Aerosol-Based Self-Assembly of a Ag-ZnO Hybrid Nanoparticle Cluster with Mechanistic Understanding for Enhanced Photocatalysis.

A gas-phase-controlled synthetic approach is demonstrated to fabricate Ag-ZnO hybrid nanostructure as a high-performance catalyst for photodegradation of water pollutants. The degradation of rhodamine B (RhB) was used as representative, which were tested and evaluated with respect to the environmental pH and the presence of dodecyl sulfate corona on the surface of the catalyst. The results show that a raspberry-structure Ag-ZnO hybrid nanoparticle cluster was successfully synthesized via gas-phase evaporation-induced self-assembly. The photodegradation activity increased significantly (20×) by using the Ag-ZnO hybrid nanoparticle cluster as a catalyst. A surge of catalytic turnover frequency of ZnO nanoparticle cluster (>20×) was observed through the hybridization with silver nanoparticles. The dodecyl sulfate corona increased the photocatalytic activity of the Ag-ZnO hybrid nanoparticle cluster, especially at the acidic and neutral pH environments (maximum 6×), and the enhancement in catalytic activity was attributed to the improved colloidal stability of ZnO-based nanoparticle cluster under the interaction with RhB. Our work provides a generic route of facile synthesis of the Ag-ZnO hybrid nanoparticle cluster with a mechanistic understanding of the interface reaction for enhancing photocatalysis toward the degradation of water pollutants.

SnFe2 O4 Nanocrystals as Highly Efficient Catalysts for Hydrogen-Peroxide Sensing.

SnFe2 O4 nanocrystals (NC), prepared with a simple one-step carrier-solvent-assisted interfacial reaction process, were developed as highly efficient catalysts for hydrogen peroxide sensing. These NCs, with a size of around 7 nm, served as the sensing catalyst and were decorated onto the pore surfaces of a porous fluorine-doped tin oxide (PFTO) host electrode, prepared from commercial FTO glass with a simple anodic treatment, to form the sensing electrode for hydrogen peroxide. The SnFe2 O4 NCs-loaded PFTO electrode exhibited an ultra-high sensitivity of 1027 mA m(-1) cm(-2) toward hydrogen peroxide, outperforming Pt NCs-loaded PFTO electrodes. The SnFe2 O4 NCs-loaded PFTO electrode proved a promising relatively low cost, high performance sensing electrode for hydrogen peroxide.

Manganese oxide/graphene aerogel composites as an outstanding supercapacitor electrode material.

Graphene aerogels (GA), prepared with an organic sol-gel process, possessing a high specific surface area of 793 m(2) g(-1) , a high pore volume of 3 cm(3) g(-1) , and a large average pore size of 17 nm, were applied as a support for manganese oxide for supercapacitor applications. The manganese oxide was electrochemically deposited into the highly porous GA to form MnO2 /GA composites. The composites, at a high manganese oxide loading of 61 wt. %, exhibited a high specific capacitance of 410 F g(-1) at 2 mV s(-1) . More importantly, the high rate specific capacitances measured at 1000 mV s(-1) for these composites were two-fold higher than those obtained with samples prepared in the absence of the GA support. The specific capacitance retention ratio, based on the specific capacitance obtained at 25 mV s(-1) , was maintained high, at 85 %, even at the high scan rate of 1000 mV s(-1) , in contrast with the significantly lower value of 67 % for the plain manganese oxide sample. For the cycling stability, the specific capacitance of the composite electrode decayed by only 5 % after 50,000 cycles at 1000 mV s(-1) . The success of this MnO2 /GA composite may be attributed to the structural advantages of high specific surface areas, high pore volumes, large pore sizes, and three-dimensionally well-connected network of the GA support. These structural advantages made possible the high mass loading of the active material, manganese oxide, large amounts of electroactive surfaces for the superficial redox events, fast mass-transfer within the porous structure, and well-connected conductive paths for the involved charge transport.

NiMo-MOF-Derived Carbon-Armored Ni4Mo Alloy of an Interwoven Nanosheet Structure as an Outstanding pH-Universal Catalyst for Hydrogen Evolution Reaction at High Current Densities.

