Search

While inheriting the exceptional merits of single atom catalysts, diatomic site catalysts (DASCs) utilize two adjacent atomic metal species for their complementary functionalities and synergistic actions. Herein, a DASC consisting of nickel-iron hetero-diatomic pairs anchored on nitrogen-doped graphene is synthesized. It exhibits extraordinary electrocatalytic activities and stability for both CO2 reduction reaction (CO2RR) and oxygen evolution reaction (OER). Furthermore, the rechargeable Zn-CO2 battery equipped with such bifunctional catalyst shows high Faradaic efficiency and outstanding rechargeability. The in-depth experimental and theoretical analyses reveal the orbital coupling between the catalytic iron center and the adjacent nickel atom, which leads to alteration in orbital energy level, unique electronic states, higher oxidation state of iron, and weakened binding strength to the reaction intermediates, thus boosted CO2RR and OER performance. This work provides critical insights to rational design, working mechanism, and application of hetero-DASCs., Diatomic site catalysts utilize two adjacent atomic metal species for their complementary functionalities and synergistic actions. Here, the authors report the orbital coupling of hetero-diatomic nickel-iron site boosts CO2 reduction reaction and oxygen evolution reaction.

The single-metal atoms coordinating with the surface atoms of the support constitute the active centers of as-prepared single-atom catalysts (SACs). However, under hash electrochemical conditions, (1) supports' surfaces may experience structural change, which turn to be distinct from those at ambient conditions; (2) during catalysis, the dynamic responses of a single atom to the attack of reaction intermediates likely change the coordination environment of a single atom. These factors could alter the performance of SACs. Herein, we investigate these issues using Mo2C(100)-supported single transition-metal (TM) atoms as model SACs toward catalyzing the oxygen reduction reaction (ORR). It is found that the Mo2C(100) surface is oxidized under ORR turnover conditions, resulting in significantly weakened bonding between single TM atoms and the Mo2C(100) surface (TM@Mo2C(100)_O* term for SAC). While the intermediate in 2 e- ORR does not change the local structures of the active centers in these SACs, the O* intermediate emerging in 4 e- ORR can damage Rh@ and Cu@Mo2C(100)_O*. Furthermore, on the basis of these findings, we propose Pt@Mo2C(100)_O* as a qualified ORR catalyst, which exhibits extraordinary 4 e- ORR activity with an overpotential of only 0.33 V, surpassing the state-of-the-art Pt(111), and thus being identified as a promising alternative to the commercial Pt/C catalyst.

Electrocatalytic (EC) and photoelectrocatalytic (PEC) water splitting are effective processes for mass production of hydrogen; however, they usually endure large overpotentials or low conversion efficiency. Therefore, the exploration of efficient and scale-up feasible electrocatalysts and cocatalysts is highly desirable. The novel and emerging single Pt atom catalyst is promising to perfectly satisfy the strict requirements of the EC and PEC hydrogen evolution reaction (HER). Herein, a partial nitridation modification strategy is proposed to anchor single platinum (Pt) atoms through the coordination of lone pair electrons of nitrogen atoms with the unoccupied d orbital electrons of Pt atoms. The fixed single Pt atoms are located on a porous nickel framework on nickel foam (Pt1/N–Ni/NF), and show excellent EC activity with a low overpotential of 33 mV at −10 mA cm−2 for the HER in neutral solution, which is superior to that of the bench-mark commercial electrocatalyst. In addition, the Pt1/N–Ni has been integrated into a Cu2O nanowire photocathode as a cocatalyst, and presents a high photocurrent density of −11.9 mA cm−2 at 0 V vs. RHE and PEC conversion efficiency of 1.75%. The rational design and preparation of a single atom catalyst both as an electrocatalyst for EC and a cocatalyst for PEC water splitting opens up a new avenue for boosting EC and PEC water splitting performance.

Catalytic decomposition of hydrazine monohydrate (N2H4·H2O) represents a promising hydrogen storage/production technology. Nowadays, the development of N2H4 decomposition catalysts is sluggish due primarily to a lack of universal screening rules. Herein, with the aid of first-principles calculations, we identify a reliable descriptor, N2H4 adsorption energy, to properly address N2H4 decomposition kinetics on TM catalysts. Particularly, we extract linear scaling relations between the adsorption energetics of key intermediates, H (and N2H3) vs. NH2 that exert a fundamental limitation on the H2 selectivity of N2H4 decomposition over elementary transition metal catalysts. We propose that catalysts with optimum H2 selectivity may be achieved by independently stabilizing and destabilizing adsorption of H (or N2H3, or both) and NH2, respectively. The disclosed dissociation behaviors of N2H4 and their physical origins are expected to advance the rational design of advanced catalysts for selectively promoting H2 production from N2H4.

