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Developing highly active catalysts for hydrogen evolution reaction based on earth-abundant materials is challenging. Nitrogen doping has recently been reported to improve catalytic properties by modifying the electrochemical properties of titanium carbide MXene. However, systematic doping engineering, such as optimization of doping concentration, doping site, and thermodynamic phase stabilization have not been systematically controlled, which retards the reliable production of high-activity MXene catalysts. In this study, the optimum doping concentration of nitrogen and doping process conditions on O-functionalized Ti2C MXene for hydrogen evolution reaction were investigated using density functional theory with thermodynamics. To confirm the optimum nitrogen concentration, the catalytic properties are examined considering the Gibbs free energy of hydrogen adsorption and conductivity for 2.2-11.0 at % nitrogen concentration. It was confirmed that 8.8 at % nitrogen-doped Ti2CO2 had optimum catalytic properties under standard conditions. Moreover, when the doping concentration was higher, the decrease in the adsorption energies of hydrogen and the transition in the energy dispersion of the conduction band led to deterioration of the catalytic properties. Through theoretical results, the feasible process conditions for optimum nitrogen concentration while maintaining the structure of MXene are presented using a thermodynamics model taking into account chemical reactions with various nitrogen sources. This study provides further understanding of the nitrogen-doping mechanism of Ti2CO2 for hydrogen evolution reactions.

Borosulfates are classified as silicate analogue materials. The number of crystallographically characterized compounds is still limited, whereas the structural diversity is already impressive. The anionic substructures of borosulfates exhibit vertex-connected (BO4 )- and (SO4 )-tetrahedra, whereas bridging between two (SO4 )- or even between two (BO4 )-tetrahedra is scarce. The herein presented compound Sr[B3 O(SO4 )4 (SO4 H)] is the first borosulfate with a triple-vertex linkage of three (BO4 ) tetrahedra via one common oxygen atom. DFT calculations complement the experimental studies. Bader charges (calculated for all atoms) as well as charge-density calculations give hint to the electron distribution within the anionic substructure and density-of-states calculations support the interpretation of the bonding situation.

In this study, vertically aligned p–n junction TiO2 nanotubes can be formed by anodization of the Ti substrate following the addition of a high electric field in a Pd precursor-containing electrolyte. The bottom region of the TiO2 nanotubes with a high concentration of Pd because of the high electric field which was induced to be p-type. In contrast, the region with a low Pd concentration (top of the TiO2 nanotubes) was determined to be n-type, similar to the pristine TiO2. The concentration profile of the dopant in TiO2 nanotubes was investigated via TOF-SIMS and XPS. Defect formation energies in TiO2 nanotubes were estimated using density-functional theory calculation to understand the p–n junction formation. The p–n junction TiO2 nanotubes showed a high photocatalytic hydrogen production rate of 25.2 μL cm−2 h−1 under solar light irradiation as a result of the enhancement of visible light photoactivity.

Brookite, the least studied crystalline phase of TiO2, recently has been found to have excellent photocatalytic activities, comparable to that of anatase TiO2. However, its activity is highly depen...

Here we report the miscibility gap of CuSb1-xBixS2 (CABS), a promising photo energy conversion material for band gap engineered solar cells, and evaluate its applicability via a combination of theoretical predictions and experimental verifications. Our ab initio calculations and thermodynamic modeling revealed that the CABS random alloy system has optimal band gap values in the range of 1.1–1.5 eV when synthesized at room temperature. The CABS system, synthesized by mechanochemical methods, exhibited optical band gap values in very good agreement with theoretical predictions, as well as lowered kinetic energy barriers for enhanced nucleation.

To overcome the reliability issue of phase-change memory, the development of stable phase-change materials is extremely important. In this study, we analyze 13 dopants for Ge2Sb2Te5 (GST) based on two criteria: i) the change in thermal stability by doping and ii) a lattice distortion. After doping the elements, 11 elements showed a negative doping formation energy compared with pure GST. The angular distortion of the Zn dopant is the largest. The hole carrier decreases, and the electrical resistance increases through Zn-doping in GST. The increased resistance of the material can lead to low power consumption with a high energy effectiveness.

In order to be able to control the phase transition of engineered phase-change materials, the specific understanding of phase transition processes is essential. To understand the effect of dopant o...

Methods for oxygen vacancy engineering usually require high-temperature heating processes, which are substantially time-consuming. Laser irradiation techniques are a viable alternative to conventional methods, as they enable room-temperature tuning of material functionality using a simple, swift, and inexpensive process. In this report, the effects of pulsed laser irradiation on the formation of oxygen vacancies and its positive relationship with the sensing performance of SnO2 have been investigated. Based on density functional theory calculations, we suggest that the formation of laser-induced bridging oxygen defects and the resulting excess electrons on the SnO2 surface change the surface orbital structures of the Sn atoms in a manner favorable for NO2 adsorption, thus playing a key role in improving its sensing performance.