Development of highly efficient and stable non-precious metal-based pH-universal catalysts for hydrogen evolution reaction (HER) at high current densities remains challenging for water electrolysis-based green hydrogen production. Herein, a simple solvothermal process was developed to synthesize a NiMo metal-organic framework (MOF), from which a carbon-armored Ni4Mo alloy of an interwoven nanosheet structure was derived with a two-stage thermal treatment, to serve as a high-performance pH-universal HER catalyst. It requires low overpotentials of 22, 48, and 98 mV to achieve a current density of -10 mA cm-2 and 192, 267, and 360 mV to deliver an ultrahigh current density of -500 mA cm-2 in alkaline, acidic, and neutral media, respectively, and exhibits remarkable operational stability at an ultrahigh initial current density of -500 mA cm-2 for over 50 h, making it promising for applications in large-scale green hydrogen production. The success can be attributed to the unique catalyst design of a carbon-armored, composition-optimized NiMo alloy of an advantageous nanostructure of interwoven nanosheets for enhanced utilization of active sites and mass transfer of electrolytes and gaseous products, made possible with a MOF-derivation fabrication approach.

Tetragonal/orthorhombic-bismuth tungstate homojunction formed through in situ bismuth induced phase transformation as highly efficient photocatalyst for pollutant degradation.

Tetragonal/orthorhombic-bismuth tungstate (t/o-Bi2WO6) homojunctions of high photocatalytic efficiencies were fabricated through a novel in situ Bi induced phase transformation. The photocatalytic efficiencies of t-Bi2WO6 were greatly enhanced via formation of the homojunction. Photocatalytic degradation of rhodamine B (RhB), a recalcitrant organic pollutant, under simulated sunlight illumination was investigated as a demonstration for the efficiency enhancement. A 6.22 folds improvement was achieved with formation of the homojunction in terms of reaction rate constants. The homojunction catalyst was demonstrated to be photocatalytically stable over a five cycles operation. The t/o-Bi2WO6 homojunction enhances separation and utilization efficiency of photo-generated charge carriers and thus greatly boosts the catalytic efficiency. Trapping tests and electron spin resonance spectroscopy were conducted to reveal that singlet oxygen (1O2), hole (h+), electrons (e-), and superoxide anion radical (O2-) are the main working reactive species for RhB degradation. Density functional theory (DFT) calculations were performed to prove the feasibility of Bi induced phase transformation of t-Bi2WO6 to o-Bi2WO6. The present development offers a new design route for high efficiency photocatalysts for water pollution control.

Nitrogen-doped carbon armored Cobalt oxide hollow nanocubes electrochemically anchored on fluorine-doped tin oxide substrate for acidic oxygen evolution reaction.

Developments of non-precious metal based active and stable catalysts are of great importance and challenge to green hydrogen production from acidic electrocatalytic water splitting. Design of composite catalysts with synergy between active and stable components proves to be a promising approach. Herein, N-doped carbon armored Co3O4 hollow nanocubes electrochemically anchored on fluorine-doped tin oxide (FTO) substrates are developed as efficient and stable catalysts for acidic oxygen evolution reactions. Co3O4 acts as the active component with N-doped carbon coating layer serving as the stable protection component, shielding Co3O4 from direct attack of anodic dissolution. Electrochemical fixation offers firm holding of the composite catalyst onto acid-tolerant FTO substrates and hollow nanocubes serve as nano-reactors for confined fast reactions. Under optimal conditions, the composite catalyst achieves an overpotential of 465 mV at 10 mA cm-2 in 0.5 M H2SO4, and stays stable for 12 hr with a 10% increment in applied potentials.

Pyrolytic carbon from an aromatic precursor and its application as a counter electrode in dye-sensitized solar cells.