The emerging two-dimensional tellurene has been demonstrated to be a promising candidate for photoelectronic devices. However, there is a lack of comprehensive insight into the effects of vacancies and common adsorbates (i.e., O2 and H2O) in ambient conditions, which play a crucial role in semiconducting devices. In this work, with the aid of first-principles calculations, we demonstrate that H2O and O2 molecules behave qualitatively differently on tellurene, while water adsorption can be remarkably promoted by adjacent preadsorbed O2. Upon the formation of Te vacancies, the adsorption of both O2 and H2O molecules is enhanced. More importantly, the existence of H2O and Te vacancies can dramatically facilitate the dissociation of O2, suggesting that tellurene may be readily oxidized in humid conditions. In addition, it is found that the electronic properties of tellurene are well preserved upon either H2O or O2 adsorption on the surface. In sharp contrast, vacancies enable significant modification on the band structure. Specifically, an indirect-to-direct band gap transition is found at a vacancy concentration of 5.3%.

In this study, FeOx and CoOx thin films were successively and uniformly coated on high-surface-area carbon nanotubes by plasma-enhanced atomic layer deposition, which formed a heterojunction of the two oxides. As an electrocatalyst in the oxygen evolution reaction (OER), CoOx/FeOx/CNTs showed excellent electrocatalytic performance in terms of catalytic activity and stability. The overpotential of CoOx/FeOx/CNTs in OER was only 308 mV at 10 mA cm−2, which was lower than those of the pure oxides: CoOx/CNTs (392 mV) and FeOx/CNTs (406 mV). The as-prepared electrocatalyst also displayed better stability than the reference RuO2 material, with almost no attenuation of current density in contrast to the 10% loss seen with RuO2. The OER performance of CoOx/FeOx/CNTs was superior to those of its oxide components due to the formation of heterojunction, which led to a smoother reaction path and a lower overpotential for OER compared to pure oxides, as supported by the density-functional theory (DFT) calculations. These results provide a new direction for the preparation of electrocatalysts for metal–air batteries and water splitting reactions.

The separation efficiency and recombination rate of photogenerated electron–hole pairs play a vital role in photocatalytic activity. We report the incorporation of ultra-small subnano TiOx clusters as outstanding cocatalysts on the surface of plasmonic Ag/AgCl to boost the simulated sunlight photoreactivity. The formed structure is in favour of an efficient photoinduced electron–hole transfer process and suppressing the recombination rate, yielding an enhanced photocatalytic activity. The experimental results show that TiOx@Ag/AgCl exhibits an extraordinarily high activity in the photodegradation of tetracycline (TC) under simulated sunlight irradiation, which is about 21.7 times higher than that of bare Ag/AgCl. Their corresponding apparent quantum efficiency (AQE) is up to 0.917% at 435 nm and 17.459% at 350 nm during the photodegradation of TC, respectively. Moreover, TiOx@Ag/AgCl shows an outstanding long-term stability without obvious performance loss even after ten cycles of photodegradation. The possible enhancement mechanism of the reaction rate is proposed based on the compositional, structural, and optical property analysis. We found that h+ was the major reactive species in the photodegradation process of TC. Our findings on ultra-small TiOx cocatalysts would shed light on the design and development of highly efficient oxide cluster cocatalysts.

Currently, development of metal sulfide anodes for sodium-ion batteries (SIBs) with high capacity, fast charging/discharging, and good cycling performance continues to present a great challenge. He...

The exploration of novel two-dimensional semimetallic materials is always an attractive topic. We propose a series of two-dimensional silicon carbides with a tetragonal lattice. The band structure of silicon carbides with tetragonal carbon rings and silicon rings exhibits Dirac cones. Interestingly, the Dirac cone of tetragonal SiC originates from a “ring coupling” mechanism. This mechanism refers to the mutual coupling between the four carbon atoms in the tetragonal C ring, and the same coupling in the tetragonal Si ring. Additionally, the “ring coupling” mechanism is applicable to other group IV binary compounds such as monolayer GeC and SnC. This work provides reliable evidence for the argument that two-dimensional tetragonal materials can produce Dirac cones.

Developing earth-abundant transition metal (TM)-based electrocatalysts toward oxygen reduction reaction (ORR) is significant in overcoming the high cost of fuel cells. Herein, using an as-synthesized proton-conductive coordination polymer (termed TM-DHBQ) as a template, we investigate the ORR performance of a series of such TM-DHBQs via screening 3d, 4d, and 5d TMs. We find that most 3d TM-DHBQs exhibit distinguished durability under ORR turnover conditions. The formation energies of these TM-DHBQs and adsorption free energies of ORR intermediates show a good correlation with the number of outer electrons of TM ions in TM-DHBQs, enabling the formation energy as a robust ORR activity descriptor. The Sabatier-type volcano plot and microkinetic modeling coidentify Fe- and Co-DHBQs as two promising alternatives to Pt-based ORR electrocatalysts. For those TM-DHBQs showing strong bonding to oxygen species, the ORR intermediate is found to combine with the TM ion serving as the active center.

Rechargeable Zn–air battery is a promising alternative to the widely used lithium–ion battery. Its practical use, however, is hindered by low power density, unsatisfactory energy efficiency, poor durability, and unstable voltage output. Here, we demonstrate a bifunctional catalyst for oxygen evolution and oxygen reduction reactions based on 3D MXene coupled with nitrogen-doped cobalt selenide nanocrystals (N-CoSe2/3D Ti3C2Tx). Combining experimental characterizations and density functional theory (DFT), the excellent performance is ascribed to enhanced intrinsic activity of CoSe2 due to electron transfer from MXene, N doping which lowers the reaction energy barriers, and 3D MXene architecture which provides large specific surface area, high porosity, and good conductivity. Moreover, Zn–air battery equipped with the developed N-CoSe2/3D MXene as the air cathode exhibits better power/energy densities and long-lasting cycling life (over 500 cycles) compared with that of mixed Pt/C and RuO2.

