Modulating Ti t2g Orbital Occupancy in a Cu/TiO2 Composite for Selective Photocatalytic CO2 Reduction to CO.

The electronic structure of composites plays a critical role in photocatalytic conversion, whereas it is challenging to modulate the orbital for an efficient catalyst. Herein, we regulated the t2g orbital occupancy state of Ti to realize efficient CO2 conversion by adjusting the amount of photo-deposited Cu in the Cu/TiO2 composite. For the optimal sample, considerable electrons transfer from the Cu d orbital to the Ti t2g orbital, as proven by X-ray absorption spectroscopy. The Raman spectra results also corroborate the electron enrichment on the Ti t2g orbital. Further theoretical calculations suggested that the orbital energy of the Ti 3d orbital in TiO2 is declined, contributing to accepting Cu 3d electrons. As a result, the Cu/TiO2 composite exhibited an extremely high selectivity of 95.9 % for CO, and the productivity was 15.27 µmol g-1 h-1 , which is almost 6 times that of the original TiO2 . Our work provides a strategy for designing efficient photocatalysis as a function of orbital regulation.

Understanding the Doping Chemistry of High Oxidation States in Scheelite CaWO4 by Hydrothermal Conditions.

Doping chemistry has become one of the most effective means of tuning materials' properties for diverse applications. In particular for scheelite-type CaWO4, high-oxidation-state doping is extremely important, since one may expand the scheelite family and further create prospective candidates for novel applications and/or useful spectral signatures for nuclear forensics. However, the chemistry associated with high-valence doping in scheelite-type CaWO4 is far from understanding. In this work, a series of scheelite-based materials (Ca1-x-y-zEuxKy□z)WO4 (□ represents the cation vacancy of the Ca2+ site) were synthesized by hydrothermal conditions and solid-state methods and comparatively studied. For the bulk prepared by the solid-state method, occupation of high-oxidation-state Eu3+ at the Ca2+ sites of CaWO4 is followed by doping of the low-oxidation-state K+ at a nearly equivalent molar amount. The Eu3+ local symmetry is thus varied from the original S4 point group symmetry to C2v point group symmetry. Surprisingly different from the cases in bulk, for the nanoscale counterparts prepared by hydrothermal conditions, the high-oxidation-state Eu3+ was incorporated in CaWO4 at two distinct sites, and its amount is higher than that of the low-oxidation-state K+ even though KOH was used as a mineralizer, creating a certain amount of cation vacancies. Consequently, an apparent split emission of 5D0 → 7F0 was first demonstrated for (Ca1-x-y-zEuxKy□z)WO4. The doping chemistry of high oxidation states uncovered in this work not only provides an explanation for the commonly observed spectral changes in rare-earth-ion-modified scheelite structures, but also points out an advanced direction that can guide the design and synthesis of novel functional oxides by solution chemistry routes.

Cation-Exchange-Induced Metal and Alloy Dual-Exsolution in Perovskite Ferrite Oxides Boosting the Performance of Li-O2 Battery.

A temperature-controlled cation-exchange approach is introduced to achieve a unique dual-exsolution in perovskite La0.8 Fe0.9 Co0.1 O3-δ where both CoFe alloy and Co metal are simultaneously exsolved from the parent perovskite, forming an alloy and metal co-decorated perovskite oxide. Mossbauer spectra show that cation exchange of Fe atoms in CoFe alloy and Co cations in the perovskite is the key to the co-existence of Co metal and CoFe alloy. The obtained composite exhibits an enhanced catalytic activity as Li-O2 battery cathode catalysts with a specific discharge capacity of 6549.7 mAh g-1 and a cycling performance of 215 cycles without noticeable degradation. Calculations show that the combination of decorated CoFe alloy and Co metal synergistically modulated the discharge reaction pathway that improves the performance of Li-O2 battery.

Carbon Nitride Supramolecular Hybrid Material Enabled High-Efficiency Photocatalytic Water Treatments.

Surface defects in relation to surface compositions, morphology, and active sites play crucial roles in photocatalytic activity of graphitic carbon nitride (g-C3N4) material for highly reactive oxygen radicals production. Here, we report a high-efficiency carbon nitride supramolecular hybrid material prepared by patching the surface defects with inorganic clusters. Fe (III) {PO4[WO(O2)2]4} clusters have been noncovalently integrated on surface of g-C3N4, where the surface defects provide accommodation sites for these clusters and driving forces for self-assembly. During photocatalytic process, the activity of supramolecular hybrid is 1.53 times than pure g-C3N4 for the degradation of Rhodamine B (RhB) and 2.26 times for Methyl Orange (MO) under the simulated solar light. Under the mediation of H2O2 (50 mmol L-1), the activity increases to 6.52 times for RhB and 28.3 times for MO. The solid cluster active sites with high specific surface area (SSA) defect surface promoting the kinetics of hydroxide radicals production give rise to the extremely high photocatalytic activity. It exhibits recyclable capability and works in large-scale demonstration under the natural sunlight as well and interestingly the environmental temperature has little effects on the photocatalytic activity.