Pyrolytic carbon thin films were deposited on quartz plates through a chemical vapor deposition process, by using a biphenyl precursor, 4,4'-bis(chloromethyl-1,1'-biphenyl). The pyrolytic carbons were microporous and catalytic toward reduction of tri-iodide, and the films thus obtained possessed a metallic appearance with good mirror reflections, hydrophilic surfaces, and low sheet resistances. The pyrolytic carbon-coated quartz plates were used, in place of the commonly used Pt-coated fluorine-doped tin oxide glass, as the counter electrode for dye-sensitized solar cells (DSSCs). The light to electricity conversion efficiency of the cell thus obtained was reasonably high, achieving 78% of that obtained by using the conventional but much more expensive Pt counter electrode. From the electrochemical impedance spectroscopic analysis, one found that the minor reduction in the conversion efficiency came from the relatively higher resistance and lower catalytic activity of the pyrolytic carbon. This work demonstrates that the newly developed pyrolytic carbon films may be a promising alternative to Pt as the counter electrode material for DSSCs.

N-doped carbon armored metal phosphides grown in-situ on nickel foam as chainmail catalysts toward high efficiency electrolytic water splitting.

N-doped carbon armored metal phosphides were anchored in-situ on backbones of nickel foam (NF), by using polydopamine as both carbon source and targeted precursor delivery vehicle. The product served as a highly efficient chainmail bi-functional catalyst toward electrolytic water splitting, requiring small operating cell voltages of only 1.65, 1.73, and 1.79 V at 50, 100, and 250 mA/cm2, respectively for overall water splitting. More importantly, the product catalyst also possessed excellent long-term stability, even tested at 250 mA/cm2. The excellent activity and durability may be ascribed to the positive synergistic effects between well-mixed metal phosphides, advantageous nanostructure of interwoven thin vertical nanosheets, protective and conductive N-doped carbon matrix, hydrophilic and aerophobic characteristics, and well dispersed and utilized active materials on skeleton surfaces of the highly conductive NF. This synergistic effect boosting and protective layer armored design for highly efficient and stable electrocatalysts is unique and scalable, suitable for large-scale applications not only in the field of electrocatalysis, but also in other heterogeneous catalysis.

Mixed Metal Phosphide Chainmail Catalysts Confined in N-Doped Porous Carbon Nanoboxes as Highly Efficient Water-Oxidation Electrocatalysts with Ultralow Overpotentials and Tafel Slopes.

Electrocatalytic hydrogen production driven by surplus electric energies is considered a promising sustainable process for hydrogen supply. The high overpotential and low energy-conversion efficiency caused by the slow kinetics of the four-electron transfer oxygen-evolution reaction (OER), however, hamper its competitiveness. Herein, a highly stable, efficient OER catalyst was developed, taking the effects of both composition and nanostructure into account for the catalyst design. N-doped carbon-armored mixed metal phosphide nanoparticles confined in N-doped porous carbon nanoboxes, a particle-in-box nanostructure, were synthesized from monodisperse Ni3[Fe(CN)6]2·H2O nanocubes through sequential conformal polydopamine coating, ammonia etching, and thermal phosphorization. The product exhibited outstanding catalytic abilities for the OER in 1.0 M KOH, delivering 10, 100, and 250 mA/cm2 at ultrasmall overpotentials of 203, 242, and 254 mV, respectively, with an ultrasmall Tafel slope of 38 mV/dec, outperforming most recently reported top-notch iron-group-based OER catalysts. The long-term stability was also excellent, showing a small chronopotentiometric decay of 2.5% over a 24 h operation at 50 mA/cm2. The enhanced catalytic efficiency and stability may be attributable to the unique particle-in-box structure as a nanoreactor offering a local, fast reaction environment, the conductive N-doped porous carbon shell for fast charge and mass transport, the synergistic effect between multicomponent metal phosphides for enhanced intrinsic activities, and the carbon protection layer to prevent/delay the catalyst core from being deactivated. This combined particle-in-box and chainmail design concept for electrocatalysts is unique and advantageous and may be readily applied to the general field of heterogeneous reactions.

Open-mouth N-doped carbon nanoboxes embedded with mixed metal phosphide nanoparticles as high-efficiency catalysts for electrolytic water splitting.