Rechargeable zinc-ion batteries (ZIBs) appear to be a promising candidate for large-scale energy storage system because of the abundance and inherent safety of the zinc negative electrode. Despite these benefits, huge polarization caused by the intercalation of multivalent charge carrier Zn2+ into the cathodic hosts remains a long-standing challenge impeding the development of high-performance ZIBs. Herein, we demonstrate the viability of the V2O5 nanorods constructed 3D porous architectures (3D-NRAs-V2O5) as cathode for ZIBs. Notably, the 3D-NRAs-V2O5 can be scaled up to kilo-gram production based on a simple sol-gel reaction followed by an annealing process. The synergic contributions from the 3D porous framework and layered structures of the 3D-NRAs-V2O5 lead a more facile Zn2+ ions (de)intercalation storage process. Consequently, it offers high reversible capacity of 336 mAh g−1 at a high current density of 50 mA g−1 and exhibits excellent long-term cyclic stability with a capacity retention of 85% over 5000 cycles at a high current density of 10 A g−1. Furthermore, the use of various ex-situ characterization techniques and first-principles calculations has successfully unravelled the Zn2+ ions storage mechanism of the 3D-NRAs-V2O5. Besides the excellent electrochemical performance of the 3D-NRAs-V2O5, it can also be easily scaled up based on the simple synthetic protocol, which shows great potential to be practically used for the next-generation large-scale energy storage applications.

The rhenium-based transition metal dichalcogenides (TMDs), as new members in the TMDs family, have raised great interests recently. Due to the anisotropic structure and unique photoelectric properties, they have potential applications for electrochemical energy conversion and storage. In this work, we performed density functional theory (DFT) calculations on pristine 1T′-ReSe2 toward hydrogen evolution reaction (HER). The results indicated that the Gibbs free energy of the 1T′-ReSe2 edge site for HER could be as small as 0.01 eV, superior to other reported TMDs. Experimentally, we developed a strategy to fabricate sub-5 nm sized 1T′-ReSe2 nanoflakes on carbon nanotubes. Such a small size for the nanoflakes brought abundant edge exposure, which boosted the catalytic activity in the HER. Specifically, the 1T′-ReSe2 nanoflakes needed only 23 and 60 mV overpotentials to achieve −1 and −10 mA cm−2 current densities, along with a low Tafel slope of 37 mV dec−1 and a high exchange current density of 0.3 mA cm−2. The edge-rich and layered 1T′-ReSe2 was also explored as an anode for sodium ion battery. The in operando X-ray absorption near edge structure (XANES) technique was applied to investigate the TMD behavior in real-time during the sodiation/desodiation process. The in situ results revealed that the nanosized 1T′-ReSe2 is electrchemically reversible during discharge/charge cycles. The electrochemical test results demonstrated that 1T′-ReSe2 could be a promising anode material for alkaline batteries.

In this paper, van der Waals heterostructures consisting of MX2 (M = Zr, Hf and X = S, Se) monolayers are modeled. The favorable stacking and stability of the modeled monolayer heterostructures are confirmed through binding energy and phonon dispersion calculations. After confirming stability, the electronic and thermoelectric properties of these compounds are explored using the first-principles calculations combined with semiclassical Boltzmann transport theory. It is found that type-II band alignment in ZrS2 HfSe2 facilitates charge separation for optoelectronics and solar energy conversion. All studied heterostructures show remarkably higher electrical conductivity than corresponding monolayers, responsible for large power factor values, especially at 1200 K. These findings indicate that the creation of van der Waals heterostructures from MX2 may be promising for efficient optoelectronic and thermoelectric devices.

Synthesis of high-performance and cost-effective electrocatalysts towards hydrazine electrooxidation is vital to develop the direct hydrazine fuel cell (DHFC) as a viable energy conversion technology. Herein, we report a combined experimental and theoretical study of nickel phosphides (NixP) as promising catalysts for hydrazine electrooxidation. NixP nanowire array supported on a Ni foam (NF) was synthesized by a one-step phosphorization method using hypophosphite as a P-source. Ni12P5 and Ni2P phases are observed as the products of the direct phosphorization of commercial NF under the applied conditions with Ni2P nanoparticles exclusively distributing on the surface of Ni12P5. The NixP/NF catalyst exhibits a synergetic capabilities of exceptionally high activity, excellent durability and nearly 100% selectivity towards the complete electrooxidation of hydrazine in alkaline condition, which is among the best performance reported on hydrazine electrooxidation catalysts. First-principles calculations have been conducted to gain insight into the catalytic mechanism of Ni phosphides towards hydrazine electrooxidation.

