in the reaction solutions. The blue TiO 2 has a distinct improvement of visible light harvesting and remarkable enhancement in photocatalytic activity, which could be ascribed to the presence of Ti 3+ centers and the synergetic effect between the two phases. In addition, the as-prepared blue TiO 2 shows an excellent stability during the photocatalytic reactions. A photocatalytic mechanism of the self-doped blue TiO 2 was proposed at last.

CO 2 has a potentially bright future as a carbon resource because it is very cheap and abundant. The conversion technology of CO 2 into useful chemicals therefore has gained growing attention over recent years. Despite many attempts, there have not yet been revolutionary successes for commercialization of such technology. One of the main challenges in this field is to catalytically activate the CO 2 molecule on the surfaces of catalysts. Although many researchers have studied the catalytic reactions involving CO 2 on the surfaces, the activation process of CO 2 is still controversial. Here, we performed density functional theory calculations to understand the CO 2 activation and dissociation on a wide range of bimetallic alloy surfaces. To begin with, the adsorption process of CO 2 on pure metal surfaces was carefully examined with the analyses of adsorption energetics, geometries, vibrational frequencies, charge transfers, and density of states. From the activated CO 2 on the surfaces, we could precisely capture the transition state of the dissociation reaction. On the basis of the…

Metal–organic frameworks (MOFs) have attracted significant interest as a class of adsorbent materials for gas storage applications, including hydrogen storage for fuel cell vehicles. Here, we evaluated 137 953 hypothetical MOFs for hydrogen storage at cryogenic conditions (77 K) by determining the deliverable storage capacity between 100 and 2 bar, using grand canonical Monte Carlo simulations. The highest predicted volumetric capacity for a structure in this study is 50 g/L, and the highest gravimetric capacity is almost 25 wt %. We find that the optimal void fraction is 0.9 and the optimal pore diameter is 12–15 Å. MOFs with larger pores have significant regions in the center of the pore with low hydrogen density, which lowers the storage efficiency. We give examples of MOFs with very large pores in which the hydrogen capacity can potentially be increased using catenation or functional groups to reduce the pore size and increase the surface area per volume. We also introduce a screening parameter, the binding fraction, which is the fraction of the unit cell volume within a given…

In this paper, we reported that oxygen vacancies could be introduced in tungsten oxide hierarchical nanostructures through air treatment at certain temperatures. The production of oxygen vacancies may be due to two mechanisms, i.e., critical phase transition and nanoscale inhomogeneous deformation, depending on the annealing temperature or time and the size of the building block. The oxygen vacancies can be introduced at 300 and 350 °C when critical phase transformation from orthorhombic WO 3 ·0.33H 2 O to hexagonal WO 3 takes place or 350 and 400 °C when nanoscale inhomogeneous deformation occurs in the nanobelts. Moreover, the oxygen vacancy concentration is also influenced by the annealing time. For comparison, the oxygen vacancies are also introduced by hydrogen treatment. It is found that a certain amount of oxygen vacancies introduced by air treatment could trap and transfer electrons, thus decreasing the electron–hole recombination rate and improving the conductivity, while an abundance of oxygen vacancies introduced by hydrogen treatment could facilitate the electron–hole…

CO and CO 2 chemisorption on uniform CeO 2, TiO 2, and Cu 2 O nanocrystals with various morphologies were comprehensively studied with in-situ diffuse reflectance infrared Fourier transform spectroscopy. The formed adsorbates were observed to be morphology dependent. CO or CO 2 chemisorbed at the metal cation sites, and bidentate and bridged carbonates involving the O sites are sensitive to the surface composition and the local coordination environments of surface metal cations and O anions and can be correlated well with the surface structures of facets exposed on oxide nanocrystals. Carbonate and carbonite species formed by CO chemisorption can probe the different facets of CeO 2 . Carbonate species formed by CO chemisorption can probe the different facets of TiO 2 . Adsorbed CO and carbonate species formed by CO chemisorption can probe the different facets of Cu 2 O, and adsorbed CO 2 formed by CO 2 chemisorption can also probe the different facets of Cu 2 O. These results demonstrate chemisorption of probing molecules as a convenient technique to identify surface structures of…

Erbium (Er 3+ ) substituted nanocrystalline, cobalt-rich ferrites, which can be represented chemically as Co 1.1 Fe 1.9– x Er x O 4 (CFEO; x = 0.0–0.2), were synthesized by the sol–gel autocombustion method. The structural, dielectric, and electrical transport properties of CFEO were investigated in detail. CFEO materials crystallize in a spinel cubic structure for x ≤ 0.10; formation of orthoferrite (ErFeO 3 ) secondary phase was noted for x ≥ 0.15. Microstructural and compositional studies revealed the formation of spherical, elongated grains with stoichiometric presence of Co, Fe, Er, and O. The dielectric constant (ε′) dispersion fits to the Debye’s function for all CFEO ceramics. The relaxation time and spread factor obtained from ε′ dispersion are ∼10 –3 s and ∼0.50 (±0.10), respectively. The complex impedance analyses confirm a grain-interior mechanism contributing to the dielectric properties. Semiconducting behavior and small polaron conduction mechanism were evident in electrical transport properties of CFEO materials.