It has recently been demonstrated that the OER activity of transition metal sulfides (TMSs) could be enhanced by the introduction of a thin amorphous layer on a pristine surface. We report here a novel strategy to enhance the OER by developing cobalt nickel sulfide (CoxNi1−xS2, CNS) with a high density of crystalline and amorphous phase boundaries. Electrochemical activation (ECA) can partially amorphize hollow CNS nanoparticles derived from surface-selective sulfidation. The ECA-treated CNS (ECA-CNS) electrocatalyst, which is comprised of CNS nanodots separated by thin amorphous layers, shows high densities of crystalline and amorphous phase boundaries. This catalyst shows superior OER catalytic performance with a current density of 10 mA cm−2 at a small overpotential of 290 mV, a low Tafel slope of 46 mV dec−1, a high mass activity of 217 A g−1, a high turnover frequency of 0.21 s−1 at an overpotential of 340 mV, and excellent stability in alkaline media.

The development of cost-effective and high-performance electrocatalysts for water oxidation has attracted intense research interest. It was reported recently that the interface between the amorphous and crystalline phases plays a significant role in the electrocatalytic activity of transition metal compounds. It was reckoned therefore that an increase in the density of the crystalline–amorphous phase boundary would enhance the electrochemical water oxidation on the catalyst. In this work we develop a new and facile strategy for inducing high density crystalline–amorphous phase boundaries via selective fluorination surface doping. This resulted in excellent characteristics of the engineered material for electrochemical water splitting. An initial computational simulation is carried out to design the crystalline–amorphous phase boundary material and an experimental verification follows for demonstration and optimization of the impact of surface doping. We conclude that the engineering of the interface using this facile and cost-effective strategy maximizes the crystalline and amorphous phases of metal–metalloids, which can be used to fabricate low-cost and efficient electrocatalysts for water oxidation.

Poor optical absorbance and charge recombination are the major drawbacks of polymeric graphitic carbon nitride (g-C3N4)-based photocatalysts. In this paper, we show for the first time a single-step in situ technique to control the porosity of two-dimensional g-C3N4 sheets and exfoliate them by introducing ascorbic acid (AA) molecules. The AA simultaneously acts as the carbon (C) source and deposits amorphous C onto g-C3N4 sheets. Nanosized pores are also introduced into the g-C3N4 sheets, leading to a large number of active sites. The as-prepared C-doped porous g-C3N4 nanosheets demonstrate a high visible light-photocatalytic H2 production activity of 793 μmol g−1 with the optimum structure, which is almost 25 times higher than the value obtained with bulk g-C3N4 (31 μmol g−1). This exceptional photocatalytic performance arises from the C-doped conjugated system and porous nanosheets. The enhanced photocatalytic H2 evolution was attributed to the effective separation and transport of charge carriers by the deposition of C onto the nanosheets and an increased number of active sites resulting from the nanopores created inside the g-C3N4 sheets. Moreover, molecular dynamics (MD) simulations confirm that the interaction between AA and melamine molecules at elevated temperatures results in the formation of C-doped porous and exfoliated g-C3N4 structures. Therefore, the present approach is very promising for application to the design of new and efficient photocatalysts for photocatalytic H2 evolution under visible irradiation.

Highlights Single-atom M-N4/N-doped carbons (M = Fe, Co) prepared as OER/ORR catalysts.Ultrasonication-assisted plasma engineering used for catalyst synthesis.Co-N4/NC outperformed benchmark commercial catalysts in practical Zn–air battery test.DFT calculations provided insights into the origin of superior ORR/OER performance. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-020-00581-4, As bifunctional oxygen evolution/reduction electrocatalysts, transition-metal-based single-atom-doped nitrogen–carbon (NC) matrices are promising successors of the corresponding noble-metal-based catalysts, offering the advantages of ultrahigh atom utilization efficiency and surface active energy. However, the fabrication of such matrices (e.g., well-dispersed single-atom-doped M-N4/NCs) often requires numerous steps and tedious processes. Herein, ultrasonic plasma engineering allows direct carbonization in a precursor solution containing metal phthalocyanine and aniline. When combining with the dispersion effect of ultrasonic waves, we successfully fabricated uniform single-atom M-N4 (M = Fe, Co) carbon catalysts with a production rate as high as 10 mg min−1. The Co-N4/NC presented a bifunctional potential drop of ΔE = 0.79 V, outperforming the benchmark Pt/C-Ru/C catalyst (ΔE = 0.88 V) at the same catalyst loading. Theoretical calculations revealed that Co-N4 was the major active site with superior O2 adsorption–desorption mechanisms. In a practical Zn–air battery test, the air electrode coated with Co-N4/NC exhibited a specific capacity (762.8 mAh g−1) and power density (101.62 mW cm−2), exceeding those of Pt/C-Ru/C (700.8 mAh g−1 and 89.16 mW cm−2, respectively) at the same catalyst loading. Moreover, for Co-N4/NC, the potential difference increased from 1.16 to 1.47 V after 100 charge–discharge cycles. The proposed innovative and scalable strategy was concluded to be well suited for the fabrication of single-atom-doped carbons as promising bifunctional oxygen evolution/reduction electrocatalysts for metal–air batteries. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-020-00581-4

