Continuous hot electron generation in Pt/TiO2, Pd/TiO2, and Pt/GaN catalytic nanodiodes from oxidation of carbon monoxide

Thin films (<10 nm) of platinum or palladium were deposited on TiO2 or GaN to form Schottky diodes. We detected and monitored the continuous electron flow across the metal-oxide interfaces of Pt/TiO2, Pd/TiO2, and Pt/GaN during the catalytic oxidation of carbon monoxide. The electron excitation and flow in the metals were due to conversion of energy released by the oxidation of carbon monoxide into the kinetic energy of free electrons in platinum and palladium. The best conversion of three electrons per four CO2 molecules was observed from 5 nm Pt/TiO2.     

The catalytic nanodiode: detecting continuous electron flow at oxide-metal interfaces generated by a gas-phase exothermic reaction.

Continuous flow of ballistic charge carriers is generated by an exothermic chemical reaction and detected using the catalytic metal-semiconductor Schottky diode. We obtained a hot electron current for several hours using two types of catalytic nanodiodes, Pt/TiO2 or Pt/GaN, during carbon monoxide oxidation at pressures of 100 Torr of O2 and 40 Torr of CO at 413-573 K. This result reveals that the chemical energy of an exothermic catalytic reaction is directly converted into hot electrons flux in the catalytic nanodiode. By heating the nanodiodes in He, we could measure the thermoelectric current which is in the opposite direction to the flow of the hot electron current. The chemicurrent is well correlated with the turnover rate of CO oxidation, which is separately measured with gas chromatography. The influence of the flow of hot charge carriers on the chemistry at the oxide-metal interface, and the turnover rate in the chemical reaction are discussed.     

Dynamics of surface catalyzed reactions; the roles of surface defects, surface diffusion, and hot electrons

The mechanism that controls bond breaking at transition metal surfaces has been studied with sum frequency generation (SFG), scanning tunneling microscopy (STM), and catalytic nanodiodes operating under the high-pressure conditions. The combination of these techniques permits us to understand the role of surface defects, surface diffusion, and hot electrons in dynamics of surface catalyzed reactions. Sum frequency generation vibrational spectroscopy and kinetic measurements were performed under 1.5 Torr of cyclohexene hydrogenation/dehydrogenation in the presence and absence of H(2) and over the temperature range 300-500 K on the Pt(100) and Pt(111) surfaces. The structure specificity of the Pt(100) and Pt(111) surfaces is exhibited by the surface species present during reaction. On Pt(100), pi-allyl c-C6H9, cyclohexyl (C6H11), and 1,4-cyclohexadiene are identified adsorbates, while on the Pt(111) surface, pi-allyl c-C6H9, 1,4-cyclohexadiene, and 1,3-cyclohexadiene are present. A scanning tunneling microscope that can be operated at high pressures and temperatures was used to study the Pt(111) surface during the catalytic hydrogenation/dehydrogenation of cyclohexene and its poisoning with CO. It was found that catalytically active surfaces were always disordered, while ordered surface were always catalytically deactivated. Only in the case of the CO poisoning at 350 K was a surface with a mobile adsorbed monolayer not catalytically active. From these results, a CO-dominated mobile overlayer that prevents reactant adsorption was proposed. By using the catalytic nanodiode, we detected the continuous flow of hot electron currents that is induced by the exothermic catalytic reaction. During the platinum-catalyzed oxidation of carbon monoxide, we monitored the flow of hot electrons over several hours using a metal-semiconductor Schottky diode composed of Pt and TiO2. The thickness of the Pt film used as the catalyst was 5 nm, less than the electron mean free path, resulting in the ballistic transport of hot electrons through the metal. The electron flow was detected as a chemicurrent if the excess electron kinetic energy generated by the exothermic reaction was larger than the effective Schottky barrier formed at the metal-semiconductor interface. The measurement of continuous chemicurrent indicated that chemical energy of exothermic catalytic reaction was directly converted into hot electron flux in the catalytic nanodiode. We found the chemicurrent was well-correlated with the turnover rate of CO oxidation separately measured by gas chromatography.     