A Graphene-like Oxygenated Carbon Nitride Material for Improved Cycle-Life Lithium/Sulfur Batteries.

Novel sulfur (S) anchoring materials and the corresponding mechanisms for suppressing capacity fading are urgently needed to advance the performance of Li/S batteries. Here, we designed and synthesized a graphene-like oxygenated carbon nitride (OCN) host material that contains tens of micrometer scaled two-dimensional (2D) rippled sheets, micromesopores, and oxygen heteroatoms. N content can reach as high as 20.49 wt %. A sustainable approach of one-step self-supporting solid-state pyrolysis (OSSP) was developed for the low-cost and large-scale production of OCN. The urea in solid sources not only provides self-supporting atmospheres but also produces graphitic carbon nitride (g-C3N4) working as 2D layered templates. The S/OCN cathode can deliver a high specific capacity of 1407.6 mA h g(-1) at C/20 rate with 84% S utilization and retain improved reversible capacity during long-term cycles at high current density. The increasing micropores, graphitic N, ether, and carboxylic O at the large sized OCN sheet favor S utilization and trapping for polysulfides.

Ultrasonic reduction: an unconventional route to exsolute Ag from perovskite La(Ag)FeO3-δ for enhanced catalytic oxidation activity.

We explore an uncommon ultrasonic reduction method to exsolute Ag from perovskite La0.87Ag0.03FeO3-δ, forming a composite with enhanced catalytic oxidation activity. Such a mild exsolution is based on the coupling effect of ultrasonic cavitation and reducible BH4-, and holds great potential in the fields of energy and environment catalysis.

Activating Octahedral Center in Co-Doped NiFe2 O4 via Bridging Amorphous MoSx for Electrocatalytic Water Oxidation: A Case for eg Orbital Regulation in Spinel Oxide.

Moderate eg filling for octahedral metal cations (MOh ) is strongly correlated with the electrocatalytic water oxidation performance in the oxides system. Here, the eg fillings of NiOh and FeOh in NiFe2 O4 -based spinel are controllably regulated by introducing an external radical of catalytically inactive MoSx as an electron acceptor via a novel ultrasonic anchored pyrolysis strategy. The electron occupied in eg orbit of MOh emigrates with the amount of MoS hanging on the apical of octahedral sites, and results in a salutary transition from high to medium eg occupancy state, as confirmed by the X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. In addition, benefiting from the abundant unsaturated S atoms in amorphous MoSx , the MOh at the surface furthest activates and consequently shows a superior water oxidation performance. Density functional theory also reveals that the eg fillings of Ni and Fe decrease to 1.4 and 1.2 after MoSx modification, which can effectively reduce the free energy of the OOH* intermediates in the oxygen evolution reaction process. This work opens an avenue for further releasing the electrocatalytic activity of octahedral sites through bridging external phases with rational electron-capturing/donating capability.

Confinement chemistry of FeOx centers for activating molecular oxygen under ambient conditions.

Activating molecular oxygen under mild conditions is highly important for developing advanced green technologies and for understanding the origin and running of life as well, which still remains a challenge. In this work, we report on the confinement chemistry for activating molecular oxygen over oxides under mild conditions by presenting the synthesis and characterization of FeOx species confined to the pores of support CeO2 nanospheres. Active catalytic materials are obtained by a controllable three-step method via the formation of porous CeO2 nanospheres that have an average diameter of 120 nm and exhibit a large surface area of 168 m2 g-1 and a pore size of 18.7 nm, confining FeOx in intimate contact with ultra-small Pt particles in pores. The optimized PtOy-FeOx/CeO2-H catalyst showed an excellent performance in the preferential oxidation of CO reactions, as featured by 100% CO conversion at room temperature with almost no attenuation in a prolonged operation, which could not be accessible without pore-confined FeOx centers. Mechanical studies prove that the reaction progresses via abnormal non-competitive adsorption associated with synergistic roles from uniform loading, stabilization of divalent Fe species, surface oxygen activation on CeO2 supports, and the reduced H2 spillover effect on Pt0, making the CO species adsorbed on Ptδ+ easier to be desorbed. The methodology demonstrated here may inspire one to explore more advanced catalysts with high activity at room temperature essential for a wide range of applications.