Although TiO 2 is generally considered to be an oxygen deficient n-type compound, the role of oxygen vacancies and Ti 3+ ions on its photocatalytic activity is not fully understood. In this study, we investigated the effects of high-temperature calcination and H 2 reduction treatment on the water oxidation activity of rutile TiO 2 under ultraviolet irradiation. Calcination above 900 °C decreased the photocatalytic activity of the TiO 2 owing to strong oxidation, but its initial activity was restored by H 2 treatment at above 500 °C. Electron spin resonance (ESR) spectra showed that the high-temperature calcination created O •– radicals (trapped hole in oxygen lattice site), while the H 2 reduction treatment created Ti 3+ ions (trapped electron in titanium lattice site) with oxygen vacancies. Diffuse reflectance ultraviolet–visible–near-infrared (UV–vis–NIR) spectroscopy indicated an increase in the amount of electrons in shallow traps and the conduction band with H 2 treatment temperature. Measurements of the sheet resistance and space charge layer capacitance of the thermally…

The characteristic differences between MoS 2 and WS 2 nanosheets and nanodots are investigated. The nanosheets were formed by liquid-phase sonication, while the nanodots were formed by breaking the nanosheets through heating the solvent ethylene glycol. The nanosheets and nanodots were approximately 0.7–2 nm thick, with slight deviation. Most of the nanosheets were longer than 100 nm, and most of the nanodots were shorter than 5 nm. As the bulk materials were transformed into nanosheets and/or nanodots, the absorption peaks and Raman peaks shifted to shorter wavelengths. Photoluminescence peaks were observed at 500 and 445 nm in the MoS 2 and WS 2 samples smaller than 100 nm. In the X-ray diffraction spectra, only the (002) peak was present in the nanosheets, while no peak was detected for the nanodots due to their small size. No detectable differences between the nanosheets and nanodots were observed in the transmission electron micrographs, synchrotron radiation photoemission spectra, or work function measurements, suggesting that exfoliation did not affect the crystal structure…

Redox pairs of transition metal complexes are often involved in small molecule activation in response to modern energy challenges as well as in other areas of electrocatalysis. Within such a family of molecular electrocatalysts, ligand substitution is a means of varying catalytic efficiency, best gauged through catalytic Tafel plots relating overpotential and turnover frequency. In practice, efficient molecular catalysis involves multielectron–multistep processes. It is in this framework that we discuss through-structure inductive substituent effects. What the best choice is for the reference thermodynamic index, how the global substituent effect may be expressed as a function of this index, and how it may be dissected into individual effects assigned to each of the reaction steps are challenging questions that are addressed and resolved here for the first time. The discussion is illustrated by the effect of successive phenyl perfluoration and of o, o ′-methoxy substitution of the Fe I/0 tetraphenylporphyrin catalysts of the CO 2 -to-CO electrochemical conversion. Consequences on…

In this work, we perform a first-principles study of graphene, nitrogen-, boron-doped graphene, and codoped graphene as the potential catalysts in nonaqueous lithium–oxygen (Li–O 2 ) batteries. Among the samples studied, boron-doped graphene exhibits the lowest discharge and charge overpotentials, suggesting that boron-doped graphene is the best catalyst for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in nonaqueous Li–O 2 batteries. Another significant finding is that codoping of nitrogen and boron atoms does not enhance the ORR/OER in the presence of lithium atoms, indicating that the synergistic effect in the presence of protons does not appear in nonaqueous Li–O 2 batteries. This behavior is attributed to the fact that the existence of lithium atoms can change the most stable adsorption sites and adsorption energies of intermediates. Finally, on the basis of our calculation results, we propose that the adsorption energy of intermediates in the rate-determining step (RDS) can be the descriptor of the overpotential, and the lower adsorption…

Fast ion transport is essential for high rate capability in rechargeable battery operation. Recently, an ultrafast rechargeable aluminum-ion battery was experimentally demonstrated through the reversible intercalation/deintercalation of chloroaluminate anions (AlCl 4 – ) in graphitic-foam cathodes. Using first-principles calculations, herein, we report that the unique structural characteristic of graphitic foam, i.e., mechanical flexibility of few-layered graphene nanomaterials, plays a key role for the ultrafast aluminum-ion battery. We found that AlCl 4 – is stored by forming doubly stacked ionic layers in the interlayer space between graphene sheets, and their diffusivity increases dramatically once graphene film is less than five layers thick; the diffusivity begins to increase when the film thickness reduces below five layers in such a way that the film thickness of four, three, and two graphene layers enables 48, 153, and 225 times enhanced diffusivity than that of the bulk graphite, respectively, and this nanoscale thickness is mainly responsible for the observed ultrafast…

Mineralization of bone and teeth involves interactions between biomolecules and hydroxyapatite. Associated complex interfaces and processes remain difficult to analyze at the 1 to 100 nm scale using current laboratory techniques, and prior apatite models for atomistic simulations have been limited in the representation of chemical bonding, surface chemistry, and interfacial interactions. In this contribution, an accurate force field along with pH-resolved surface models for hydroxyapatite is introduced to represent chemical bonding, structural, surface, interfacial, and mechanical properties in quantitative agreement with experiment. The accuracy is orders of magnitude higher in comparison to earlier models and facilitates quantitative monitoring of inorganic-biological assembly. The force field is integrated into the AMBER, CHARMM, CFF, CVFF, DREIDING, GROMACS, INTERFACE, OPLS-AA, and PCFF force fields to enable realistic simulations of apatite-biological systems of any composition and ionic strength. Specific properties that are reproduced well in comparison to experiment include…

Discharging of the aprotic Li–O 2 battery relies on the oxygen reduction reaction (ORR) producing Li 2 O 2 in the positive electrode, which remains incompletely understood. Here, we report a mechanistic study of the Li-ORR on a model system, i.e., an Au electrode in a Li + dimethyl sulfoxide (DMSO) electrolyte. By spectroscopic identification of the reaction intermediates coupled with density functional theory calculations, we conclude that the formation of O 2 – and LiO 2 in the Li-ORR critically depends on electrode potentials and determines the Li 2 O 2 formation mechanism. At low overpotentials (> 2.0 V vs Li/Li + ) O 2 – is identified to be the first surface intermediate, which diffuses into the bulk electrolyte and forms Li 2 O 2 therein via a solution-mediated disproportionation mechanism. At high overpotentials (ca. 2.0–1.6 V vs Li/Li + ) LiO 2 has been observed, which can rapidly transform to Li 2 O 2 by further electro-reduction, suggesting a surface-mediated mechanism. The solution-mediated Li 2 O 2 formation that can account for the widely observed toroid-shaped…