Enhancement of the efficiency of dye-sensitized solar cell with multi-wall carbon nanotubes/polythiophene composite counter electrodes prepared by electrodeposition

For the purpose of increasing the energy conversion efficiency of dye-sensitized solar cells (DSSCs), multi-wall carbon nanotube (MWCNT)/polythiophene (PTh) composite film counter electrode has been fabricated by electrophoresis and cyclic voltammetry (CV) in sequence. The morphology and chemical structure have been characterized by transmission electron microscopy (TEM), scanning electron microscope (SEM), and Raman spectroscopy respectively. The overall energy conversion efficiency of the DSSC employing the MWCNT/PTh composite film has reached 4.72%, which is close to that of the DSSC with a platinum (Pt) counter electrode (5.68%). Compared with a standard DSSC with MWCNT counter electrode whose efficiency is 2.68%, the energy conversion efficiency has been increased by 76.12% for the DSSC with MWCNT/PTh counter electrode. These results indicate that the composite film with high conductivity, high active surface area, and good catalytic properties for I₃⁻ reduction can potentially be used as the counter electrode in a high-performance DSSC.     

铂-氧化铈/聚苯胺/聚砜复合膜电极的制备及对甲醇的电催化氧化

采用电化学聚合法制备了掺杂CeO2纳米粒子的聚苯胺(PAN)/聚砜(PSF)复合膜电极,在其上电沉积铂粒子,制得了铂-氧化铈/聚苯胺/聚砜的复合膜修饰电极。 复合膜的形貌和化学组分通过冷场发射扫描电子显微镜（Cold FE-SEM）和能量散射X射线谱（EDS）进行了表征,用循环伏安法和电化学交流阻抗法考察了复合膜电极对甲醇的电催化氧化性能。 结果表明,复合膜的双层多孔结构使铂粒子与CeO2粒子在复合膜内层的多孔聚苯胺上均匀沉积,粒子平均尺寸约为80 nm;CeO2为铂质量的7%时,铂-氧化铈/聚苯胺/聚砜复合膜修饰电极对甲醇有很好的电催化氧化性能和高的稳定性。     

Ag作催化剂制备的GaN的形貌及其性能

用化学气相沉积法(CVD)在Si(100)衬底上以Ag纳米颗粒为催化剂制备了微纳米结构的GaN,原料是熔融态的金属Ga和气态的NH3。采用X射线衍射仪(XRD)、透射电镜(TEM)、X-ray能谱仪(EDS)、场发射扫描电子显微镜(SEM)、光致发光能谱(PL)和霍尔效应测试对样品进行了结构、成分、形貌和发光、电学性能分析。结果表明:生成的自组装GaN为六方纤锌矿的类似小梯子的微纳米单晶结构,且在不同的温度下,GaN的发光性能和电学性能也有所不同,相对于强的紫外发光峰,其它杂质发光峰很微弱,且均呈p型导电。对本实验所得到的GaN微纳米结构的可能形成机理进行了探讨。     