Stabilizing black phosphorus via inorganic small-molecular H3BO3.

Taking the degradation mechanism into consideration, inorganic small-molecular H3BO3 was selected for surface coordination of black phosphorus (BP). The successful coordination between BP and H3BO3 not only improves the ambient stability of BP, but also preserves the intrinsic properties of BP without distorting its structure.

Tuning W18O49/Cu2O{111} Interfaces for the Highly Selective CO2 Photocatalytic Conversion to CH4.

As a multiple proton-coupled electron transfer process, photocatalytic conversion of CO2 usually produces a wide variety of products. Improving the yield and selectivity of CO2 to the single product is still a significant challenge. In this work, we describe that the rationally constructed W18O49/Cu2O{111} interfaces achieve highly selective CO2 photocatalytic conversion to CH4. In situ Fourier transform infrared spectroscopy measurements reveal that the formation of W18O49/Cu2O{111} interfaces restrains the desorption of CO* intermediates in CO2 photocatalytic conversion. UPS spectra, PL spectra, and photocurrent curves show that more photogenerated electrons are excited and transferred to W18O49/Cu2O{111} interfaces. All of these play critical roles in CH4 production. As an outcome, the yield rate of CO2 photocatalytic conversion to CH4 was enhanced from 6.5 to 17.20 µmol g-1 h-1 with selectivity as high as 94.7%. The work demonstrates the feasibility and versatility of interface engineering in achieving highly selective CO2 photocatalytic conversion.

Optimizing the surface state of cobalt-iron bimetallic phosphide via regulating phosphorus vacancies.

Herein, we successfully regulated phosphorus vacancies in Co0.68Fe0.32P through Ar-plasma treatment. The Ar-plasma treated Co0.68Fe0.32P exhibits a delicate surface state where the surface Co and Fe ions show an unusual electron loss. The unique surface state enhances the oxygen evolving performance of the phosphide.

Cobalt Nanoparticles/Black Phosphorus Nanosheets: An Efficient Catalyst for Electrochemical Oxygen Evolution.

Black phosphorus (BP) nanosheet (NS) is an emerging oxygen evolution reaction (OER) electrocatalyst with both high conductivity and abundant active sites. However, its ultrathin structure suffers instability because of the lone pair electrons exposed at the surface, which badly restricts durability for achieving long-term OER catalysis. Herein, a facile solvothermal reduction route is designed to fabricate Co/BP NSs hybrid electrocatalyst by in situ growth of cobalt nanoparticles on BP NSs. Notably, electronic structure engineering of Co/BP NSs catalyst is observed by electron migration from BP to Co due to the higher Fermi level of BP than that of Co. Because of the preferential migration of the active lone pairs from the defect of BP NSs, the stability and high hole mobility can be effectively retained. Consequently, Co/BP NSs electrocatalyst exhibits outstanding OER performance, with an overpotential of 310 mV at 10 mA cm-2, and excellent stability in alkaline media, indicating the potential for the alternatives of commercial IrO2. This study provides insightful understanding into engineering electronic structure of BP NSs by fully utilizing defect and provides a new idea to design hybrid electrocatalysts.

Architecture of Biomimetic Water Oxidation Catalyst with Mn4CaO5 Clusterlike Structure Unit.

Mn4CaO5 cluster in green plant is considered as the ideal structure for water oxidation catalysis. However, this structure is difficult to be constructed in heterogeneous catalyst because of its distorted spatial structure and unique electronic state. Herein, we report the synthesis of two-dimensional biomimetic Ca-Mn-O catalyst with Mn4CaO5 clusterlike structure through ultrasonic-assisted reduction treatment toward Ca-birnessite. The synergistic effect between ultrasonic and reduction successfully reduced the Mn oxidation state in Ca-birnessite without breaking the structure of MnO2 monolayers, forming a regular two-dimensional structure with Mn4CaO5 cubanelike structure unit for the first time. The biomimetic catalyst shows a superior water oxidation activity (turnover frequency = 3.43 s-1), which is the best in manganese-based heterogeneous catalyst to date. This work provides a new strategy for the precise synthesis of specific structure and exhibits a great prospect of biomimic in heterogeneous catalyst.

Phase-Controlled Synthesis of High-Bi-Ratio Ternary Sulfide Nanocrystals of Cu1.57 Bi4.57 S8 and Cu2.93 Bi4.89 S9.