The key to develop efficient catalysts is to improve the quantity and activity of catalytic sites of the catalysts through optimal structural and compositional design. Accordingly, open-mouth N-doped carbon nanoboxes embedded with mixed metal phosphide nanoparticles are fabricated from monodisperse Ni3[Fe(CN)6]2·H2O nanocubes through conformal Ni3[Co(CN)6]2·12H2O layer coating, ammonia etching, and thermal phosphorization, sequentially. The product catalyst exhibits highly efficient electrocatalytic performances, achieving low overpotentials of 204 and 129 mV for the oxygen evolution reaction and hydrogen evolution reaction, respectively, and a small working voltage of 1.57 V for the overall water splitting, all at 10 mA cm-2. Its long-term electrocatalytic stability is also outstanding, experiencing only minor chronopotentiometric decay after a 24 h operation at 10 mA cm-2. The enhanced electrocatalytic performance may be attributed to the synergistic effects between the mixed metal phosphides, the protective function offered by the chainmail catalyst design, and the fast mass transport channels for the electrolyte and gaseous products afforded by the large openings on the nanobox shell, as well as the easy access of the inner active sites of the nanobox. The ingenious open-mouth nanobox structure together with embedded mixed metal phosphide nanoparticles is a unique design for improving the quantity and activity of catalytic sites of the catalyst for high efficiency electrolytic water splitting. The present design concept can be readily applied to the fabrication of other heterogeneous catalysts.

Bimetallic Metal-Organic Framework-Derived Hybrid Nanostructures as High-Performance Catalysts for Methane Dry Reforming.

Syngas, consisting of equimolar CO and H2, is an important feedstock for large-scale production of a wide range of commodity chemicals including aldehyde, methanol, ammonia, and other oxygenated chemicals. Dry reforming of methane (DRM), proceeding by reacting greenhouse gases, CO2 and CH4, at high temperatures in the presence of a metal catalyst, is considered one of the most environmentally friendly routes for syngas production. Nevertheless, nonprecious metal-based catalysts, which can operate at relatively low temperatures for high product yields and selectivities, are required to drive the DRM process for industrial applications effectively. Here, we developed NiCo@C nanocomposites from a corresponding NiCo-based bimetallic metal-organic framework (MOF) to serve as high-performance catalysts for the DRM process, achieving high turnover frequencies (TOF) at low temperatures (>5.7 s-1 at 600 °C) and high product selectivities (H2/CO = 0.9 at 700 °C). The incorporation of Co in Ni catalysts improves the operation stability and light-off stability. The present development for MOF-derived nanocomposites opens a new horizon for design of DRM catalysts.

Gigantic enhancement in sensitivity using Schottky contacted nanowire nanosensor.

A new single nanowire based nanosensor is demonstrated for illustrating its ultrahigh sensitivity for gas sensing. The device is composed of a single ZnO nanowire mounted on Pt electrodes with one end in Ohmic contact and the other end in Schottky contact. The Schottky contact functions as a "gate" that controls the current flowing through the entire system. By tuning the Schottky barrier height through the responsive variation of the surface chemisorbed gases and the amplification role played by the nanowire to Schottky barrier effect, an ultrahigh sensitivity of 32,000% was achieved using the Schottky contacted device operated in reverse bias mode at 275 degrees C for detection of 400 ppm CO, which is 4 orders of magnitude higher than that obtained using an Ohmic contact device under the same conditions. In addition, the response time and reset time have been shortened by a factor of 7. The methodology and principle illustrated in the paper present a new sensing mechanism that can be readily and extensively applied to other gas sensing systems.

Differential sensing of serine and tyrosine with aligned CdS nanowire arrays based on pH-dependent photoluminescence behavior.

The differential sensing of tyrosine and serine is achieved with well-aligned CdS nanowire arrays by exploring the pH-dependent photoluminescence behavior of the nanowire arrays toward exposure to the two amino acid solutions. The contrasting trend in photoluminescence (PL) intensity with respect to variations in analyte concentration observed at pH 11 served as the check point for the present differential sensing. The application format of the nanowire array is better suited for further sensing device assembly than that of nanocrystal suspensions.

NiFe Alloy Nanotube Arrays as Highly Efficient Bifunctional Electrocatalysts for Overall Water Splitting at High Current Densities.