构筑先进二维异质结构Ag/WO3−x用于提升光电转化效率

Solar energy, which is clean, affordable and reliable, can help alleviate the current environmental pollution and energy crisis efficiently. In the past few decades, great progress has been made in harvesting and converting solar energy into chemical energy. Among various technologies, plasmon-induced photoelectrochemistry has been proposed as a promising alternative for solar energy conversion. The hot electrons generated from plasmon excitation and transfer from metal nanostructures to semiconductors is a potential new paradigm for solar energy conversion. However, the ultrafast decay of the hot carriers is unfavorable for the improvement of photocatalytic efficiency. Therefore, finding more efficient photocatalysts, with enhanced light absorption and a longer carrier lifetime, is of paramount importance for improving the conversion efficiency of solar energy, but their fabrication is challenging. In this work, a plasmonic metal/semiconductor heterostructure based on Ag nanoparticles embedded in two-dimensional (2D) amorphous sub-stoichiometric tungsten trioxide (a-WO_3−x), followed by annealing, was successfully fabricated. Firstly, the peculiar nanostructure of 2D a-WO_3−x was successfully constructed from WS₂ nanosheets with supercritical CO₂ (SC CO₂) at 200 ℃. Secondly, the Ag/a-WO_3−x heterostructure was synthesized using an in situ reduction method. Finally, the obtained 2D heterostructure of Ag/WO_3−x was annealed at 400 ℃ in N₂ to further improve its stability and conductivity. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure, morphology, and composition of the material, respectively. UV-Vis spectra were also measured to evaluate light adsorption. Characterization results show that the amorphous structure can effectively anchor metal nanoparticles, and the metal nanoparticles are uniformly dispersed in the amorphous region and have a small size. The as-prepared nanocomposites showed efficient photoelectrochemical (PEC) water splitting when serving as photoelectrode materials, and efficient PEC activity towards photo-oxidation degradation currents under excitation of Ag localized surface plasmon resonance (LSPR). The photocurrent response of the Ag/WO_3−x heterostructure was approximately five times greater than that of a-WO_3−x. Moreover, the PEC degradation efficiency of Ag/WO_3−x reached 96.7% for MO under Vis light illumination (after reaction for 120 min), while the PEC degradation efficiency of WO_3−x was only 63.6%. The high PEC performance of the composite photoanode can be ascribed to the local surface plasmon resonance (LSPR) effect of the Ag nanoparticles, which can enhance the light absorption and hot electron transformation. Moreover, the construction of local crystalline-amorphous interfaces can further promote the separation efficiency of the photogenerated electron-hole pairs, and thus increase conductivity. This work provides a positive strategy for the fabrication of advanced photocatalysts, and a new perspective on understanding of the synergistic effects of structural and electronic regulations.     

激光溅射法制备Pt/CNTs催化剂用于邻氯硝基苯的液相加氢反应

采用激光溅射法制备了碳纳米管负载铂(Pt/CNTs)催化剂, 并利用透射电子显微镜(TEM)、X射线能量散射谱(EDS)、X射线衍射(XRD)、X射线光电子能谱(XPS)等表征手段研究了Pt粒子在碳纳米管表面存在的状态、组成、结构等性质. 讨论了Pt/CNTs催化剂的不同制备条件对催化剂结构的影响, 并考察了催化剂对邻氯硝基苯液相加氢合成邻氯苯胺反应的加氢性能. 研究结果表明, 在激光电压为250 V, 绝对压力为300 Pa, 载体温度为25 ℃条件下制备的Pt/CNTs催化剂, 在不加脱卤抑制剂, 反应温度为60 ℃和氢气压力为1.0 MPa的条件下, 邻氯硝基苯转化率可以达到99.7%以上, 邻氯苯胺选择性可达到98.2%以上, 表现出较高的加氢性能和抑制脱卤性能.     

Physical and electrical properties of chemical vapor grown GaN Nano/microstructures

Wurtzitic gallium nitride nano- and microleaves were controlled grown through a facile chemical vapor deposition method. This is the first report of GaN nanoleaves, a new morphology of GaN nanostructures. The as-grown GaN structures were characterized by means of X-ray powder diffraction, scanning electron microscopy, energy dispersive X-ray, transmission electron microscopy, and selected area electron diffraction. Raman scattering spectra of the GaN leaves were studied. Field effect transistors based on individual GaN nanoleaves were fabricated, and the electrical transport results revealed a pronounced n-type gating effect of the GaN nanostructures.     