The three-dimensional (3D) MoS 2 hierarchical nanospheres which assembled spontaneously by two-dimensional (2D) lamina have been successfully designed and fabricated in large scale via a simple hydrothermal process. Subsequently, the electromagnetic wave absorption properties of hierarchical MoS 2 nanospheres compounded with polyvinylidene fluoride (PVDF) were investigated in a broad frequency range of 2–40 GHz. The results indicated that the MoS 2 /PVDF nanocomposites possess adjustable and enhanced wave absorption performance. Furthermore, the MA performance can be effectively tuned by the absorber’s thickness and filler content. In addition, the peculiar hierarchical nanostructure of MoS 2 is beneficial to microwave absorption property compared with the bulk MoS 2 and micrometer-sized MoS 2 . Moreover, the main microwave absorption mechanism including various polarization, destructive interference theory, and multiple reflection has been described in detail.

Abstract With the recent explosion in computational catalysis and related microkinetic modeling, the need for a fast yet accurate way to predict equilibrium and rate constants for surface reactions has become more important. Here we present a fast and accurate new method to estimate the partition functions and entropies of adsorbates based on quantum mechanical estimates of the potential energy surface. As with previous approaches, it uses the harmonic oscillator (HO) approximation for most of the modes of motion of the adsorbate. However, it uses hindered translator and hindered rotor models for the three adsorbate modes associated with motions parallel to the surface and evaluates these using an approach based on a method that has proven accurate in modeling the internal hindered rotations of gas molecules. The adsorbate entropies were calculated with this method for four adsorbates (methanol, propane, ethane, and methane) on Pt(111) using density functional theory (DFT) to evaluate the potential energy surface and are shown to be in very good agreement with experiments, better…

Abstract Full derivations of Heyrovsky–Volmer (HV), Tafel–Volmer (TV), Heyrovsky–Tafel (HT), and Heyrovsky–Tafel–Volmer (HTV) mechanisms under steady state conditions are provided utilizing a new theoretical framework which allows better understanding of each of the mechanistic currents and part currents. Simple and easily implemented equations are presented, which provide both the hydrogen coverage and electrochemical current as a function of overpotential and relevant kinetic parameters. It is shown how these responses are governed by a set of dimensionless parameters associated with the ratio of electrokinetic parameters. For each of the different mechanisms, an “atlas” of Hads coverage with overpotential and corresponding current density is provided, allowing an understanding of all possible responses depending on the dimensionless parameters. Analysis of these mechanisms provides the limiting reaction orders of the exchange current density for protons and bimolecular hydrogen for each of the different mechanisms, as well as the possible Tafel slopes as a function of the…

The focus of the present article is on understanding the insight that X-ray photoelectron spectroscopy (XPS) measurements can provide when studying self-assembled monolayers. Comparing density functional theory calculations to experimental data on deliberately chosen model systems, we show that both the chemical environment and electrostatic effects arising from a superposition of molecular dipoles influence the measured core-level binding energies to a significant degree. The crucial role of the often overlooked electrostatic effects in polar self-assembled monolayers (SAMs) is unambiguously demonstrated by changing the dipole density through varying the SAM coverage. As a consequence of this effect, care has to be taken when extracting chemical information from the XP spectra of ordered organic adsorbate layers. Our results, furthermore, imply that XPS is a powerful tool for probing local variations in the electrostatic energy in nanoscopic systems, especially in SAMs.

Cesium and methylammonium bismuth iodides (Cs 3 Bi 2 I 9 and MA 3 Bi 2 I 9 ) are new low-toxic and air stable compounds in the perovskite solar cell family with promising characteristics. Here, the electronic structure and the nature of their optical transitions, dielectric constant, and charge carrier properties are assessed for photovoltaic applications with density functional theory (DFT) calculations and experiments. The calculated direct and indirect band gap values for Cs 3 Bi 2 I 9 (2.17 and 2.0 eV) and MA 3 Bi 2 I 9 (2.17 and 1.97 eV) are found to be in good agreement with the experimental optical band gaps (2.2, 2.0 eV and 2.4, 2.1 eV for Cs 3 Bi 2 I 9 and MA 3 Bi 2 I 9, respectively) estimated for solution-processed films. There is an error cancelation in the DFT calculated band gap similar to that for lead perovskites. However, fully relativistic DFT calculations indicate that the size of the spin orbit coupling (SOC) error cancelation for bismuth perovskite (0.5 eV) is less than for lead perovskite (1 eV), and other factors are therefore also important. Band structure…

Lithium ion batteries (LIB) can feature reactive anodes that operate at low potentials, such as lithium metal or silicon, passivated by solid electrolyte interphase (SEI) films. SEI is known to evolve over time as cycling proceeds. In this modeling work, we focus on the stability of two main SEI components, lithium carbonate (Li 2 CO 3 ) and lithium ethylene dicarbonate (LEDC). Both components are electrochemically stable but thermodynamically unstable near the equilibrium Li + /Li(s) potential. Interfacial reactions represent one way to trigger the intrinsic thermodynamic instability. Both Li 2 CO 3 and LEDC are predicted to exhibit exothermic reactions on lithium metal surfaces, and the barriers are sufficiently low to permit reactions on battery operation time scales. LEDC also readily decomposes on high Li-content Li x Si surfaces. Our studies suggest that the innermost SEI layer on lithium metal surfaces should be a thin layer of Li 2 O, the only thermodynamically and kinetically stable component (in the absence of a fluoride source). This work should also be relevant to…