A bubble-releasing assisted pulse electrodeposition method was developed to create metallic alloy, NiFe, nanotube arrays in one step. The NiFe alloy nanotube array exhibited excellent bifunctional electrolytic activities, achieving low overpotentials of 100 mV for the hydrogen evolution reaction and 236 mV for the oxygen evolution reaction at 10 mA cm-2, both in 1 M KOH at room temperature. For overall water splitting, the NiFe alloy nanotube array delivered 10 mA cm-2 at an ultralow cell voltage of 1.58 V, among the top tier of the state-of-the-art bifunctional electrocatalysts. The NiFe alloy nanotube array also exhibited ultrastability at high current densities, experiencing only a minor chronoamperometric decay of 6.5% after a 24 h operation at 400 mA cm-2. The success of the present binder-free nanotube array-based electrode can be attributed to the much enlarged reaction surface area, one-dimensionally guided charge transport and mass transfer offered by the nanotube structure, and improved product crystallinity provided by the pulse current electrodeposition. The nanotube array structure proves to be a promising new architecture design for electrocatalysts.

Enhancement of catalytic activity by UV-light irradiation in CeO2 nanocrystals.

Ultraviolet (UV) light irradiation on CeO2 nanocrystals catalysts has been observed to largely increase the material's catalytic activity and reactive surface area. As revealed by x-ray absorption near edge structure (XANES) analysis, the concentration of subvalent Ce3+ ions in the irradiated ceria samples progressively increases with the UV-light exposure time. The increase of Ce3+ concentration as a result of UV irradiation was also confirmed by the UV-vis diffuse reflectance and photoluminescence spectra that indicate substantially increased concentration of oxygen vacancy defects in irradiated samples. First-principle formation-energy calculation for oxygen vacancy defects revealed a valence-hole-dominated mechanism for the irradiation-induced reduction of CeO2 consistent with the experimental results. Based on a Mars-van Krevelen mechanism for ceria catalyzed oxidation processes, as the Ce3+ concentration is increased by UV-light irradiation, an increased number of reactive oxygen atoms will be captured from gas-phase O2 by the surface Ce3+ ions, and therefore leads to the observed catalytic activity enhancement. The unique annealing-free defect engineering method using UV-light irradiation provides an ultraconvenient approach for activity improvement in nanocrystal ceria for a wide variety of catalytic applications.

Transparent, hydrophobic composite aerogels with high mechanical strength and low high-temperature thermal conductivities.

A facile one-step polymer-incorporation sol-gel process, together with a surface modification and an ambient pressure drying processes, was developed to prepare silica-poly(vinylpyrrolidone) composite aerogels. These composite aerogels are with high hydrophobicity (static contact angle >120 degrees), good mechanical strength (Young's modulus of bending >30 MPa), and low high-temperature thermal conductivity (0.063 W/m-K at 300 degrees C), which are critical characteristics for practical applications of aerogels, particularly in energy saving areas, for long-term usage and large scale production.

TiO2 nanocrystals decorated Z-schemed core-shell CdS-CdO nanorod arrays as high efficiency anodes for photoelectrochemical hydrogen generation.

TiO2 nanocrystals decorated core-shell CdS-CdO nanorod arrays, TiO2@CdO/CdS NR, were fabricated as high efficiency anodes for photoelctrochemical hydrogen generation. The novel sandwich heterostructure was constructed from first growth of CdS nanorod arrays on a fluorine doped tin oxide (FTO) substrate with a hydrothermal process, followed by in situ generation of CdO thin films of single digit nanometers from the CdS nanorod surfaces through thermal oxidation, and final decoration of TiO2 nanocrystals of 10-20 nm via a successive ionic layer absorption and reaction process. The core-shell CdS-CdO heterostructure possesses a Z-scheme band structure to enhance interfacial charge transfer, facilitating effective charge separation to suppress electron-hole recombination within CdS for much improved current density generation. The final decoration of TiO2 nanocrystals passivates surface defects and trap states of CdO, further suppressing surface charge recombination for even higher photovoltaic conversion efficiencies. The photoelectrochemical performances of the plain CdS nanorod array were significantly improved with the formation of the sandwich heterostructure, achieving a photo current density of 3.2 mA/cm2 at 1.23 V (vs. RHE), a 141% improvement over the plain CdS nanorod array and a 32% improvement over the CdO/CdS nanorod array.