α-SiAlON ceramics have been in use as engineering ceramics in the most arduous industrial environments such as molten metal handling, cutting tools, gas turbine engines, extrusion molds, thermocouple sheaths, protective cover for high-temperature sensors, etc., owing to their outstanding mechanical, thermal and chemical stability. Taking advantage of the intrinsic properties of α-SiAlONs, we investigate, in this paper, the possibility of using the Er-doped α-SiAlON (Er-α-SiAlON) ceramic as a high-temperature sensing material via its unique near-infrared to visible upconversion property. We first use neutron diffraction and density functional theory calculations to study the electronic structure and thermodynamic stability of Er-α-SiAlON. It is found that the interstitial doping of Er stabilizes the α-SiAlON structure via chemical bonds with O-atoms with N:O ratio of 5:2 in the seven-fold coordination sites of the Er3+ ion. Temperature-dependent upconversion emissions are then studied under 980 and 793 nm excitations over a temperature range of 298–1373 K and the fluorescence intensity ratio (FIR) technique has been employed to investigate the temperature sensing behavior. Temperature-dependent Raman behavior is also investigated. We demonstrate that using Er-α-SiAlON as a sensing material, the limit of temperature measurement via the FIR technique can be pushed well beyond 1200 K.

Organic–inorganic hybrid lead halide perovskites have gained significant attention as light-harvesting materials in thin-film photovoltaics due to their exceptional optoelectronic properties and simple fabrication process. The power conversion efficiency of perovskite solar cells (PSCs) has surged beyond 25% in a short time span. Their transition to commercial market is a “work in progress” due to limited long-term operational stability and the persisting environmental concern due to the presence of lead. Comprehensive investigations on the interplay of material composition and interfacial effects on the device performance of PSCs based on methylammonium lead iodide have shown the crucial role of an A-site cation in incipient deterioration of the material through external stimuli (moisture, light, oxygen, or heat). Consequently, a partial or complete replacement of A-site cations by up to four isoelectronic substituents has resulted in many new perovskite compositions. The correlations between the chemical composition and the optoelectronic properties are, however, not always easy to determine. A-site cation management is governed by stability and charge neutrality of the lattice, and the choices include Cs+-cations and organic cations such as CH3NH3+ or CH(NH2)2+ and combinations thereof. Since the size of the cations is an important structural parameter, an adequate compositional engineering of the A-site could effectively optimize the stability by reducing non-radiative defect sites and enhancing carrier lifetimes. This Perspective reflects on the experimental strategies for A-site cation management and their direct impact on the stability and device performance. It also highlights the opportunities and challenges for further research and industrial commercialization of PSCs.INTRODUCTION

Low-crystalline TiNb2O7 shows a high practical capacity of ∼280 mAh g−1 by size and morphology controls, composites, doping foreign atoms. However, the defect chemistry of TiNb2O7 has rarely been investigated. We report an effect of point/line defects on a crystal structure and lithium storage properties of TiNb2O7−x engineered by different calcination temperatures and vacuum conditions. X-ray diffraction studies reveal an evolution of oxygen vacancies aligned to ( h ¯ 0 l ) and (0k0) planes can cause an increase in lattice constants and a cell volume expansion to ∼3%. Transmission electron microscopy observation shows calcination at 800℃ in vacuum results in not only a size reduction of particles but also a development of edge/screw dislocations generated on ( 3 ¯ 03 ) plane as a slip system, which can drive microstrain correlated to the lattice parameter change. TiNb2O7−x shows remarkable lithium storage properties including charge (lithiation)/discharge (delithiation) capacities of 324/284 mAh g− 1 and capacity retentions of 100% at a 2C rate over 500 cycles and at a 5C rate over 1500 cycles, respectively. The achievement is attributed to a synergetic effect of short Li+ ion path by a size reduction and reduce Li+ ion migration by an expansion of the lattice constants and the cell volume.

Charge storage in supercapacitors is strongly related to the bond characteristics and electronic structure of electrode materials. Graphene-based materials are widely used in a supercapacitor due to the easily tunable properties and high surface/volume ratios. However, we claim that the typical covalent bond characteristics of 2D carbon materials originating from this 2pπ orbital is not very suitable to the application in supercapacitor. Here, we suggest an efficient way to improve the supercapacitor performance by tuning the covalency of bonding between the graphene-based electrode and potassium ion. We, for the first time, also introduce a simple solvent-free scale-up doping technique to prepare fluorine-doped graphene oxide (FGO) by direct plasma treatment on graphene oxide (GO) powder at ambient pressure. The FGO enabled fast electrochemical charge transfer and provided a large number of active sites for redox reactions during supercapacitor operation, and those mechanisms were thoroughly studied by various electrochemical analyses. As a result, the fabricated symmetric supercapacitor using FGO electrodes exhibited a maximum power density (~3200 W/kg) and energy density (~25.87 Wh/kg) with superior cycle stability (20000 cycles) without capacitance loss. Furthermore, the computational calculation results clarified the roles of semi-ionic C–F bonding of FGO: huge charge accumulation on the electrodes and superior electrical conductivity. Thus, our study demonstrates a facile strategy to develop promising functionalized materials, which can enhance the viability of supercapacitor for the next generation of energy storage systems.