Abstract The mechanism of the initial steps of pyrite (100) surface oxidation was investigated in detail by means of density functional theory/plane-wave calculations. Pyrite oxidation is related to many environmental and technological issues, and its mechanism has not been completely understood. A chemical picture of the pyrite oxidation process in the presence of oxygen and water was proposed in the present investigation. The reaction steps of the oxidation mechanism can be separated into two types. Type I reactions present lower activation energies and are redox processes that involve oxidation of two Fe(II) sites on the surface to form predominantly the Fe(III)–OH–. This species is formed from hydrogen transfer between the adsorbed water to the adsorbed oxygen molecule on the Fe(II) sites. Type II reactions present higher activation energies and lead to the formation of a S═O bond through the hydrogen atom transference from a water molecule to the Fe(III)–OH– species, forming Fe(II)–OH2. These reactions present higher activation energies. The determinant step of this oxidation…

The use of high-concentration salts in electrolyte solutions of lithium–sulfur (Li–S) batteries has been shown to be beneficial for mitigating some effects such as polysulfide shuttle and dendrite growth at the Li metal anode. Such complex solutions have structural-, dynamical-, and reactivity-associated issues that need to be analyzed for a better understanding of the reasons behind such beneficial effects. A passivation interfacial layer known as solid–electrolyte interphase (SEI) is generated during battery cycling as a result of electron transfer from the metal anode causing electrolyte decomposition. Here, using density functional theory and ab initio molecular dynamics simulations, we investigate the salt decomposition, solvation effects, interactions among intermediate products and other species, and potential components of the SEI layer as a function of chemical nature and concentration of the salt for lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI) at 1 and 4 M concentrations in dimethoxyethane. It is found that LiTFSI…

The photoactivity of semiconductor nanostructures makes them potentially useful for environmental remediation and antibacterial applications. Understanding the mechanism underlying the photochemical and photobiological activities of photoexcited semiconductors is of great importance for developing applications and assessing associated risks. In the current work, using electron spin resonance spectroscopy coupled with spin trapping and spin labeling techniques, we comparatively and systematically investigate the abilities of ZnO and ZnS to generate hydroxyl radical, superoxide, singlet oxygen, photoinduced electrons, and oxygen consumption during irradiation. It was found that although ZnO and ZnS, when photoexcited, can produce hydroxyl radical, superoxide, and singlet oxygen, ZnO is more effective than ZnS in producing hydroxyl radical and singlet oxygen while ZnS is more effective than ZnO in generating superoxide. The characterization with ESR spin labeling and oximetry indicates ZnS is about 4 times more active than ZnO in production of photoinduced electrons and consumption of…

The adsorption, dissociation, and diffusion of CO 2 on the anatase (001) surface was studied using DFT by means of the generalized gradient approximation using the Perdew–Burcke–Ernzerhof (PBE)-functional and applying corrections for long-range dispersion interactions. Different stable adsorption configurations were identified for the fully oxidized surface. The most stable adsorption configuration is the monodentated carbonate-like structure. Small energy barriers were identified for the conversion of a physisorbed to a chemisorbed configuration. CO 2 dissociation is found to be unfeasible on the stoichiometric surface. The introduction of oxygen vacancy defects gives rise to new highly stable adsorption configurations with a stronger activation of the C–O bonds. This leads to the possibility of exothermic dissociation of CO 2 with barriers up to 22.2 kcal/mol, corresponding to chemical lifetimes of less than 4 s at 300 K. These reactions cause a CO molecule to be formed, which will easily desorb, and the reduced surface to become oxidized. It is clear that oxygen vacancy defects…

We developed a ReaxFF reactive force field for Al/C interactions to investigate carbon coating and its effect on the oxidation of aluminum nanoparticles (ANPs). The ReaxFF parameters were optimized against quantum mechanics-based (QM-based) training sets and validated with additional QM data and data from experimental literature. ReaxFF-molecular dynamics (MD) simulations were performed to determine whether this force field description was suitable to model the surface deposition and oxidation on complex materials (i.e., carbon-coated ANPs). Our results show that the ReaxFF description correctly reproduced the Al/C interaction energies obtained from the QM calculations and qualitatively captured the processes of the hydrocarbons’ binding and their subsequent reactions on the bare ANPs. The results of the MD simulations indicate that a carbon coating layer was formed on the surface of the bare ANPs, while H atoms were transferred from the hydrocarbons to the available Al binding sites typically without breaking C–C bonds. The growth of the carbon layer depended strongly on the…

Methylammonium lead iodide (MAPbI 3 ) perovskite shows an outstanding performance in photovoltaic devices. However, certain material properties, especially the possible ferroic behavior, remain unclear. We observed distinct nanoscale periodic domains in the piezoresponse of MAPbI 3 (Cl) grains. The structure and the orientation of these striped domains indicate ferroelasticity as their origin. By correlating vertical and lateral piezoresponse force microscopy experiments performed at different sample orientations with X-ray diffraction, the preferred domain orientation is suggested to be the a 1 –a 2 -phase. The observation of these ferroelastic fingerprints appears to strongly depend on the film texture and thus the preparation route. The ferroelastic twin domains could form due to the introduction of strain during the cubic−tetragonal phase transition.