High Bi-ratio ternary sulfides have been recently reported as superior thermoelectric materials. However, the synthesis of high Bi-ratio Cu-Bi-S nanocrystal remains a challenge. Reported here are the synthesis and characterization of three-phase Cu-Bi-S nanocrystals with the nominal chemical formulae of Cu1.57 Bi4.57 S8 , Cu2.93 Bi4.89 S9 and Cu3 BiS3 . The samples were prepared using a Bi2 S3 precursor by varying the amount and type of Cu2-x S (i. e. Cu2 S or Cu7.2 S4 ) reactants. TEM images reveal that two new samples crystalized having nanorod morphology with radii of approximately 50 nm and lengths of 200 nm. XPS results indicate that the valence states of Bi in both the two new phases are +3 with viable oxidation states for Cu. UV-Vis-NIR absorption spectroscopy reveals that narrow direct bandgaps are 1.12 and 1.27 eV for Cu1.57 Bi4.57 S8 and Cu2.93 Bi4.89 S9 , respectively. Besides, this method could also be applied to synthesize the Cu3 BiS3 phase with a new nanoplate morphology. The as-synthesized Cu-Bi-S samples show Cu/Bi ratio-dependent photoresponsive properties. This study not only reports the structure and bandgap of two ternary sulfides, which have only been discovered in the mineral previously, but also provides an efficient method for synthesizing Bi-rich ternary chalcogenide nanocrystals.

Activation of Surface Oxygen Sites in a Cobalt-Based Perovskite Model Catalyst for CO Oxidation.

Anionic redox chemistry is becoming increasingly important in explaining the intristic catalytic behavior in transition-metal oxides and improving catalytic activity. However, it is a great challenge to activate lattice oxygen in noble-metal-free perovskites for obtaining active peroxide species. Here, we take La0.4Sr0.6CoO3-δ as a model catalyst and develop an anionic redox activity regulation method to activate lattice oxygen by tuning charge transfer between Co4+ and O2-. Advanced XAS and XPS demonstrate that our method can effectively decrease electron density of surface oxygen sites (O2-) to form more reactive oxygen species (O2- x), which reduces the activation energy barriers of molecular O2 and leads to a very high CO catalytic activity. The revealing of the activation mechanism for surface oxygen sites in perovskites in this work opens up a new avenue to design efficient solid catalysts. Furthermore, we also establish a correlation between anionic redox chemistry and CO catalytic activity.

Cation Segregation of A-Site Deficiency Perovskite La0.85FeO3-δ Nanoparticles toward High-Performance Cathode Catalysts for Rechargeable Li-O2 Battery.

Cation segregation of perovskite oxide is crucial to develop high-performance catalysts. Herein, we achieved the exsolution of α-Fe2O3 from parent La0.85FeO3-δ by a simple heat treatment. Compared to α-Fe2O3 and La0.85FeO3-δ, α-Fe2O3-LaFeO3- x achieved a significant improvement of lithium-oxygen battery performance in terms of discharge specific capacity and cycling stability. The promotion can be attributed to the interaction between α-Fe2O3 and LaFeO3- x. During the cycling test, α-Fe2O3-LaFeO3- x can be stably cycled for 108 cycles at a limited discharge capacity of 500 mAh g-1 at a current density of 100 mA g-1, which is remarkably longer than those of La0.85FeO3-δ (51 cycles), α-Fe2O3 (21 cycles), and mechanical mixing of LaFeO3 and α-Fe2O3 (26 cycles). In general, these results suggest a promising method to develop efficient lithium-oxygen battery catalysts via segregation.

δ-MnO2-Mn3O4 Nanocomposite for Photochemical Water Oxidation: Active Structure Stabilized in the Interface.

Pure phase manganese oxides have been widely studied as water oxidation catalysts, but further improvement of their activities is much challenging. Herein, we report an effective method to improve the water oxidation activity by fabricating a nanocomposite of Mn3O4 and δ-MnO2 with an active interface. The nanocomposite was achieved by a partial reduction approach which induced an in situ growth of Mn3O4 nanoparticles from the surface of δ-MnO2 nanosheets. The optimum composition was determined to be 38% Mn3O4 and 62% δ-MnO2 as confirmed by X-ray photoelectron spectra (XPS) and X-ray absorption spectra (XAS). The δ-MnO2-Mn3O4 nanocomposite is a highly active water oxidation catalyst with a turnover frequency (TOF) of 0.93 s-1, which is much higher than the individual components of δ-MnO2 and Mn3O4. We consider that the enhanced water oxidation activity could be explained by the active interface between two components. At the phase interface, weak Mn-O bonds are introduced by lattice disorder in the transition of hausmannite phase to birnessite phase, which provides active sites for water oxidation catalysis. Our study illustrates a new view to improve water oxidation activity of manganese oxides.