The electrocatalytic performance of transition metal sulfide (TMS)–graphene composites has been simply regarded as the results of high conductivity and the large surface/volume ratio. However, unavoidable factors such as degree of oxidation of TMSs have been hardly considered for the origin of this catalytic activity of TMS–graphene composites. To accomplish the reliable application of TMS-based electrocatalytic materials, a clear understanding of the thermodynamic stability of TMS and effects of oxidation on catalytic activity is necessary. In addition, the mechanism of charge transfer at the TMS–graphene interface must be studied in depth to properly design composite materials. Herein, we report a comprehensive study of the physical chemistry at the junction of a Co1–xNixS2–graphene composite, which is a prototype designed to unravel the mechanisms of charge transfer between TMS and graphene. Specifically, the thermodynamic stability and the effects of oxidation of TMSs during the oxygen evolution react...

A key issue with Na-ion batteries is the development of active materials with stable electrochemical reversibility through the understanding of their sodium storage mechanisms. We report a sodium storage mechanism and properties of a new anode material, digenite Cu1.8S, based on its crystallographic study. It is revealed that copper sulfides (CuxS) can have metal-rich formulas (x ≥ 1.6), due to the unique oxidation state of +1 found in group 11 elements. These phases enable the unit cell to consist of all strong Cu–S bonds and no direct S–S bonds, which are vulnerable to external stress/strain that could result in bond cleavage as well as decomposition. Because of its structural rigidness, the Cu1.8S shows an intercalation/deintercalation reaction mechanism even in a low potential window of 0.1–2.2 V versus Na/Na+ without irreversible phase transformation, which most of the metal sulfides experience through a conversion reaction mechanism. It uptakes, on average, 1.4 Na+ ions per unit cell (∼250 mAh g–1) and exhibits ∼100% retention over 1000 cycles at 2C in a tuned voltage range of 0.5–2.2 V through an overall solid solution reaction with negligible phase separation.

Black anatase TiO2 has surprisingly enhanced solar energy harvesting efficiency and electrical conductivity, which makes it a promising material in a wide range of energy and environmental applications. Several experimental and theoretical studies have successfully revealed the mechanisms of band gap reduction by surface hydrogenation of anatase TiO2. However, recent experimental evidence suggests the existence of bulk point defects that yield infrared (∼1.0 eV) photoabsorption and high conductivity of black anatase TiO2. In the current study, using a combination of ab initio molecular dynamics simulations and electronic structure calculations, we successfully explain the physical properties, metallicity, and infrared/microwave absorption (i.e., black color) of highly reduced anatase TiO2 crystal in a hydrogenated state with a newly found pair defect (Tii-VO)4+. Hydrogen atoms in the bulk are unnecessary to understand the observed properties.

A combined solvothermal method and post-annealing to synthesize highly active ZnS-ZnO nanosheets is presented. ZnO was produced on ZnS nanosheets by thermal annealing ZnS/ethylenediamine in air. From combined studies of experimental and computational works, we revealed that during annealing, carbon (C) element was a major dopant from the decomposition of ethylenediamine, which was unintentionally doped into the ZnS-ZnO, and gave major impacts on enhanced visible-light photocatalysis. Rhodium (Rh) was deposited by in situ photoreduction to form a ZnS-ZnO/Rh catalyst composite. This multijunctioned photocatalyst was outstanding for H2 generation from pure water under solar simulating light due to effective charge separation by Z-scheme heterojunction of ZnS and ZnO and Schottky junction of Rh-cocatalysts/semiconductors. This facile method realizes multijunctioned ZnS-ZnO/Rh photocatalysts with substantial defects that are very promising for solar-energy harvesting applications.

Indium vacancies in Bi-doped In3SbTe2 (BIST) cause local distortion or and faster phase transition of BIST with good stability. The formation energy of the In vacancy in the BIST is relatively lower compared to that in IST due to triple negative charge state of the In vacancy (V 3− In) and higher concentration of the V 3− In in BIST. The band gap of BIST is substantially reduced with increasing concentrations of the V 3− In and the hole carriers, which results in a higher electrical conductivity. The phase-change memory (PRAM) device fabricated with the BIST shows very fast, stable switching characteristics at lower voltages.

Impurity doping and synthesizing polymorphic particles are the common strategies to improve activity of TiO 2 photocatalyst by lowering the band gap and enhancing electron-hole separation rate. However, these two approaches have side effects. Doping of impurities make space charge region (SCR) thinner near the surface, which requires smaller sized particles than undoped TiO 2 for the optimal performance. Polymorphic TiO 2 particles, in which rutile and anatase phases coexist in a particle, are usually large due to energetic unstability of the rutile phase in a fine particle. For this contradiction that one needs small size while the other needs large size, two effects are not easy to be combined. In this study, we suggest a dual-doping strategy to solve the contradictory problem of SCR reduction by donor doping and inevitable size growth in polymorphic particles. We successfully dope W, a band gap narrower, into fine size of polymorphic particles by Sn-codoping, a promoter of the anatase-to-rutile transformation (ART), and demonstrate greatly improved photocatalytic activity. The accelerated ART by Sn-doping could keep the size of polymorph junctioned TiO 2 small (∼10 nm) as lower temperature annealing become able to induce the ART. The concept of dual doping with a band gap narrower and an ART promoter provides a way to synthesize highly active photocatalysts by overcoming the drawback from shortened SCR length.