In this work, the photophysics of 3-(9,9-dimethylacridin-10(9H)-yl)-9H-xanthen-9-one (ACRXTN) has been investigated by combined density functional theory and multireference configuration interaction quantum chemical methods. ACRXTN was recently utilized as an assistant dopant in a green emitting organic light-emitting diode (OLED), increasing substantially the external electroluminescence quantum efficiency of the OLED. The efficient triplet-to-singlet upconversion, found experimentally in ARCXTN, cannot be explained solely on the basis of a small singlet–triplet energy gap. We find five interacting electronically excited states in a small energy interval: a charge-transfer excitation (triplet and singlet) from acridine to xanthone, a local xanthone triplet ( ππ *) excitation, and a pair of triplet and singlet states originating from an nπ * excitation on xanthone. On the basis of calculated spin–orbit coupling constants and potential energy profiles, we propose here that the triplet ( ππ *) state mediates the triplet-to-singlet upconversion in ACRXTN and that the carbonyl bond…

Sodium is first introduced to modify Ni-rich LiNi 0.8 Co 0.15 Al 0.05 O 2 cathode material in this work, and Li 1– x Na x Ni 0.8 Co 0.15 Al 0.05 O 2 ( x = 0, 0.01, 0.02, 0.05) is successfully synthesized by using Na 2 CO 3 as sodium resource through coprecipitation and solid state calcination route. The morphology of the samples analyzed with SEM, EDS. and TEM show that all the samples maintain sphere-like morphology and the main elements are uniformly distributed. X-ray diffraction (XRD) results show that all the synthesized materials have typical hexagonal structure without impurities. The lattice parameters calculated from the XRD data are also refined by Rietveld refinement methods, confirming that the position of Na in the NCA is occupying the Li slab as designed. Despite a slight decrease in the initial discharge capacities, the sodium doped materials display improved capacity retention as well as superior performance at high rates. The Li 0.99 Na 0.01 Ni 0.8 Co 0.15 Al 0.05 O 2 exhibits an initial discharge specific capacity of 184.6 mAh g –1 at 0.1 C and a capacity retention…

To enhance the photocatalytic activity of monoclinic BiVO 4 for O 2 evolution from water, Ce-doped BiVO 4 was prepared using the one-pot facile solvothermal method and characterized via XRD, Raman, XPS, and electrochemical impedance spectroscopy (EIS). The XPS spectra confirm that Ce component is Ce 3+ ions instead of Ce 4+ ions. From the structural characterization and the calculations of formation energies it has been stated that the doping of Ce 3+ ions takes place at Bi 3+ sites without changing the host structure. The as-prepared Ce-doped BiVO 4 samples display significantly enhanced photocatalytic O 2 evolution activities from water compared to pristine BiVO 4 . Density of states calculations indicate that Ce 3+ ions act as hole traps, thereby delaying the recombination of photogenerated electrons and holes. The results demonstrate that the substitution of the remaining monoclinic crystal structure may offer an attractive alternative approach for the doping of BiVO 4 to enhance the evolution activity of photocatalytic O 2 .

Abstract The surface functionalization of TiO2-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO2 surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO2. We demonstrate that monomeric APTES chemisorption should proceed through covalent Si–O–Ti bonds. Although dimerization of the silane through Si–O–Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO2, resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through…

A widely used adsorption energy scaling relation between OH* and OOH* intermediates in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), has previously been determined using density functional theory and shown to dictate a minimum thermodynamic overpotential for both reactions. Here, we show that the oxygen–oxygen bond in the OOH* intermediate is, however, not well described with the previously used class of exchange-correlation functionals. By quantifying and correcting the systematic error, an improved description of gaseous peroxide species versus experimental data and a reduction in calculational uncertainty is obtained. For adsorbates, we find that the systematic error largely cancels the vdW interaction missing in the original determination of the scaling relation. An improved scaling relation, which is fully independent of the applied exchange–correlation functional, is obtained and found to differ by 0.1 eV from the original. This largely confirms that, although obtained with a method suffering from systematic errors, the previously obtained scaling…

Oxidative aging causes major changes in asphalt binder’s physiochemical and rheological properties, giving rise to pavement distress and failure. Asphalt aging occurs due to two main processes: a loss of volatile components and a reduction of the malthene phase and oxidation of certain functional groups in asphalt, increasing the concentration of asphalt’s polar components. Performing SARA (saturate, aromatic, resin, and asphaltene) analysis and FTIR (Fourier transform infrared) spectroscopy on the crude oil shows that the ratio of polar components to nonpolar ones is higher in oxidized asphalt compared to virgin asphalt. However, when a biomodifier is introduced to virgin asphalt, the rate of carbonyl (as a polar functional) formation is reduced, which may indicate a delayed oxidation due to the presence of biomodifier molecules in the asphalt. To understand the fundamental origin of oxidative aging at the molecular level, this paper provides a comprehensive computational chemistry analysis conducted in conjunction with laboratory experiments. On the basis of the results of our…

Recent reports focus on the experimental preparation of metal monolayer-stanene which is a zero-gap semiconductor with buckled honeycomb structure. Owing to the outstanding properties of stanene, its promising applications in nanoelectronics are widely concerned and studied. Using the first-principles calculations, we investigate the adsorption behavior of CO, NH 3, H 2 O, H 2 S, O 2, NO, and NO 2 molecules on a stanene sheet based on the energetics, charge transfer, and work function. We determine the optimal adsorption sites of small molecule on the stanene sheet and the styles of molecule doping, and discuss the interaction mechanism between the molecule and stanene. The results indicate that the sensing performance of stanene is superior to other 2D materials such as silicene and germanene. It is found that CO, O 2, NO, and NO 2 molecules act as charge acceptors, whereas NH 3, H 2 O, and H 2 S molecules serve as charge donors. For nonpolarized molecules, the molecule–stanene interaction is mainly ascribed to the electrostatic attractions effect. In contrast, for polarized…