The development of advanced electrocatalysts for ambient NH3 production is increasingly attractive but severely plagued by low activity and poor selectivity. Herein, we reported first-principles results to explore the potential of transition metal (Sc– Cu, Mo, Ru, Rh, Pd, Re, Ir and Pt) atoms embedded in monolayer g-C3N4 as efficient single-atom electrocatalysts for NH3 production. We found that embedding single Pt atoms in g-C3N4 enables robust stability and good electrical conductivity and particularly results in excellent activity (with a low limiting potential of −0.24 V) at room temperature. Furthermore, the dominant N2 adsorption over that of H atoms suggests superior NH3 selectivity. Mechanistic analyses disclosed that single Pt atoms and g-C3N4 work in concert to address the significant deviation from the nitrogen linear scaling relationships, synergistically providing highly active sites for ambient electrochemical NH3 synthesis.

To gain fundamental understanding of how catalyst composition and structure affects catalytic reaction chemistry is of paramount importance in the search of high-performance electrocatalysts for practical applications. Herein, we report an in-depth study of Ni–Mo–O derived electrocatalysts, a promising but less well investigated electrocatalytic material for the HER, with a focus on the correlation of phase/microstructure and HER activity. A series of Ni foam-supported Ni–Mo–O derived electrocatalysts were prepared using a simple hydrothermal method, followed by annealing treatment under a H2 atmosphere. Depending upon the annealing temperature, different Ni–Mo alloys were formed and the resulting Ni–Mo/MoO3−x nanocomposites exhibited varied HER activities under alkaline conditions following the order of Ni10Mo/MoO3−x > Ni4Mo/MoO3−x > Ni3Mo/Ni4Mo/MoO3−x. A combination of phase/structure/chemical state analyses and first-principles calculations were conducted to gain insight into the variation of apparent catalytic activity. Our study found that the variation of HER activity under alkaline conditions should stem from the intrinsic activity change, which was associated with the change of the amount of MoO3 sites and the variation of Had–alloy binding strength.

Synthesis of high-performance and cost-effective catalysts towards the hydrogen evolution reaction (HER) is critical in developing electrochemical water-splitting as a viable energy conversion technique. For non-precious metal Co- and Ni-based catalysts, hydroxides were found to form on the surface of the catalysts under alkaline environments and benefit the catalytic performance, whereas there is limited systematic study on the explicit influence of hydroxides on the electrocatalytic mechanism and performance of these catalysts. Herein, we report a close correlation observed between the amount of the surface hydroxides formed and the resulting electrocatalytic performance of a Co-Mo-O nanocatalyst through careful comprehensive structural and property characterizations. We found that an appropriate amount of hydroxide can be moderated by simply coating the catalyst surface with carbon shells to optimize the catalytic properties. As a result, a carbon-coated Co-Mo-O nanocatalyst was successfully developed and is among the best reported non-precious HER catalysts with a superior electrocatalytic activity and outstanding durability for the HER under alkaline environment. First-principles calculations were further conducted to probe the nature of the active sites and the role of hydroxides in the Co-Mo-O@C/NF catalyst towards the HER.

Photocatalytic carbon dioxide (CO2) reduction is a sustainable and energy-consumption-free route to directly convert the greenhouse gas into chemicals. Given the vast amount of greenhouse gases, numerous efforts have been devoted to developing inorganic photocatalysts due to their stable, low-cost and environmental-friendly properties. However, more efficient titanium dioxide (TiO2) without noble metal or sacrifice/organic agent is highly desirable for CO2 reduction practical application, and it is also difficult and urgently in demand for TiO2 producing selectively valuable compounds, i.e. industrial chemicals and fuels. Here, we develop a novel “adatom at step” strategy via anchoring single tungsten atom oxide (STAO) site on intrinsic steps of classic TiO2 nanoparticles. The composition of single-sites can be controlled by tuning the ratio of adatom W5+ to neighboring Ti3+, resulting in significant CO2 reduction efficiency and selectively yield of carbon monoxide (CO) or methane (CH4) as main products. The W5+-dominated catalysts can achieve an ultrahigh photocatalytic CH4 production of 59.3 μmol/g/h, while the Ti3+-dominated catalysts can achieve a CO production of 181.4 μmol/g/h, which both exceed those of pristine TiO2 by more than one order of magnitude. The mechanism relies on the accurate control of atomic sites with high coverage and the subsequent excellent electron-hole separation along with favorable adsorption-desorption of intermediates on sites. This approach not only provides a novel strategy for inorganic catalytic single-sites with superior performance, but also identifies the rational design mechanisms of the efficient site with controllable production.

Optimization of solid electrolytes (SEs) is of great significance for lithium-based solid state batteries (SSBs). However, insufficient Li ion transport, deficient interfacial compatibility and formation of lithium dendrites lead to poor cycling performance. Based on Li+ conductive metal-organic frameworks (LCMOFs), herein a multiscale optimization strategy is put forward to facilitate Li+ transport within the MOFs (molecular scale), between the MOFs' boundaries (nanoscale) and across the SE/electrode interface (microscale) in SSBs. LCMOFs are obtained by binding Li+ onto ionogenic chemical groups (-CO2H, -SO3H and -OH) in nanoscale dispersed MOFs. Both experimental results and DFT simulations confirm the key role of ionogenic groups for Li+ transport. Furthermore, benefiting from the optimized interfaces between LCMOF crystals, SEs with excellent electrochemical properties are obtained, including a high ionic conductivity of 1.06 × 10-3 S cm-1 at 25 °C, a wide electrochemical window from 2.0 to 4.5 V, low interfacial resistances and stable Li plating/stripping. The fabricated Li|SE|LiFePO4 SSB exhibits high and stable charge/discharge capacities under wide operation temperatures ranging from -20 to 60 °C.