Li4Ti5O12 (LTO) has attracted tremendous attention as a stationary Li-ion battery anode material due to its excellent stability. However, the poor rate capability caused by the low electrical conductivity limits its practical use. Previously, Mg-doping in LTO has been used to improve the electrical conductivity and electrochemical properties, but the Mg-doped LTO system generally exhibits large anomalies in the electrical properties and capacities, which limits the reliable mass-production of engineered LTO. In this study, on the basis of first-principles calculations and related experiments, we systematically study the effects of charge-compensating point defects of the Mg-doped LTO on the electrical properties. A combination of first-principles calculations with thermodynamic modeling shows that high-temperature annealing under reducing conditions could effectively alter the Mg-doping site from a Ti4+ to Li+ site and increase the electrical conductivity significantly due to reduced electron effective ma...

Understanding the roles of point defects in optical transitions is a key to the desirable engineering of photochemical materials. In this study, the origins of the significantly varying optical and photochemical properties of Nb-doped anatase TiO2 were systematically investigated, using density functional theory (DFT) calculations and experimental verifications. We found from DFT calculations that the desirable band gap reduction of anatase TiO2 by ∼0.1 eV reported in many of experimental reports and the resultant improvements of photocatalytic and photovoltaic efficiencies of Nb5+-doped anatase TiO2 are due to the formation of complex (NbTi-VTi)3− as the compensator of NbTi+. Our experiments demonstrated that the O2-rich annealing, which is expected to increase the concentration of desirable (NbTi-VTi)3− complex, narrows band gap of TiO2 and strongly enhances the photocatalytic activity of Nb-doped TiO2 particle. On the contrary, pure TiO2 showed rather worse photocatalytic performances when annealed in O2-rich atmosphere, which is due to the formation of deep level by O-interstitial defect (Oi). Theoretically obtained charge effective masses could further explain the different photocatalytic activities of undoped and Nb-doped TiO2.

Titanium dioxide (TiO2) with exposed (001) facets (TiO2(001)) has attractive photocatalytic properties. However, the high recombination rate of the photo-excited charge carriers on this surface often limits its application. Here, we report that a few-layered 1T-MoS2 coating on TiO2(001) nanosheets (abbreviated as MST) can be a promising candidate that overcomes some of the challenges of TiO2(001). Computational and experimental results demonstrate that MST as a photocatalyst exhibits a significantly low-charge recombination rate as well as excellent long-term durability. The synthesized MST 2D nanocomposites show a 31.9% increase in photocatalytic activity for hydrogen (H2) production relative to the counterpart TiO2(001). MST offers a new route for further improvement of the photocatalytic activity of TiO2 with exposed high energy facets.

Photoelectrochemical (PEC) water splitting reactions are promising for sustainable hydrogen production from renewable sources. We report here, the preparation of α-Fe2O3/Fe3O4 composite films via a single-step chemical vapor deposition of [Fe(OtBu)3]2 and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidation activity (0.48 mA/cm2 at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm2) standard illumination was found for hybrid films with a thickness of 11 μm. This phenomenon is attributed to an improved electron transport resulting from a higher magnetite content toward the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of α-Fe2O3/Fe3O4 nanocomposite photoanodes is attributed to the close pr...

In application of graphene to real electronics, understanding the mechanism of the electrical breakdown of the graphene in harsh environments should precede many activities in tamed conditions. In this article, we report the unusual structural evolution of microbridge graphene in air near the electrical current-breakdown limit. In-situ micro-Raman study revealed that Joule heating near the electrical breakdown gave rise to a substantial structural evolution: a previously unknown broad amorphous-like and partially reversible phase at an on- and off-current of ∼3.0 × 10 8 A/cm 2 , which finally drove the phase to the electrical current-breakdown. Our calculations suggest that the phase originates from the broken symmetry caused by defect formations during Joule heating. In particular, these formations are bonds of carbon-oxygen and vacancies-oxygen. A collection of energetically favorable vacancies-oxygen pairs results in porous graphene, and its evolution can be the key to understanding how the breakdown starts and propagates in graphene under high current density in air.

Tungsten–nitrogen (W–N) codoping has been known to enhance the photocatalytic activity of anatase TiO2 nanoparticles by utilizing visible light. The doping effects are, however, largely dependent on calcination or annealing conditions, and thus, the massive production of quality-controlled photocatalysts still remains a challenge. Using density functional theory (DFT) thermodynamics and time-dependent DFT computations (TDDFT), we investigate the atomic structures of N doping and W–N codoping in anatase TiO2, as well as the effect of the thermal processing conditions. We find that W and N dopants predominantly constitute two complex structures: an N interstitial site near a Ti vacancy in the triple charge state ((VTi-Ni)3–) and the simultaneous substitutions of Ti by W and the nearest O by N ((WTi-NO)+). The latter case induces highly localized shallow in-gap levels near the conduction band minimum (CBM) and the valence band maximum (VBM), whereas the (VTi-Ni)3– defect complex yielded deep levels (1.9 eV a...