Surface-enhanced Raman spectroscopy (SERS) has attracted tremendous interest as a label-free highly sensitive analytical method. For optimization of SERS activity, it is highly important to systematically investigate the size effect of nanoparticles on the SERS enhancement, which appears to be challenging in experiment, as the localized surface plasmon resonance (LSPR) of nanoparticles also changes with the change of the particle size. This challenge can be overcome by utilizing the unique property of gold nanorods, whose LSPR wavelength can be controlled to be the same by properly choosing the size and aspect ratio of the nanorods. We obtained the correlated SEM images, scattering spectra, and SERS spectra on a home-built single nanoparticle spectroscopy system and systematically investigate the size effect on SERS of individual gold nanorods using the adsorbed malachite green isothiocyanate (MGITC) molecule as the probe molecule. The dark field scattering intensity was found to increase with the increase of the size of nanoparticles, whereas the SERS intensity increases with the…

To improve the gas-sensing performance of metal-oxide-semiconductors, the effect of defects on gas-sensing properties has been widely investigated. Nevertheless, although the metal cation defect is the dominative acceptor defect in p-type semiconductors, its effect on the gas-sensing properties remains blank, which leads to a hindrance for further developing p-type semiconductor-based gas sensors. Accordingly, to eleborate the effect of metal cation defects on the sensing properties, mesoporous NiO nanosheets with different amounts of nickel vacancies were prepared by annealing at different temperatures. It was found that the amount of nickel vacancies increased with increasing the annealing temperature. Gas-sensing studies revealed that the NiO with a higher concentration of nickel vacancies exhibited higher sensitivity to NO 2 at room temperature. With further increasing the annealing temperature to 600 °C, although the rapid decrease in the specific surface area of the NiO might limit the physisorption of NO 2, the NiO could also present a better sensitivity to NO 2 due to the…

This work reports the synthesis of Co 3 O 4 particles that can be used as effective electrode materials for both the oxygen reduction (ORR) and evolution (OER) reactions. The development of such catalysts, free from precious group metals and capable of decreasing overpotentials in fuel cells, metal–air batteries, and water electrolyzers, also requires stable supporting and conducting substrates in order to deposit low metal oxide loadings. This challenging approach led us to prepare Co 3 O 4 materials on graphene-based composites more stable than common used Vulcan carbon. Co 3 O 4 particles synthesized from a solvothermal method were thereby deposited onto reduced graphene oxide (RGO) and N-doped reduced graphene oxide (NRGO) prepared from the Hummers method. The structural properties and surface composition of the different materials characterized by X-ray diffraction, transmission electron microscopy, and X-ray induced photoelectron spectroscopy measurements were combined to cyclic voltammetry experiments for revealing the charge transfer from cobalt to nitrogen, which greatly…

We systematically studied the Raman spectra of graphyne (GY) and graphdiyne (GDY), analyzing their features under mechanical strain by group theory and first-principles calculations. The G bands in GY and GDY were softened compared with that in graphene, which provides a fingerprint useful in detecting their synthesis. We established a unified formulation to describe the effects of both uniaxial and shear strains, and combined this with calculated results to reveal the relationship underlying the changes in Raman evolution under various strains. Each doubly degenerate mode splits into two branches under strain, both of which are red-shifted with tensile uniaxial strain, but one is red-shifted and the other is blue-shifted under shear strain. The splitting under shear strain is double that under uniaxial strain.

Molybdenum disulfide (MoS 2 ) is a promising alternative hydrogen evolution reaction (HER) catalyst to high-cost platinum (Pt) due to its large surface area, low cost, easy preparation, and earth abundance. The HER efficiency of MoS 2 remains low because only the edge S-sites are active for the HER. In this work, two practical strategies, heteroatom doping (Rh, Pd, and Ag) and strain engineering, are proposed to activate the inert in-plane S-site for the HER. The density functional theory calculations demonstrate that doping MoS 2 with heteroatom can trigger the HER activity of the S atoms next to the doping atoms, with a negative hydrogen adsorption free energy (Δ G H* 0 ). The negative Δ G H* 0 can be further significantly optimized by a small compressive strain. Therefore, the combination of heteroatom doping and a small compressive strain can yield an ideal value of hydrogen-binding free energy (Δ G H* 0 = 0 eV) for HER. These results highlight an innovative avenue to optimize the HER performance of MoS 2 .

In this study, we performed both electrochemical measurements, such as the rotating disk electrode and the rotating ring-disk electrode techniques, and density functional theory (DFT) calculations to investigate oxygen reduction reaction (ORR) on nonprecious transition metal/nitrogen doped carbon nanofiber (Fe/N/C and Co/N/C) catalysts in acid medium. The nanofiber catalysts were synthesized by electrospinning and subsequent heat treatment procedures. Our electrochemical measurements showed that the pyrolyzed Fe/N/C catalyst possesses higher activity for ORR than the pyrolyzed Co/N/C catalyst and could promote four-electron (4e − ) ORR. In comparison, O 2 electroreduction was found to proceed mainly with 2e − pathway on the Co/N/C catalyst. To gain insights into underlying catalytic mechanisms, we calculated the adsorption energies of all possible chemical species and the activation energies for O–O bond dissociation reactions involved in ORR on the FeN 4 and CoN 4 active sites embedded in graphene. Our DFT calculations predicted that the ORR could happen through 4e − associative…

The most commonly used redox mediators in dye-sensitized solar cells (DSCs), iodide/triiodide and cobalt trisbipyridine ([Co(bpy) 3 ] 2+/3+ ), were successfully replaced by bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp) 2 ] 1+/2+ ). The use of the copper complex based electrolyte led to an exceptionally high photovoltaic performance of 8.3% for LEG4-sensitized TiO 2 solar cells, with a remarkably high open-circuit potential of above 1.0 V at 1000 W m –2 under AM1.5G conditions. The copper complex based redox electrolyte has higher diffusion coefficients and is considerably faster in dye regeneration than comparable cobalt trisbipyridine based electrolytes. A driving force for dye regeneration of only 0.2 eV is sufficient to obtain unit yield, pointing to new possibilities for improvement in DSC efficiencies. The interaction of the excited dye with components of the electrolyte was monitored using steady-state emission measurements and time-correlated single-photon counting (TC-SPC). Our results indicate bimolecular reductive quenching of the excited LEG4 dye by the…