质子交换膜燃料电池Pt纳米线电催化剂研究现状

质子交换膜燃料电池(PEMFC)能直接将化学能转换为电能,具有能量转换效率高、环境友好、启动快等优点.其中电催化剂是决定PEMFC性能、寿命及成本的关键材料之一.目前所采用的Pt催化剂成本较高,是阻碍其商业化的主要因素.而Pt纳米线电催化剂的Pt利用率和催化剂活性高,抗CO毒性以及耐久性好.本文综述了Pt纳米线电催化剂的制备及其电化学催化性能的研究现状.     

Photoinduced Betaine Generation for Efficient Photothermal Energy Conversion

The conversion of solar energy to thermal, chemical, or electrical energy attracts great attention in chemistry and physics. There has been a considerable effort for the efficient extraction of photons throughout the entire solar spectrum. In this work light energy was efficiently harvested by using a long-lived betaine photogenerated from an acridinium-based electron donor–acceptor dyad. The photothermal energy-conversion efficiency of the dyad is significantly enhanced by simultaneous illumination with blue (420–440 nm) and yellow (>480 nm) light in comparison with the sum of the conversion efficiencies for individual illumination with blue or yellow light. The enhanced photothermal effect is due to the photogenerated betaine, which absorbs longer-wavelength light than the dyad, and thus the dyad–betaine combination is promising for efficient photothermal energy conversion. The mechanisms of betaine generation and energy conversion are discussed on the basis of steady-state and transient spectral measurements.     

Platinum nanoparticles on gallium nitride surfaces: effect of semiconductor doping on nanoparticle reactivity

Platinum nanoparticles supported on n- and p-type gallium nitride (GaN) are investigated as novel hybrid systems for the electronic control of catalytic activity via electronic interactions with the semiconductor support. In situ oxidation and reduction were studied with high pressure photoemission spectroscopy. The experiments revealed that the underlying wide-band-gap semiconductor has a large influence on the chemical composition and oxygen affinity of supported nanoparticles under X-ray irradiation. For as-deposited Pt cuboctahedra supported on n-type GaN, a higher fraction of oxidized surface atoms was observed compared to cuboctahedral particles supported on p-type GaN. Under an oxygen atmosphere, immediate oxidation was recorded for nanoparticles on n-type GaN, whereas little oxidation was observed for nanoparticles on p-type GaN. Together, these results indicate that changes in the Pt chemical state under X-ray irradiation depend on the type of GaN doping. The strong interaction between the nanoparticles and the support is consistent with charge transfer of X-ray photogenerated free carriers at the semiconductor-nanoparticle interface and suggests that GaN is a promising wide-band-gap support material for photocatalysis and electronic control of catalysis.     

Microstructure and properties of sol-gel derived Pb(Zr<Subscript>0.3</Subscript>Ti<Subscript>0.7</Subscript>)O<Subscript>3</Subscript> thin films on GaN/sapphire for nanoelectromechanical systems

Highly (111) oriented, phase-pure perovskite Pb(Zr_0.3Ti_0.7)O₃ (or PZT 30/70) thin films were deposited on single-crystal, (0001) wurtzite GaN/sapphire substrates using the sol-gel process and rapid thermal annealing. The phase, crystallinity, and stoichiometry of annealed PZT films were evaluated by X-ray diffraction and Rutherford backscattering spectroscopy. The atomic force microscopy revealed a smooth PZT surface (rms roughness ∼1.5 nm) with striations and undulations possibly influenced by the nature of the underlying GaN surface. The cross-sectional field-emission scanning electron microscopic images indicated a sharper PZT/GaN interface compared to that of sol-gel derived PZT on (111) Pt/TiO₂/SiO₂/(100) Si substrates. The capacitance-voltage (C-V) characteristics for PZT in the Pt/PZT/GaN (metal-ferroelectric-semiconductor or MFS) configuration were evaluated as a function of annealing temperature and applied voltage. The observed C-V hysteresis stemmed from trapped charge at defect sites within PZT. Also, the lower capacitance density (C/A = 0.35 μF/cm², where A is the area of an electrode) and remnant polarization (P _r ∼ 4 μC/cm²) for PZT in the MFS configuration, compared to the values for PZT in the MFM configuration (Pt/PZT/Pt), were attributed to the high depolarization field within PZT.     