Highlights 3D flower-like architecture assembled by NH4V4O10 nanobelts (3D-NVO) was fabricated.The Zn2+ ion was intercalated into NVO cathode within the interlayer region (NH4V4O10 ↔ ZnxNH4V4O10).The 3D-NVO cathode could deliver a large reversible capacity of 485 mAh g−1 at a current density of 100 mA g−1 for zinc-ion battery. Electronic supplementary material The online version of this article (10.1007/s40820-020-0401-y) contains supplementary material, which is available to authorized users., Given the advantages of being abundant in resources, environmental benign and highly safe, rechargeable zinc-ion batteries (ZIBs) enter the global spotlight for their potential utilization in large-scale energy storage. Despite their preliminary success, zinc-ion storage that is able to deliver capacity > 400 mAh g−1 remains a great challenge. Here, we demonstrate the viability of NH4V4O10 (NVO) as high-capacity cathode that breaks through the bottleneck of ZIBs in limited capacity. The first-principles calculations reveal that layered NVO is a good host to provide fast Zn2+ ions diffusion channel along its [010] direction in the interlayer space. On the other hand, to further enhance Zn2+ ion intercalation kinetics and long-term cycling stability, a three-dimensional (3D) flower-like architecture that is self-assembled by NVO nanobelts (3D-NVO) is rationally designed and fabricated through a microwave-assisted hydrothermal method. As a result, such 3D-NVO cathode possesses high capacity (485 mAh g−1) and superior long-term cycling performance (3000 times) at 10 A g−1 (~ 50 s to full discharge/charge). Additionally, based on the excellent 3D-NVO cathode, a quasi-solid-state ZIB with capacity of 378 mAh g−1 is developed. Electronic supplementary material The online version of this article (10.1007/s40820-020-0401-y) contains supplementary material, which is available to authorized users.

Understanding on the photogenerated charge separation from a microscopic level remains a challenge and is highly desirable as it provides a cornerstone for designing high-performance photocatalysts. Herein, facet engineering is chosen as a tool to reveal the relationship between the charge separation/transfer and crystal structure. A series of BiOBr nanosheets with dominantly exposed facets of (001) or (010) as well as different lateral facet exposure ratios are constructed via adjusting pH value during the hydrothermal process. It is found that exposure of anisotropic crystal facets allows the separative transfer of photogenerated electrons and holes onto the lateral facet and dominantly exposed facets, respectively, which is attributed to the junction formed between distinct facets (i.e., facet junction). In the case of BiOBr-5 with (010)/(102) facet junction, the electron transfer rate (kET) and efficiency (ηET) are 3.658 × 106 s-1 and 54.09%, which are superior than the counterpart of BiOBr-1 with (001)/(110) facet junction. The fast electron transfer rate and high transfer efficiency of BiOBr-5 result in the high CO evolution rate from CO2 photoreduction under artificial sunlight. Our work may bring some new insights into the mechanism understanding of facet junction and rational design of photocatalysts with high performance for solar energy storage in future.

Quantitatively simultaneous detection of hydroquinone (HQ) and catechol (CC) with low limit of detection (LOD) and wide detection range is of critical significance in water quality control but faces great challenges due to their isomer similarity and a large range of contamination concentrations. Herein, high density of boron atoms is introduced into nitrogen-doped nanocarbon (BNC) to realize simultaneous detection of HQ and CC with a large separation of peak potential (ΔEp) of 111 mV between oxidations of HQ and CC by differential pulse voltammetry. Besides, extremely low LODs of 33.3 and 16.3 nM for HQ and CC were achieved through amperometric I-t curve with wide linear ranges of 0.099 ∼ 43340 µM and 0.049 ∼ 5110 µM, respectively. The systematical investigation of the enhancement mechanism by spectroscopy and DFT calculations clearly reveal that the boron-nitrogen co-doping creates rich electroactive sites for high sensing performance toward HQ and CC, thereby shedding lights on the great effect of a unique nanostructure on electrocatalysis in sensing applications.

Interface engineering in heterogeneous catalysis is fascinating because of the modulation of charge-transfer processes and catalytic activity. Herein, by constructing Fe(III)–TiO2 systems with diff...