Roughness of a surface as characterized by an atomic force microscope (AFM) is typically expressed using conventional statistical measurements including root-mean-square, peak-to-valley ratio, and average roughness. However, in these measurements only the vertical distribution of roughness ( z -axis) is considered. Additionally, roughness of a surface as determined by AFM is a function of the scanning scale, sampling interval and/or scanning methods; therefore, the consideration and quantification of the lateral distribution ( x and y ) is necessary. Power spectral density (PSD) analysis provides both lateral and vertical signals captured from AFM images. By applying one of the commonly adopted models to the PSD data, the fractal model and k -correlation model, the equivalent root mean squared roughness, correlation length, fractal dimension and Hurst exponent are quantified. These parameters describe the spatial distribution of roughness and spatial length scale of the roughness values. Longer correlation length is preferred for the comprehensive measurement of roughness of surface…

Abstract Two-dimensional Cr2N MXene as well as surface-passivated Cr2NF2, Cr2N­(OH)2, and Cr2NO2 have been investigated using density functional theory. Cr2N is an antiferromagnetic metal. The F atom or OH group-passivation does not change the antiferromagnetic characteristics; however, Cr2NO2 has a ferromagnetic ground state that acts a half-metal. The half-metallic structure is verified by phonon dispersion and ab initio molecular dynamics simulations. Furthermore, the half-metallicity of Cr2NO2 is still robust when bias is applied to a nanometer-sized device, implying that the half-metallicity is intrinsic and does not require atomically clean surfaces. Therefore, the stable surface-oxygen-passivated MXene is a good candidate for spintronics.

We report a detailed mechanistic study of the oxygen reduction reaction (ORR) on Pt(111) in alkaline solution, combining density functional theory and kinetic Monte Carlo simulations. A complex reaction network including four possible pathways via either 2e – or 4e – transfer is established and is able to reproduce the experimental measured polarization curve at both low- and high-potential regions. Our results show that it is essential to account for solvation by water and the dynamic coverage of *OH to describe the reaction kinetics well. In addition, a chemisorbed water (*H 2 O)-mediated mechanism including 4e – transfers is identified, where the reduction steps via *H 2 O on the surface are potential-independent and only the final removal of *OH from the surface in the form of OH – (aq) contributes to the current. For the ORR in alkaline solutions, such a mechanism is more competitive than the associative and dissociative mechanisms typically used to describe the ORR in acid solution. Finally, *OH and **O 2 intermediates are found to be critically important for tuning the ORR…

A simple, mild and green route is developed for the preparation of uniform polystyrene-methyl acrylic acid/silver (PSMAA/Ag) nanocomposite spheres with high catalytic activities. In this approach, monodisperse polystyrene-methyl acrylic acid (PSMAA) spheres are used as polymeric matrixes; silver precursor-[Ag(NH 3 ) 2 ] + ions are then adsorbed onto the surfaces of PSMAA spheres due to the strong electrostatic attraction between carboxyl groups (electronegative) and [Ag(NH 3 ) 2 ] + ions (electropositive). Meanwhile, [Ag(NH 3 ) 2 ] + ions are reduced to Ag nanoparticles and simultaneously protected by PVP on the surface of PSMAA spheres. In this way, the PSMAA/Ag nanocomposite spheres with loading Ag nanoparticles on PSMAA spheres are prepared in aqueous media without any toxic reagents used during the preparation process. Ag nanoparticles uniformly distribute on the surfaces of PSMAA spheres with its size ranging from 8 to 24 nm. Moreover, the coverage degree of Ag nanoparticles on PSMAA spheres increases with the increasing of concentration of silver precursor-[Ag(NH 3 ) 2 ] +…

HKUST-1 is a metal–organic framework (MOF) which plays a significant role in both applicative and basic fields of research, thanks to its outstanding properties of adsorption and catalysis but also because it is a reference material for the study of many general properties of MOFs. Its metallic group comprises a pair of Cu 2+ ions chelated by four carboxylate bridges, forming a structure known as paddle-wheel unit, which is the heart of the material. However, previous studies have well established that the paddle-wheel is incline to hydrolysis. In fact, the prolonged exposure of the material to moisture promotes the hydrolysis of Cu–O bonds in the paddle-wheels, so breaking the crystalline network. The main objective of the present experimental investigation is the determination of the details of the structural defects induced by this process in the crystal, and it has been successfully pursued by coupling the electron paramagnetic resonance spectroscopy with other more commonly considered techniques, such as X-ray diffraction, surface area estimation, and scanning electron…

Two-dimensional (2D) stable CdS monolayer sheets are proposed using the state-of-the-art theoretical calculations. Three different conformers (planar, distorted, and buckled) are predicted which are separated by low energy barriers. These monolayer sheets are not only thermodynamically, mechanically, and dynamically stable but also can withstand temperature as high as 1000 K. Band edge alignment of these monolayer sheets and bulk CdS is done with respect to the water oxidation and reduction potential to evaluate their photocatalytic activities. Here we show a planar CdS monolayer sheet is the most promising material for visible light photocatalysis and can be used for electronic and optoelectronic devices.

