Zhengfei Dai

Phase Diagram, Design of Monolayer Binary Colloidal Crystals, and Their Fabrication Based on Ethanol-Assisted Self-Assembly at the Air/Water Interface

Flexible structural design and accurate controlled fabrication with structural tunability according to need for binary or multicomponent colloidal crystals have been expected. However, it is still a challenge. In this work, the phase diagram of monolayer binary colloidal crystals (bCCs) is established on the assumption that both large and small polystyrene (PS) colloidal spheres can stay at the air/water interface, and the range diagram for the size ratio and number ratio of small to large colloidal spheres is presented. From this phase diagram, combining the range diagram, we can design and relatively accurately control fabrication of the bCCs with specific structures (or patterns) according to need, including single or mixed patterns with the given relative content. Further, a simple and facile approach is presented to fabricate large-area (more than 10 cm(2)) monolayer bCCs without any surfactants, using differently sized PS spheres, based on ethanol-assisted self-assembly at the air/water interface. bCCs with different patterns and stoichiometries are thus designed from the established phase diagram and then successfully fabricated based on the volume ratios (V(S/L)) of the small to large PS suspensions using the presented colloidal self-assembling method. Interestingly, these monolayer bCCs can be transferred to any desired substrates using water as the medium. This study allows us to design desired patterns of monolayer bCCs and to more accurately control their structures with the used V(S/L).

Fabrication of Gold Nanoparticles by Laser Ablation in Liquid and Their Application for Simultaneous Electrochemical Detection of Cd2+, Pb2+, Cu2+, Hg2+

In this paper, we demonstrated the fabrication of high active and high sensitive Au nanoparticles by laser ablation in liquid (LAL) method, and their application in electrochemical detection of heavy metal ions. First, LAL method are used to fabricate Au nanoparticles in water in a clean way. Second, the Au nanoparticles were assembled onto the surface of the glassy carbon (GC) electrode by an electrophoretic deposition method to form an AuNPs/GC electrode for electrochemical characterization and detection. Through differential pulse anodic stripping voltammetry method, it shows that the AuNPs/GC electrode could be used for the simultaneous and selective electrochemical detection of Cd(2+), Pb(2+), Cu(2+), and Hg(2+). By studying the influence of test conditions to optimize the electrochemical detection, we can detect Cd(2+), Pb(2+), Cu(2+), and Hg(2+) simultaneously with a low concentration of 3 × 10(-7) M in the experiments.

Fast-Response, Sensitivitive and Low-Powered Chemosensors by Fusing Nanostructured Porous Thin Film and IDEs-Microheater Chip

The chemiresistive thin film gas sensors with fast response, high sensitivity, low power consumption and mass-produced potency, have been expected for practical application. It requires both sensitive materials, especially exquisite nanomaterials, and efficient substrate chip for heating and electrical addressing. However, it is challenging to achieve repeatable microstructures across the films and low power consumption of substrate chip. Here we presented a new sensor structure via the fusion of metal-oxide nanoporous films and micro-electro-mechanical systems (MEMS)-based sensing chip. An interdigital-electrodes (IDEs) and microheater integrated MEMS structure is designed and employed as substrate chip to in-situ fabricate colloidal monolayer template-induced metal-oxide (egg. SnO2) nanoporous sensing films. This fused sensor demonstrates mW-level low power, ultrafast response (~1 s), and parts-per-billion lever detection for ethanol gas. Due to the controllable template strategy and mass-production potential, such micro/nano fused high-performance gas sensors will be next-generation key miniaturized/integrated devices for advanced practical applications.