Catalytic decomposition of hydrazine monohydrate (N2H4·H2O) over Ni-based catalysts has been extensively investigated as a promising hydrogen generation means for onboard or portable applications. Despite the flourishing experimental development of Ni-based catalysts, the mechanistic study of catalytic decomposition process is still in its infancy. Herein, we report a first-principles study of the elementary steps in N2H4 decomposition over the Ni(111) surface. The calculations show that the decomposition behaviors of N2H4 strongly depend on the surface coverage. At a higher coverage of 0.25 ML, the calculation results are in excellent agreement with the experimental observation. Our calculations, for the first time, presented a complete picture of catalytic selective H2 generation from N2H4 decomposition over Ni surface. Such mechanistic understanding may lay the foundation for the development of high-performance Ni-based catalyst for promoting hydrogen generation from N2H4.

Based on (hybrid) first-principles calculations, material properties (structural, electronic, vibrational, optical, and photocatalytic) of van der Waals heterostructures and their corresponding monolayers (transition metal dichalcogenides and MXenes) are investigated. MoSe2/Zr2CO2 and WSe2/Zr2CO2 heterostructures are found to be indirect band gap semiconductors with type-I and type-II band alignment, respectively, while MoSe2/Zr2CF2 and WSe2/Zr2CF2 are metals. A transition from type-I to type-II band alignment is achieved in MoSe2/Zr2CO2 by moderate compressive and tensile strain. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, whereas red and blue shifts are observed in the positions of the excitonic peaks under tensile and compressive strain in the heterostructures. Photocatalytic studies show that MoSe2/Zr2CO2 and WSe2/Zr2CO2 heterostructures can oxidize H2O/O2 to O2, but unlike their parent monolayers (MoSe2, WSe2 and Zr2CO2) these heterostructures fail to reduce H+ to H2.

The edge site of two-dimensional (2D) transition metal dichalcogenides (TMDs) is active towards the hydrogen evolution reaction (HER). Herein, a feasible synthesis of sub-100 nm molybdenum/tungsten diselenide [Mo(1−x)WxSe2] crystals is described. The abundant edge exposure and heteroatom-doping synergistically boost the catalysis of HER by this material. In this work, sub-100 nm Mo(1−x)WxSe2 single crystals were grown on a nitrogen-doped multiwall carbon nanotube before being applied as HER electrocatalysts. At x = 0.13 ± 0.02, the Mo(1−x)WxSe2 shows optimal HER catalytic performance with low overpotentials (70 and 129 mV) required to achieve current densities of −1 and −10 mA cm−2, respectively, along with a Tafel slope of 53.6 mV dec−1 and an exchange current density of 49.5 μA cm−2. Density functional theory (DFT) calculations indicate that the Gibbs free energy of the HER process at the edge site of the crystals reaches a minimum value of 0.06 eV, which is lower than when the reaction is catalysed on Pt active sites. This study provides a general approach to increasing the edge proportion of the catalyst material and activating the terrace of the 2D materials for catalysis, which may be of benefit to the design and fabrication of other TMDs-based compounds.

The stacking of monolayers in the form of van der Waals heterostructures is a useful strategy for band gap engineering and the control of dynamics of excitons for potential nano-electronic devices. We performed first-principles calculations to investigate the structural, electronic, optical and photocatalytic properties of the SiC–MX2 (M = Mo, W and X = S, Se) van der Waals heterostructures. The stability of most favorable stacking is confirmed by calculating the binding energy and phonon spectrum. SiC–MoS2 is found to be a direct band gap type-II semiconducting heterostructure. Moderate in-plane tensile strain is used to achieve a direct band gap with type-II alignment in the SiC–WS2, SiC–MoSe2 and SiC–WSe2 heterostructures. A difference in the ionization potential of the corresponding monolayers and interlayer charge transfer further confirmed the type-II band alignment in these heterostructures. Furthermore, the optical behaviour is investigated by calculation of the absorption spectra in terms of e2(ω) of the heterostructures and the corresponding monolayers. The photocatalytic response shows that the SiC–Mo(W)S2 heterostructures can oxidize H2O to O2. An enhanced photocatalytic performance with respect to the parent monolayers makes the SiC–Mo(W)Se2 heterostructures promising candidates for water splitting.

The rational design of efficient electrocatalysts based on comprehensive mechanistic insights is crucial to widespread penetration of future sustainable and eco-friendly energy technologies. Herein, via systematic first-principles calculations, we propose a general two-step strategy for developing highly active single-atom catalyst (SACs) supported on a prototypical substrate (g-C3N4) for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Our results show that the intrinsic activity can be properly evaluated by a robust screening rule, and, particularly, the descriptor can be readily calculated by a few handy and basic properties. According to the knowledge, we propose an effective approach (i.e., creating an N vacancy in the cavity of g-C3N4) that enables further activity improvement. Specifically, the predicted ORR/OER activity on Ag and Rh based SACs are comparable or even outperforms that of respective benchmark catalysts. This study not only provides several promising candidates for bifunctional ORR/OER, but also directs a new avenue for rational design of high-performance catalysts.

In this work, the MoSe 2 nanosheets were firstly prepared by hydrothermal method. We found the magnetism of MoSe 2 nanosheets obviously enhance with the decrease of temperature. Further, we fabricated a memristor using as-prepared MoSe 2 nanosheets, and observed temperature can regulate the memristor memory behavior of the device. In a word, this work reveals the potential multifunctional applications of MoSe 2 nanosheets with magnetism and memristive memory effects in future device technology.