The growth of ZnO nanowires by chemical bath deposition (CBD) is of great potential for their integration into nanoscale devices. However, the effects of the chemical precursors in solution are still under debate, such as the role of hexamethylenetetramine (HMTA). In order to tackle this issue, these effects are thoroughly disentangled from the effects of the structural morphology of the ZnO seed layer and investigated through a large number of nonequimolar CBDs over a broad range of chemical precursor concentrations and ratios. The analysis is further supported by thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) species. It is shown that the ZnO deposited volume and, to some extent, the length of ZnO nanowires are directly related to the supersaturation in solution, which strongly depends on the chemical precursor concentration and pH. A slight excess of HMTA with respect to zinc nitrate is required to reach the largest axial growth rate of ZnO nanowires. In addition to act as a source of HO – ions, HMTA is found to act as a pH…

As a compound analogue of black phosphorus, a new 2D semiconductor of SnS layers is proposed. Based on state-of-the-art theoretical calculations, we confirm that such 2D SnS layers are thermally and dynamically stable and can be mechanically exoliated from α-phase SnS bulk materials. The 2D SnS layer has an indirect band gap that can be tuned from 1.96 eV for the monolayer to 1.44 eV for a six-layer structure. Interestingly, the decrease of the band gap with increasing number of layers is not monotonic but shows an odd–even quantum confinement effect, because the interplay of spin–orbit coupling and lack of inversion symmetry in odd-numbered layer structures results in anisotropic spin splitting of the energy bands. It was also found that such 2D SnS layers show high in-plane anisotropy and high carrier mobility (tens of thousands of cm 2 V –1 s –1 ) even superior to that of black phosphorus, which is dominated by electrons. With these intriguing electronic properties, such 2D SnS layers are expected to have great potential for application in future nanoelectronics.

A novel fiber-shaped supercapacitor is created with high volumetric energy density by designing an asymmetric configuration and integrating multiple functional components for electrodes. More specifically, a ternary hybrid fiber is prepared as a positive electrode by growing MnO 2 nanosheets onto a conducting polymer-coated carbon nanotube fiber; an ordered microporous carbon/carbon nanotube hybrid fiber is made as a negative electrode; and the hybrid positive and negative fibers are assembled into a fiber-shaped asymmetric supercapacitor. It shows a high operating voltage of 1.8 V, compared with 0.8–1.0 V of their conventional counterparts based on aqueous gel electrolytes; in particular, it produces an energy density as high as 11.3 mWh/cm 3 which is on par with the thin-film lithium-ion batteries. It also exhibits remarkable cyclic stability and good rate performance. Due to the unique fiber shape, it can be woven or knitted into flexible power textiles.

Organometal halide perovskites have attracted tremendous attention for optoelectronic applications. Charge carrier trapping is one of the dominant processes often deteriorating the performance of devices. Here, we investigate the details of trap behavior in colloidal nanoparticles (NPs) of CH 3 NH 3 PbBr 3 perovskites with mean size of 8 nm and the corresponding bulk crystals (BCs). We use excitation intensity dependence of photoluminescence (PL) dynamics together with comprehensive simulation of charge carrier trapping and the trap-state dynamics. In the bulk at very low excitation intensities the PL is quenched by trapping. A considerable fraction of the traps become filled if excitation fluence is increased. We identified two different traps, one exhibiting ultralong lifetime (∼70 μs) which leads to efficient accumulation of trap filling even at relatively low excitation intensities. In colloidal NPs, the average number of surface traps is estimated to be 0.7 per NP. It means about 30% excitation would undergo trap-free radiative recombination. The trapping time constant of 7 ns…

Zeolitic imidazolate frameworks (ZIFs) are a set of nanoporous metal–organic frameworks (MOFs) with tunable porosity and functionality. Among MOFs, they also show relatively good stability with respect to temperature and humidity. These characteristics lead to their possible applications in separation processes. In many practical separation processes, adsorbents are exposed to a variety of molecular species including acid gases. However, there is little knowledge of the effects of such acid gas exposure on the adsorption and separation properties of ZIFs. Here, the stability of a model ZIF material (ZIF-8) under SO 2 exposure in dry, humid, and aqueous environments has been investigated in detail. Combined characterization by several techniques (PXRD, N 2 physisorption, EDX, XPS, and FTIR) allowed us to track the structural and compositional properties of ZIF-8 before and after SO 2 exposure. ZIF-8 is stable after prolonged exposure in dry SO 2 and in humid air without SO 2 . However, exposure to 10–20 ppm concentrations of SO 2 in the presence of high relative humidity led to its…

A series of Pt/TiO 2 catalysts with various Pt particle sizes and valence states were prepared and tested for CO oxidation at room temperature. Field-emission transmission electron microscopy and X-ray photoelectron spectroscopy analyses confirmed that the activity of the Pt/TiO 2 catalyst was influenced by the particle size and valence state of the catalyst particles. Excellent CO oxidation activity was observed at room temperature using highly dispersed, small metallic Pt particles. Increasing the Pt metallic /Pt total ratio resulted in an increase of turnover frequencies. According to the Fourier-transform infrared spectroscopy results, the linear CO species that was adsorbed on metallic Pt sites at 25 °C reacted with atmospheric O 2 and was easily desorbed. However, linear CO species adsorbed on PtO x ( x ≥ 2) sites was only desorbed at temperatures ≥100 °C, confirming the lack of CO oxidation activity at room temperature with ionic Pt catalysts.

It was found that copper-modified covalent triazine frameworks (Cu-CTF) efficiently catalyze the electrochemical reduction of nitrate and promote N–N bond formation of nitrous oxide (N 2 O), a key intermediate for N 2 formation (denitrification). A Cu-CTF electrode exhibited an onset potential of −50 mV versus RHE for the electrochemical nitrate reduction reaction (NRR). The faradaic efficiency for N 2 O formation by Cu-CTF reached 18% at −200 mV versus RHE, whereas that for Cu metal was negligible. On the basis of density functional calculations for Cu-CTF, both solvated and surface-bound nitric oxide (NO) were generated by the NRR due to the moderate adsorption strength of Cu atoms for NO, a property that facilitated the effective dimerization of NO through an Eley–Rideal-type mechanism.