Highly reversible switching from P- to N-type NO2 sensing in a monolayer Fe2O3 inverse opal film and the associated P-N transition phase diagram

The detection of nitrogen dioxide, NO2, is currently the subject of extensive scientific and technological research, motivated by its deleterious impact on the environment and on human health and safety. However, detecting trace levels of NO2 gas in a timely, sensitive, and selective manner remains challenging, while the mechanisms governing selective NO2 sensing are still unclear. In this work, a monolayer α-Fe2O3 inverse opal (IO) film with single-crystalline 3-fold rotocenters is firstly synthesized in situ on a substrate using a sacrificial template and evaluated as a sensitive NO2 chemiresistor. Interestingly, the Fe2O3 macroporous film manifests abnormal sensing behavior with reversible transitions from p- to n-type sensing as a function of the NO2 concentration (C) and working temperature (T). Based on more familiar phase diagrams, a binary (T–C) transition diagram has been created in terms of the gas sensing response, which can be directly used to design and control the p–n transitions. Further investigations also show that such abnormal p–n transitions do not occur if a different sensing material (e.g. SnO2 IO) is used, or on exposure to gases other than NO2. The porous Fe2O3 sensor is capable of detecting trace levels of NO2, as low as 10 ppb, and shows good stability. Finally, the mechanisms underlying the unusual NO2 sensing transitions are described based on the nature of the materials and the well-known Lennard-Jones model. Reversible switching from p- to n-type sensing and the associated transition diagram carry great potential for the recognition and sensitive detection of trace levels of NO2.

Honeycomb-like Periodic Porous LaFeO3 Thin Film Chemiresistors with Enhanced Gas-Sensing Performances

The use of composite materials and polynary compounds is a promising strategy to promote conductometric sensor performances. The perovskite oxides provide various compositional combinations between different oxides for tuning gas-sensing reaction and endowing rich oxygen deficiencies for preferable gas adsorption. Herein, a sacrificial colloidal template approach is exploited to fabricate crystalline ternary LaFeO3 perovskite porous thin films, by transferring a La(3+)-Fe(3+) hybrid solution-dipped template onto a substrate and sequent heat treatment. The honeycomb-like LaFeO3 film consisted of monolayer periodic pore (size: ∼ 500 nm) array can be successfully in situ synthesized in a homogeneous layout with a single phase of perovskite. This periodic porous LaFeO3 film with p-type semiconductivity exhibits a high gas response, fast response (∼4 s), trace detection capacity (50 ppb), and favorable ethanol selectivity from similar acetone. It exhibits enhanced sensing performances compared to those of a binary n-type Fe2O3 film and a nontemplated dense LaFeO3 film. In addition, a five-axe spiderweb diagram is introduced to make a feasible evaluation of the optimal practical work condition, comprehensively regarding the response/recovery rate, gas response, selectivity and operating temperature. The enhanced ethanol sensing mechanism of honeycomb-like LaFeO3 periodic porous film is also addressed. This novel and facile route to fabricate well-ordered porous LaFeO3 thin film can also be applied to many fields to obtain special performances, such as solar cells, ion conductors, gas separation, piezoelectricity, and self-powered sensing device system.

Co9S8/MoS2 Yolk–Shell Spheres for Advanced Li/Na Storage

Uniform sized Co9 S8 /MoS2 yolk-shell spheres with an average diameter of about 500 nm have been synthesized by a facile route. When evaluated as anodes for lithium-ion and sodium-ion batteries, these Co9 S8 /MoS2 yolk-shell spheres show high specific capacities, excellent rate capabilities, and good cycling stability.

Rh-catalyzed WO3 with anomalous humidity dependence of gas sensing characteristics

The sensing of volatile organic compounds is crucial in a variety of fields including disease diagnosis, food, and homeland security. However, the significant deterioration of gas response by water vapors often hinders the sensitive and reliable gas detection in a highly humid atmosphere. Herein, we report an Rh-loaded WO3 hollow sphere chemiresistive sensor that can be potentially used for acetone gas analysis in a highly humid atmosphere. Pure WO3 and Rh-loaded WO3 hollow spheres are synthesized via a spray pyrolysis method. The Rh-loaded WO3 sensor achieved a fast acetone response (2 s), high sensitivity, good linearity, high stability, low detection limit (40 ppb) and strong selectivity to acetone even under a highly humid (80% RH) atmosphere, compared with the unloaded WO3 sensor. Interestingly, an abnormal phenomenon occurs only with the n-type Rh-loaded WO3 sensor, where the resistance and gas response increases in humid atmospheres. The sensing mechanism by Rh loading is also addressed. The unusual improvement of gas response, selectivity, responding kinetics by Rh loading shows a good potential for the detection of acetone gas.

Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting

The rational design and synthesis of nonprecious, efficient, and stable electrocatalysts to replace precious noble metals are crucial to the future of hydrogen economy. Herein, a partial sulfurization/phosphorization strategy is proposed to synthesize a nonstoichiometric pyrrhotite-type cobalt monophosphosulfide material (Co0.9S0.58P0.42) with a hexagonal close-packed phase for electrocatalytic water splitting. By regulating the degree of sulfurization, the P/S atomic ratio in the cobalt monophosphosulfide can be tuned to activate the Co3+/Co2+ couples. The synergy between the nonstoichiometric nature and the tunable P/S ratio results in the strengthened Co3+/Co2+ couples and tunable electronic structure and thus efficiently promotes the oxygen/hydrogen evolution reaction (OER/HER) processes toward overall water splitting. Especially for OER, the Co0.9S0.58P0.42 material, featured with a uniform yolk-shell spherical morphology, shows a low overpotential of 266 mV at 10 mA cm-2 (η10) with a low Tafel slope of 48 mV dec-1 as well as high stability, which is comparable to that of the reported promising OER electrocatalysts. Coupled with the high HER activity of Co0.9S0.58P0.42, the overall water splitting is demonstrated with a low η10 at 1.59 V and good stability. This study shows that phase engineering and composition control can be the elegant strategy to realize the Co3+/Co2+ couple activation and electronic structure tuning to promote the electrocatalytic process. The proposed strategy and approaches allow the rational design and synthesis of transition metal monophosphosulfides toward advanced electrochemical applications.

CuO–ZnO Micro/Nanoporous Array‐Film‐Based Chemosensors: New Sensing Properties to H2S

CuO-ZnO micro/nanoporous array-films are synthesized by transferring a solution-dipped self-organized colloidal template onto a device substrate and sequent heat treatment. Their morphologies and structures are characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectrum analysis. Based on the sensing measurement, it is found that the CuO-ZnO films prepared with the composition of [Cu(2+)]/[Zn(2+)]=0.005, 0.01, and 0.05 all show a nice sensitivity to 10 ppm H2S. Interestingly, three different zones exist in the patterns of gas responses versus H2S concentrations: a platform zone, a rapidly increasing zone, and a slowly increasing zone. Further experiments show that the hybrid CuO-ZnO porous film sensor exhibits shorter recovery time and better selectivity to H2S gas against other interfering gases at a concentration of 10 ppm. These new sensing properties may be due to a depletion layer induced by p-n junction between p-type CuO and n-type ZnO and high chemical activity of CuO to H2S. This work will provide a new construction route of ZnO-based sensing materials, which can be used as H2S sensors with high performances.

Silver Porous Nanotube Built Three-Dimensional Films with Structural Tunability Based on the Nanofiber Template-Plasma Etching Strategy

A facile and high-throughput strategy is presented to fabricate three-dimensional (3D) hierarchically porous Ag films, with clean surfaces, via plasma etching Ag-coated electrospun nanofiber template. The films are built of Ag porous nanotubes and are homogeneous in macro-size but rough and porous in nanoscale. Each nanotube-block is micro/nanostructured with evenly distributed nanopores on the tube walls. The film architecture (or the shape, arrangement, and distribution density of porous nanotubes; the number and size of nanopores) can be easily controlled by the nanofiber-template configuration, Ag coating, and plasma etching conditions. Such hierarchically porous films could be very useful, such as in catalysis, sensors, and nanodevices. They have exhibited significantly structurally enhanced surface-enhanced Raman scattering performance with good stability and reproduction, and shown the possibility of molecule-level detection. Also, the strategy is universal for fabricating other hierarchically structured 3D metal porous films, such as porous Ag hollow sphere arrays.