We use first-principles calculations to systematically explore the potential of transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au) embedded in buckled monolayer g-C3N4 as single-atom catalysts. We show that clustering of Sc and Ti on g-C3N4 is thermodynamically impeded and that V, Cr, Mn, and Cu are much less susceptible to clustering than the other TM atoms under investigation. Strong bonding of the transition metal atoms in the cavities of g-C3N4 and high diffusion barriers together are responsible for single-atom fixation. Analysis of the CO oxidation process indicates that embedding of Cr and Mn in g-C3N4 gives rise to promising single-atom catalysts at low temperature.

Recently, a two-dimensional Pma2-SiS monolayer has been predicted to show promising electronic properties [Nano Lett., 2015, 16, 1110]. However, it is suggested that Pma2-SiS is not suitable as an anode for Li-ion batteries [J. Power Sources, 2016, 331, 391]. By employing density functional theory calculations, we find that an ultrahigh theoretical specific capacity of 893.4 mA h g−1 can be achieved in Pma2-SiS due to the strong bonding between Li and the S atoms released from Si–S bond breakage. Additionally, the low barrier of Li-diffusion (0.08 eV) along the Si–Si bond direction and the moderate average voltage (1.12 V) of the Li intercalation suggest that Pma2-SiS is promising as an anode material for Li-ion battery applications.

Herein, we systematically explored the electronic properties of Sc-based MXenes via first-principles calculations, with the aim to extend their applicability. OH-Functionalized carbides and OH/SH-terminated nitrides manifest ultralow work functions, potential in field-effect transistors. Furthermore, we identified three novel semiconductors (Sc2CCl2, Sc2C(SH)2, and Sc2NO2). Specifically, Sc2NO2 is a spin gapless semiconductor, promising for spintronics. Type-II heterojunctions are readily available between Sc-based semiconducting MXenes, facilitating charge separation for optoelectronics and solar energy conversion. Further photocatalytic analysis indicates that Sc2CCl2 is capable of oxidizing H2O into O2.

The platinum single-atom-catalyst is verified as a very successful route to approach the size limit of Pt catalysts, while how to further improve the catalytic efficiency of Pt is a fundamental scientific question and is challenging because the size issue of Pt is approached at the ultimate ceiling as single atoms. Here, a new route for further improving Pt catalytic efficiency by cobalt (Co) and Pt dual-single-atoms on titanium dioxide (TiO2 ) surfaces, which contains a fraction of nonbonding oxygen-coordinated Co-O-Pt dimers, is reported. These Co-Pt dimer sites originate from loading high-density Pt single-atoms and Co single-atoms, with them anchoring randomly on the TiO2 substrate. This dual-single-atom catalyst yields 13.4% dimer sites and exhibits an ultrahigh and stable photocatalytic activity with a rate of 43.467 mmol g-1 h-1 and external quantum efficiency of ≈83.4% at 365 nm. This activity far exceeds those of equal amounts of Pt single-atom and typical Pt clustered catalysts by 1.92 and 1.64 times, respectively. The enhancement mechanism relies on the oxygen-coordinated Co-O-Pt dimer coupling, which can mutually optimize the electronic states of both Pt and Co sites to weaken H* binding. Namely, the "mute" Co single-atom is activated by Pt single-atom and the activity of the Pt atom is further enhanced through the dimer interaction. This strategy of nonbonding interactive dimer sites and the oxygen-mediated catalytic mechanisms provide emerging rich opportunities for greatly improving the catalytic efficiency and developing novel catalysts with creating new electronic states.

The fluorinated boron nitride (F-BN) nanostructures are found to be fully spin polarized and half-metallic by means of first-principles calculations based on the Heyd-Scuseria-Ernzerhof hybrid functional. It is found that the full spin polarization and 1 μB local moment in F-BN nanotubes are independent of tube radius and it is also robust in planar ribbons and sheets. The long-ranged ferromagnetic coupling between local moments decreases with decreasing tube radius. This suggests that F-BN systems with small local curvatures could be more easily experimentally observed and have greater potential applications in spin devices.

The progress on sodium-ion battery technology faces many grand challenges, one of which is the considerably lower rate of sodium insertion/deinsertion in electrode materials due to the larger size of sodium (Na) ions and complicated redox reactions compared to the lithium-ion systems. Here, it is demonstrated that sodium ions can be reversibly stored in Zn-Sb intermetallic nanowires at speeds that can exceed 295 nm s-1. Remarkably, these values are one to three orders of magnitude higher than the sodiation rate of other nanowires electrochemically tested with in situ transmission electron micro­scopy. It is found that the nanowires display about 161% volume expansion after the first sodiation and then cycle with an 83% reversible volume expansion. Despite their massive expansion, the nanowires can be cycled without any cracking or facture during the ultrafast sodiation/desodiation process. Additionally, most of the phases involved in the sodiation/desodiation process possess high electrical conductivity. More specifically, the NaZnSb exhibits a layered structure, which provides channels for fast Na+ diffusion. This observation indicates that Zn-Sb intermetallic nanomaterials offer great promise as high rate and good cycling stability anodic materials for the next generation of sodium-ion batteries.