Due to its high redox potential, zinc sulfide (ZnS) is considered an excellent semiconductor photocatalyst. However, the rapid recombination rate of the photogenerated electron-hole pairs limits the efficiency of ZnS for photocatalytic reactions. Herein, we suggest a design rule of heterojunction structure of ZnS for improvement of its photocatalytic performance. Two specific properties are specially emphasized: phase immiscibility and the different growth rates of the component materials. The phase immiscibility not only guarantees a well-separated interface, it also enables the technical convenience of one-pot synthesis. The different growth rates help form wide heterojunctions that foster the efficient consumption of materials. We found that the ZnS/NixSy composite not only meets the aforementioned requirements but also has proper band alignment. Ready-to-use heterojunction photocatalysts (ZnS/NixSy) were obtained via one-pot synthesis, thanks to the different growth rates and the immiscibility of the two sulfides. Combining ZnS with NixSy resulted in substantially improved photocatalytic activity in regard to dye decomposition and H2 production via water splitting. Both band position measurements and DFT simulations indicated that NixSy is a co-catalyst for ZnS, allowing sufficient band offset for electron-hole separation. Abundant and mass-producible, the ZnS/NixSy composite can effectively substitute for noble metal photocatalysis when it comes to organic pollutant degradation and water splitting.

The pulsed laser fragmentation in liquid (PLFL) process is a promising technique for the synthesis of carbon‐based functional materials. However, a fundamental deep understanding of the formation of graphene quantum dots (GQDs) from multiwalled carbon nanotubes (MWCNTs) by PLFL has still not been achieved despite the high demand. In article number 2003538, Jeong Ho Ryu, Hyuksu Han, Kang Min Kim, and co‐workers report a mechanism for the formation of GQDs from MWCNTs by the PLFL process, through the combination of experimental and theoretical studies.

The pulsed laser fragmentation in liquid (PLFL) process is a promising technique for the synthesis of carbon-based functional materials. In particular, there has been considerable attention on graphene quantum dots (GQDs) derived from multiwalled carbon nanotubes (MWCNTs) by the PLFL process, owing to the low cost and rapid processing time involved. However, a fundamental deep understanding of the formation of GQDs from MWCNTs by PLFL has still not been achieved despite the high demand. In this work, a mechanism for the formation of GQDs from MWCNTs by the PLFL process is reported, through the combination of experimental and theoretical studies. Both the experimental and computational results demonstrate that the formation of GQDs strongly depends on the pulse laser energy. Both methods demonstrate that the critical energy point, where a plasma plume is generated on the surface of the MWCNTs, should be precisely maintained to produce GQDs; otherwise, an amorphous carbon structure is favorably formed from the scattered carbons.

Charge separation is the most important factor in determining the photocatalytic activity of a 2D/2D heterostructure. Despite the exclusive advantages of 2D/2D heterostructure semiconductor systems such as large surface/volume ratios, their use in photocatalysis is limited due to the low efficiency of charge separation and high recombination rates. As a remedy for the weak interlayer binding and low carrier transport efficiency in 2D/2D heterojunctioned semiconductors, we suggested an impurity intercalation method for the 2D/2D interface. PtS2/C3N4, as a prototype heterojunction material, was employed to investigate the effect of anion intercalation on the charge separation efficiency in a 2D/2D system using density functional theory. With oxygen intercalation at the PtS2/C3N4 interface, a reversed and stronger localized dipole moment and a built-in electric field were induced in the vertical direction of the PtS2/C3N4 interface. This theoretical work suggests that the anion intercalation method can be a way to control built-in electric fields and charge separation in designs of 2D/2D heterostructures that have high photocatalytic activity.

Anchoring nanoparticles on the surfaces of metal oxide nanosensors is a promising strategy to improve sensing performance. However, several issues regarding the surface coverage and uniform distribution of nanoislands over base sensing materials have yet to be resolved using conventional synthetic routes. Herein, we present a viable alternative for the decoration of V2O5 nanoislands on SnO2 nanowires: a new two-step process that combines atomic layer deposition (ALD) and successive thermal post-treatment. This process enables us to control the surface coverage of V2O5 nanoislands by varying the number of ALD cycles and to homogeneously disperse the nanoislands on the SnO2 nanowire surfaces. The NO2 response of the V2O5-decorated SnO2 sensor improved as the number of ALD-V2O5 cycles increased; the highest response, obtained by the sensor prepared with 50 ALD-V2O5 cycles, was more than 50 times greater than that of the pristine SnO2 nanowires. However, the sensing performance degraded beyond 50 ALD-V2O5 cycles as there was an oversupply of V2O5 nanoislands. Based on density functional theory calculations, we determined that V2O5 nanoisland loading is in competition with the exposed SnO2 surface to increase sensing performance, which implies that the surface coverage of V2O5 nanoislands must be precisely optimized.