Ultrasonic assisted polyol synthesis of highly dispersed Pt/MWCNT electrocatalyst for methanol oxidation

A novel chemical method based on ultrasonic assisted polyol synthesis for the fabrication of highly dispersed Pt nanoparticles on multi-walled carbon nanotubes (MWCNTs) was developed. The simple and green method took only about 10 min at ambient temperature. The structure and chemical nature of the resulting Pt/MWCNT composites were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy dispersive X-ray spectrometry (EDS). The results showed that the prepared Pt nanoparticles were uniformly dispersed on the MWCNT surface. The mean size of Pt nanoparticles was about 2.8 nm. Electrochemical properties of Pt/MWCNT electrode for methanol oxidation were examined by cyclic voltammetry (CV) and excellent electrocatalytic activities could be observed. The possible formation mechanism of Pt/MWCNTs was also discussed.     

Characterization and Catalytic Behavior of Nano Pt/MgO Catalysts

"Pt/MgO catalysts were prepared by using wet impregnation method. X-ray diffraction, transmission electron microcopy (TEM), electron spectroscopy for chemical analysis, and temperature programmed surface reaction (TPSR) were used to characterize the calcined and used catalysts. Partial Oxidation of Methane (POM) to synthesis gas was carried out in a fixed bed micro-reactor to test the catalytic activity. TEM images revealed that Pt particles have the size of less than 10 nm, the size of the support MgO is about 50-200 nm. At 800 oC, the Pt/MgO catalyst exhibited a very high activity and stability for POM reaction. Both the conversion of methane and selectivities to CO and H2 remained almost constant up to 120 h. The dispersed Pt existed in the metallic state. The state and the dispersion of active Pt and the structure of support MgO were very stable, together with the high ability to resist carbon deposition, made the catalysts show a very high stability for POM under stoichiometric reaction conditions."     

One-step synthesis of Pt nanoparticles/reduced graphene oxide composite with enhanced electrochemical catalytic activity

A one-step electrochemical approach for synthesis of Pt nanoparticles/reduced graphene oxide(Pt/RGO) was demonstrated.Graphene oxide(GO) and chloroplatinic acid were reduced to RGO and Pt nanoparticles(Pt NPs) simultaneously,and Pt/RGO composite was deposited on the fluorine doped SnO 2 glass during the electrochemical reduction.The Pt/RGO composite was characterized by field emission-scanning electron microscopy,Raman spectroscopy and X-ray photoelectron spectroscopy,which confirmed the reduction of GO and chloroplatinic acid and the formation of Pt/RGO composite.In comparison with Pt NPs and RGO electrodes obtained by the same method,results of cyclic voltammetry and electrochemical impedance spectroscopy measurements showed that the composite electrode had higher catalytic activity and charge transfer rate.In addition,the composite electrode had proved to have better performance in DSSCs than the Pt NPs electrode,which showed the potential application in energy conversion.     

Facile Fabrication of Platinum Nanoparticles Supported on Nitrogen-doped Carbon Nanotubes and Their Catalytic Activity

A facile impregnation method under mild condition is designed for synthesis of highly dispersed Pt nanoparticles with a narrow size of 4-7 nm on nitrogen-doped carbon nanotubes （CNx）. CNx do not need any pre-surface modification due to the inherent chemical activity. The structure and nature of Pt/CNx were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and energy dispersive spectroscopy spectrum. All the experimental results revealed that the large amount of doped nitrogen atoms in CNx was virtually effective for capturing the Pt（IV） ions. The improved surface nitrogen functionalities and hydrophilicity contributed to the good dispersion and immobi- lization of Pt nanoparticles on the CNx surface. The Pt/CNx served as active and reusable catalysts in the hydrogenation of allyl alcohol. This could be attributed to high dispersion of Pt nanoparticles and stronger interaction between Pt and the supports, which prevented the Pt nanoparticles from aggregating into less active Pt black and from leaching as well.     