Mixed NiO/NiCo2O4 Nanocrystals Grown from the Skeleton of a 3D Porous Nickel Network as Efficient Electrocatalysts for Oxygen Evolution Reactions.

Mixed NiO/NiCo2O4 nanocrystals grown in situ from the skeleton of a 3D porous nickel network (3DPNN) were prepared with a simple hydrothermal method followed by a low temperature calcination, exhibiting outstanding electrocatalytic efficiencies toward oxygen evolution reactions (OER). The 3DPNN was prepared with a novel leaven dough method and served as both the nickel source for growth of the mixed NiO/NiCo2O4 nanocrystals and the charge transport highway to accelerate the sluggish kinetics of the OER. The mixed NiO/NiCo2O4 nanocrystals exhibited pronounced synergistic effects to achieve a high mass activity of 200 A g-1 at the catalyst mass loading of 0.5 mg cm-2, largely outperforming the corresponding single component nanocrystal systems, NiO (5.87) and NiCo2O4 (9.35). The NiO/NiCo2O4@3DPNN composite electrocatalyst achieved a low overpotential of 264 mV at the current density of 10 mA cm-2 and 389 mV at the practically high current density of 250 mA cm-2, which compete favorably among the top tier of previously reported OER electrocatalysts. Moreover, it exhibited good stability even at the high current density of 250 mA cm-2, showing only 9.40% increase in working applied potential after a continuous 12 h operation. The present work demonstrates a new design for highly efficient OER catalysts with in situ growth of mixed oxide nanocrystals of pronounced synergistic effects.

Heterogeneous Fenton Reaction Enabled Selective Colon Cancerous Cell Treatment.

A selective colon cancer cell therapy was effectively achieved with catalase-mediated intra-cellular heterogeneous Fenton reactions triggered by cellular uptake of SnFe2O4 nanocrystals. The treatment was proven effective for eradicating colon cancer cells, whereas was benign to normal colon cells, thus effectively realizing the selective colon cancer cell therapeutics. Cancer cells possess much higher innate hydrogen peroxide (H2O2) but much lower catalase levels than normal cells. Catalase, an effective H2O2 scavenger, prevented attacks on cells by reactive oxygen species induced from H2O2. The above intrinsic difference between cancer and normal cells was utilized to achieve selective colon cancer cell eradication through endocytosing efficient heterogeneous Fenton catalysts to trigger the formation of highly reactive oxygen species from H2O2. In this paper, SnFe2O4 nanocrystals, a newly noted outstanding paramagnetic heterogeneous Fenton catalyst, have been verified an effective selective colon cancerous cell treatment reagent of satisfactory blood compatibility.

Few-Layer Graphene Sheet-Passivated Porous Silicon Toward Excellent Electrochemical Double-Layer Supercapacitor Electrode.

Few-layer graphene sheet-passivated porous silicon (PSi) as an outstanding electrochemical double-layer supercapacitor electrode was demonstrated. The PSi matrix was formed by electrochemical etching of a doped silicon wafer and was further surface-passivated with few-layer graphene sheets by a Ni-assisted chemical vapor deposition process where a wide range of porous PSi structures, including mesoporous, macroporous, and hybrid porous structures were created during the graphene growth as temperature increases. The microstructural and graphene-passivation effects on the capacitive performance of the PSi were investigated in detail. The hybrid porous PSi electrode, optimized in terms of capacitive performances, achieves a high areal capacitance of 6.21 mF/cm2 at an ultra-high scan rate of 1000 mV/s and an unusual progressing cyclic stability of 131% at 10,000 cycles. Besides mesopores and macropores, micropores were introduced onto the surfaces of the passivating few-layer graphene sheets with a KOH activation process to further increase the functioning surface area of the hierarchical porous PSi electrode, leading to a boost in the areal capacitance by 31.4% up to 8.16 mF/cm2. The present designed hierarchical porous PSi-based supercapacitor proves to be a robust energy storage device for microelectronic applications that require stable high rate capability.