The shape of Ni particles is important. A spherical shape is favored for an electrode material in multilayer ceramic capacitor (MLCC) due to high packing density. A cubic shape is better for applications in catalysts and bioseparation due to its higher magnetic property than a spherical shape. It was found that cubic Ni particles which has a face-centered cubic (fcc) crystal structure, can be synthesized by chemical vapor synthesis (CVS). In this study, to examine whether the cubic shape of Ni particles synthesized by CVS is the growth or the equilibrium shape, the shape change of Ni particles by annealing under H2 and NiCl2 atmospheres was observed. It was confirmed by a density function theory (DFT) method that H2 and NiCl2 vapor favor respectively spherical and cubic shapes, which indicates that NiCl2 stabilizes the {1 0 0} faces of Ni. In this computational work, we provide gas adsorption energies and relaxed atomic structures of Ni(1 0 0) and Ni(1 1 1) surface models. Using this fact, suitable synthetic conditions for cubic and spherical Ni particles could be derived. It was confirmed that the high and low reduction rate favor respectively spherical and cubic shapes during CVS.

As a strategy to improve the catalytic performances of two-dimensional transition metal dichalcogenide materials, doping and defect engineering are widely used. However, it is extremely challenging to find proper new materials and doping/defect engineering conditions relying only on experimental trial-and-error. In this study, use of vanadium disulfide (VS2) was suggested as an effective cathode catalyst for Li-air batteries (LABs), under the condition of proper doping engineering to optimize electrochemical performances. To investigate the dopant screening and doping processing optimization of VS2, a theoretical concept, which combines first-principles calculations and thermodynamic modeling, was first derived and suggested. Through our approach, it is worth noting that F- and N-doping on H-VS2 shows superior overpotential properties (0.68 V and 0.76 V) than the carbon-based cathode and NH3 and O2 as reactant for T-VS2, and N2 and H2 for H-VS2 are necessary to control the N doping on VS2. Our theoretical work provides the guideline for application of VS2 to cathode of LABs and leads to further insights for designing a new cathode materials based on two-dimensional materials.

Herein, holey, thin, conductive nickel substituted cobalt molybdenum boride (Ni-CMB) nanosheets have been designed to obtain superior electrochemical HER performance with small overpotential of 69 mV at 10 mA cm-2 current density and lower Tafel slope of 76.3 mV dec-1 in alkaline medium. Incorporation of Ni leads to improved conductivity and favorable hydrogen adsorption on Mo sites, which collectively yield efficient electrocatalytic H2 production from Ni-CMB catalyst. The ultrathin nature (thickness = 5.0 nm) of the designed material expectedly helps to attain high exposure of active sites and facile charge transportation through the nanosheets. Additionally, the decorated mesopores (average size = 3.86 nm) on nanosheets have benefitted towards faster electrolyte diffusion, easy gas escape from catalyst surface to support high electrocatalytic performance. Finally, well-maintained morphology of the sample and evolution of HER active sites in the material have guaranteed long-term, sustainable hydrogen production even at high current densities, which clearly demonstrate its superiority over an expensive electrolyzer (Pt-C) in alkaline water.

Perpendicular magnetic anisotropy (PMA) in thin film is the key factor to obtain good properties for high density storage devices. Although the importance of the properties of PMA is well-known for Co/Pd thin film, it is still unclear which surface structure and composition indicate that PMA is present. In this work, the surface structure and magnetic properties of L10-ordered Co/Pd (001) were analyzed using density functional theory calculations. It was confirmed that only Pd-rich A among the facile surface structures indicates PMA properties. However, according to the calculated surface energy, not only Pd-rich A but also Co-rich B is among the most energetically stable structures. The density of states showed a clear distinct electronic structure between Pd-rich A and Co-rich B derived from the structural difference. This result indicates that PMA is not always present in a sandwich structure such as Pd-rich A. The results provide a useful guide to magnetic devices created using L10-ordered Co/Pd (001) on substrate Pd.

H0.43Ti0.93Nb1.07O5 engineered with large d-spacing of ~8.3 Å and two-dimensional ionic channels enables easy Na+ ion uptake., Key issues for Na-ion batteries are the development of promising electrode materials with favorable sites for Na+ ion intercalation/deintercalation and an understanding of the reaction mechanisms due to its high activation energy and poor electrochemical reversibility. We first report a layered H0.43Ti0.93Nb1.07O5 as a new anode material. This anode material is engineered to have dominant (200) and (020) planes with both a sufficiently large d-spacing of ~8.3 Å and two-dimensional ionic channels for easy Na+ ion uptake, which leads to a small volume expansion of ~0.6 Å along the c direction upon Na insertion (discharging) and the lowest energy barrier of 0.19 eV in the [020] plane among titanium oxide–based materials ever reported. The material intercalates and deintercalates reversibly 1.7 Na ions (~200 mAh g−1) without a capacity fading in a potential window of 0.01 to 3.0 V versus Na/Na+. Na insertion/deinsertion takes place through a solid-solution reaction without a phase separation, which prevents coherent strain or stress in the microstructure during cycling and ensures promising sodium storage properties. These findings demonstrate a great potential of H0.43Ti0.93Nb1.07O5 as the anode, and our strategy can be applied to other layered metal oxides for promising sodium storage properties.