Photoinduced Oxygen Reduction Reaction Boosts the Output Voltage of a Zinc–Air Battery

Utilization of solar energy is of great interest for a sustainable society, and its conversion into electricity in a compact battery is challenging. Herein, a zinc–air battery with the polymer semiconductor polytrithiophene (pTTh) as the cathode is reported for direct conversion of photoenergy into electric energy. Upon irradiation, photoelectrons are generated in the conduction band (CB) of pTTh and then injected into the π_2p* orbitals of O₂ for its reduction to HO₂^?, which is disproportionated to OH^? and drives the oxidation of Zn to ZnO at the anode. The discharge voltage was significantly increased to 1.78 V without decay during discharge–charge cycles over 64 h, which corresponds to an energy density increase of 29.0 % as compared to 1.38 V for a zinc–air battery with state‐of‐the‐art Pt/C. The zinc–air battery with an intrinsically different reaction scheme for simultaneous conversion of chemical and photoenergy into electric energy opens a new pathway for utilization of solar energy.     

Preparation of Phase Pure Cubic Gallium Nitride, c-GaN, by Ammonothermal Conversion of Gallium Imide, {Ga(NH)3/2} n

Gallium imide, {Ga(NH)_3/2} _n , was sealed in quartz tubes with anhydrous NH₃ and NH₄X (X=Cl, Br, I) and heated vertically in an autoclave with the hot-zone (bottom) temperature ranging from 300–530°C. With NH₄I mineralizer, the imide was converted to primarily the zinc-blende phase of GaN (c-GaN), usually in the form of micron sized or smaller tetrahedra. With NH₄Cl mineralizer, hexagonal GaN nanoparticles formed instead, and NH₄Br facilitated the conversion of imide to irregular h-GaN nanoparticles at lower temperatures, and larger particles above 500°C. At the higher temperatures, chemical transport took place and GaN crystals grew on the walls at the middle to the top of the tube. Most of the deposited GaN was cubic, usually in a heavily etched triangular prismatic morphology. Several experiments were performed in a Pt lined pressure vessel in order to demonstrate that the synthesis method can be scaled up.     

Local studies of photoelectrochemical reactions at nanostructured oxides

The conversion of photon energy to chemical energy and vice versa requires the close arrangement of absorber/emitters and (electro)chemical reactions sites. This review considers local measurement techniques aiding in the design of efficient oxide systems for the utilization of light as energy source and as efficient detection principle. Artificial photoelectrochemical systems are often build on oxides as they are abundant and have semiconducting properties. However, no single oxide fulfills all requirements for an efficient conversion of sunlight to chemical energy and thus complex oxides are explored. These oxides might be obtained by doping oxides with other metal cations or by combining different oxides for absorbance and catalyzing the desired reaction, mainly water splitting. Due to the enormous amount of possible combinations combinatorial search for new material systems has been pursued and accelerated around the world making use of local photoelectrochemical characterization techniques in the screening step. Local detection schemes based on scanning electrochemical microscopy and scanning electrochemical cell microscopy also provide details about the kinetics for heterogeneous charge transfer and the release of soluble reaction products. During the recent years the scanning probe methods have been complemented by local detection of fluorescent reaction products that are formed by heterogeneous electron transfer reactions from and non-fluorescent precursor molecules. Such detection is possible with single molecule sensitivity and spatial resolution exceeding the diffraction limit (superresolution). Such approaches enabled the discovery of population within ensembles of metal oxide nanoparticles that are distinguished by the location and reactivity of their reaction sites. Optical techniques for measuring Faradaic currents hold great promise for the measurement of very low currents beyond the study of photoelectrochemistry of metal oxides.