MXenes are one of the most promising electrode materials in energy storage devices owing to their unique properties, but the practical applications have been severely plagued by the low accessible electroactive area due to their inevitable restacking during electrode preparation. Recent experimental studies have reported that the additivepolyaniline (PANI) enables us to greatly improve the electrochemical performance of MXene-based electrodes. Herein, we systematically investigate the electronic, electrochemical and mechanical properties of MXene/PANI composites, aiming to acquire a comprehensive insight. The results demonstrate that the insertion of PANI allows effective suppression of the restacking and enhances the electric conductivity and mechanical flexibility. Moreover, the presence of PANI maintains the high Na adsorption strength and fast Na mobility and preserves well the average open circuit voltage features and the maximum Na content of MXenes. These favorable attributes may not only collectively explain the experimentally observed outstanding electrochemical performance, but also render the MXene/PANI composites promising anode candidates in Na-ion batteries.

Non-faradaic catalytic decomposition of hydrazine (N2H4) is a vital but underappreciated problem for direct hydrazine fuel cell (DHFC). Herein, a careful study of a nickel molybdenum oxide derived catalyst for N2H4 electrooxidation is reported, with special focus on the method for effectively suppressing non-faradaic decomposition of hydrazine as well as the underlying mechanism. A hierarchically nanostructured catalyst consisting of tiny Ni10Mo alloy nanoparticles dispersed on porous Ni Mo O nanosheets is synthesized using a simple hydrothermal method followed by reductive annealing treatment. Thus-prepared catalyst shows high activity towards N2H4 electrooxidation, but is entangled with the problematic selectivity towards non-faradaic decomposition of N2H4. Our study find that this problem can be circumvented by a controlled phosphorization method. The resultant catalyst (Ni2P@Ni10Mo/Ni-Mo-O/NF) exhibits exceptionally high activity, excellent durability and nearly 100 % selectivity towards N2H4 electrooxidation. A detailed structural characterization and density functional theory calculations are conducted to understand the phosphorization-induced modification of catalytic property.

The search for large-band-gap quantum spin Hall (QSH) insulators, in which the dissipationless gapless edge states lie inside the bulk gap, is critical for their practical applications at room temperature. Based on first-principles calculations, we propose a sort of QSH insulator in a rectangular structure, i.e., AuSb, AuAs, and CuAs monolayers, which exhibits sizable bulk gaps up to 258 meV. These compounds host unexpected topological Rashba-like edge states, which exhibit strong dispersion with a nearly-free-electron feature. Such nontrivial edge states originate from spin-orbital coupling and an abrupt discontinuous potential across the surface. Furthermore, we argue that the AuSb monolayer can be epitaxially grown on a rutile $\mathrm{Ti}{\mathrm{O}}_{2}(110)$ surface. Importantly, the thermodynamic stability of AuSb is enhanced while the topological features are perfectly preserved due to the weak van der Waals interaction at the interface. Our findings provide promising large-gap QSH candidates with exotic topological Rashba-like edge states that are in favor of experimental realization at room temperature.

Recently, MXene/graphene heterostructures have been successfully fabricated and found to exhibit outstanding performance as electrodes for Li-ion batteries. However, insights into the mechanism behind the encouraging experimental results are missing. We use first-principles calculations to systematically investigate the electrochemical properties of MXene/graphene heterostructures, choosing Ti2CX2 (X = F, O, and OH) as representative MXenes. Our calculations disclose that the presence of graphene not only avoids restacking effects of MXene layers but also enhances the electric conductivity, Li adsorption strength (while maintaining a high Li mobility), and mechanical stiffness. These favorable attributes collectively lead to the excellent performance of MXene/graphene electrodes observed experimentally. While the Ti2CO2/graphene heterostructure is proposed to be the most promising candidate within the studied materials, the developed comprehensive understanding is of significance also for the future ratio...

Molecularly sized and heterogeneous Ni-Fe sites on graphene serve as efficient electrocatalysts., Molecularly well-defined Ni sites at heterogeneous interfaces were derived from the incorporation of Ni2+ ions into heteroatom-doped graphene. The molecular Ni sites on graphene were redox-active. However, they showed poor activity toward oxygen evolution reaction (OER) in KOH aqueous solution. We demonstrated for the first time that the presence of Fe3+ ions in the solution could bond at the vicinity of the Ni sites with a distance of 2.7 Å, generating molecularly sized and heterogeneous Ni-Fe sites anchored on doped graphene. These Ni-Fe sites exhibited markedly improved OER activity. The Pourbaix diagram confirmed the formation of the Ni-Fe sites and revealed that the Ni-Fe sites adsorbed HO− ions with a bridge geometry, which facilitated the OER electrocatalysis.

Based on the models built with our “cyclic replacement” method we introduced local strain into the (111) facet of the Si 〈112〉 nanowires. With ab initio approach, it is found that the electronic band structures of the nanowires are modulated efficiently by the surface strains: the indirect band gap declines by strong surface compression, while it always decreases and impressively changes to a direct band gap with surface tension. Moreover, the local deformations result in spatial separation of the valence band minimum to the compressed surface and the conduction band minimum to the tensed surface.