Recent studies on crystal facet manipulation of anatase ${\text{TiO}}_{2}$ in photocatalysis have revealed that reduction and oxidation reactions preferably occur on (100)/(101) and (001) facets, respectively; however, a fundamental understanding of their origin is lacking. Here, as a result of first-principles calculations, we suggest that a dissimilar trend in the anisotropy of electron vs hole bulk transport in anatase ${\text{TiO}}_{2}$ can be a dominant underlying mechanism for the difference in photochemical activity. Photoexcited electrons and holes are driven to different facets, i.e., electrons on (100)/(101) and holes on (001), leading to the observed preference for either reduction or oxidation. This trend of electrons vs holes found in pure ${\text{TiO}}_{2}$ applies even for cases where a variety of dopants or defects is introduced.

Pseudo-capacitive transition metal chalcogenides have recently received considerable attention as a promising class of materials for high performance supercapacitors (SCs) due to their superior intrinsic conductivity to circumvent the limitations of corresponding transition metal oxides with relatively poor conductivity. However, the important challenge associated with the utilization of such high-capacitive electrode materials is the development of desirably structured electrode materials, enabling efficient and rapid Faradaic redox reactions and ultra long-term cycling. Here, we propose a hierarchically integrated hybrid transition metal (Cu-Ni) chalcogenide shell-core-shell (HTMC-SCS) tubular heterostructure using a facile bottom-up synthetic approach. The resultant HTMC-SCS electrode exhibits a high volumetric capacitance of 25.9 F cm−3 at a current density of 2 mA cm−2. Furthermore, asymmetric SCs based on an HTMC-SCS heterostructured electrode demonstrate a high power density (770 mW cm−3) and an energy density (2.63 mW h cm−3) as well as an ultrahigh reversible capacity with a capacitance retention of 84% and a long-term cycling stability of over 10,000 cycles. Based on experimental results and density functional theory calculations, these remarkably improved electrochemical features are discussed and explained in terms of the unique combination of the conductive CuS core and active NiS shell materials, hierarchical tubular open geometry with nanoscale inner/outer shell structure, and mechanical stress-mitigating interlayer on shell-core-shell interface, allowing highly reversible and efficient electrochemical redox processes coupled with fast charge transfer kinetics and an electrochemically stable structure.

Using ab initio calculations, the effects of uniaxial, biaxial, and hydrostatic strains on the magnetocrystalline anisotropy of L10-orderd FePt and CoPt alloys were systematically investigated. Interestingly, the rates and the signs of magnetocrystalline anisotropy changes of FePt and CoPt were determined by the directions and dimensions of strains. The calculation results are consistent with the previous experimental observations and are expected to provide directions to tailor magnetic properties of various types of L10-ordered FePt and CoPt systems.

The inherently low electrical conductivity of TiO2-based electrodes as well as the high electrical resistance between an electrode and a current collector represents a major obstacle to their use as an anode for lithium ion batteries. In this study, we report on high-density TiO2 nanotubes (NTs) branched onto a carbon nanofiber (CNF) “tree” that provide a low resistance current path between the current collector and the TiO2 NTs. Compared to a TiO2 NT array grown directly on the current collector, the branched TiO2 NTs tree, coupled with the CNF electrode, exhibited ∼10 times higher areal energy density and excellent rate capability (discharge capacity of ∼150 mA·h·g−1 at a current density of 1,000 mA·g−1). Based on the detailed experimental results and associated theoretical analysis, we demonstrate that the introduction of CNFs with direct electric contact with the current collector enables a significant increase in areal capacity (mA·h·cm−2) as well as excellent rate capability.

Bi3+ as a dopant in wide-band-gap yttria (Y2O3) has been used as a green light emission center or a sensitizer of co-doped rare earth elements. Because the photoluminescence (PL) properties of Y2O3:Bi3+ vary remarkably according to heat treatment, the roles of point defects have been an open question. By using first-principles calculations and thermodynamic modeling, we have thoroughly investigated the formation of point defects in Y2O3:Bi3+ at varying oxygen partial pressures and temperatures, as well as their roles in PL. The photoabsorption energies of the Bi3+ dopant were predicted to be 3.1 eV and 3.4 eV for doping at the S6 and the C2 sites, respectively, values that are in good agreement with the experimental values. It was predicted that an oxygen interstitial (Oi) and an oxygen vacancy (VO) are the dominant defects of Y2O3:Bi3+ at ambient pressure and an annealing temperature of 1300 K (3.19 × 1016 cm−3 for 1% Bi doping), and the concentrations of these defects in doped Y2O3 are approximately two orders of magnitude higher than those in undoped Y2O3. The defect in Y2O3:Bi3+ was predicted to reduce the intensity of PL from Bi3+ at both S6 and C2 sites. We verify our computational predictions from our experiments that the stronger PL of both 410 and 500 nm wavelengths was measured for the samples annealed at higher oxygen partial pressure.