A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.

Transition metal carbides (TMCs) feature high catalytic activity and superior stability for the hydrogen evolution reaction (HER). However, their platinum‐like HER catalytic performance is heavily hindered, due to their strong interaction with hydrogen. Herein, Ni activation of TMCs (M = V, Fe, Cr, and Mo) is proposed through introducing adsorbed nickel atoms on the TMC surface (Ni/TMC). In both acidic and alkaline solutions, a sharp decrease of both overpotentials and Tafel slopes of the Ni/TMC catalysts for HER is achieved. At 10 mA cm−2, the overpotentials of the Ni/vanadium carbide (VC) and Ni/Fe3C catalysts are 128 and 93 mV in 1 m KOH, 111 and 112 mV in 0.5 m H2SO4, respectively. Even at 150 mA cm−2, they exhibit the overpotentials of as low as 270 and 291 mV, respectively. In the alkaline solutions, the performance of these Ni/TMC catalysts is even superior to a Pt/C catalyst. As confirmed from density functional theory calculations and X‐ray absorption fine structure analysis, such adsorbed Ni atoms effectively optimize the d‐electron structure and improve HER performance. As a versatile strategy, this work provides a universal route to activate TMCs for highly efficient HER in different media.

The new role of graphene (GR) in boosting the two-electron reduction of O2 to H2O2 has been first identified in the GR-WO3 nanorod (NR) nanocomposite photocatalysts, which are fabricated by a facile, solid electrostatic self-assembly strategy to integrate the positively charged branched poly(ethylenimine) (BPEI)-GR (BGR) and negatively charged WO3 NRs at room temperature. Photoactivity test shows that, as compared to WO3 NRs, BGR-WO3 NRs with an appropriate addition ratio of GR exhibit remarkably enhanced and stable visible-light photoactivity toward the degradation of Rhodamine B. Besides the common roles of GR observed in the GR-based composite photocatalysts in the literature, including enhancing the visible-light absorption intensity, improving the lifetime and transfer of photogenerated charge carriers, and increasing the adsorption capacity for reactants, we have observed the new role of GR in boosting the two-electron reduction of O2 to H2O2 in this specific BGR-WO3 NR photocatalyst system. Importantly, this new role of GR does contribute to the overall photoactivity enhancement of BGR-WO3 NR nanocomposites. The synergistic contribution of GR on improving the photoactivity of WO3 NRs and the underlying reaction mechanism have been analyzed by the structure-photoactivity correlation analysis and controlled experiments using radicals scavengers.

The electronic properties of molecular junctions of the general type carbon/molecule/TiO2Au were examined as examples of "molecular heterojunctions" consisting of a molecular monolayer and a semiconducting oxide. Junctions containing fluorene bonded to pyrolyzed photoresist film (PPF) were compared to those containing Al2O3 instead of fluorene, and those with only the TiO2 layer. The responses to voltage sweep and pulse stimulation were strongly dependent on junction composition and temperature. A transient current response lasting a few milliseconds results from injection and trapping of electrons in the TiO2 layer, and occurred in all three junction types studied. Conduction in PPFTiO2Au junctions is consistent with space charge limited conduction at low voltage, then a sharp increase in current once the space charge fills all the traps. With fluorene present, there is a slower, persistent change in junction conductance which may be removed by a reverse polarity pulse. This "memory" effect is attributed to a redox process in the TiO2 which generates TiIII and/or TiII, which have much higher conductance than TiO2 due to the presence of conduction band electrons. The redox process amounts to "dynamic doping" of the TiO2 layer by the imposed electric field. The memory effect arises from a combination of the properties of the molecular and oxide layers, and is a special property of the molecular heterojunction configuration.

Thin-layer carbon/molecule/TiO 2 /Au electronic junctions with 1.7-2.1 nm thick molecular layers exhibit voltage-induced conductance switching, which may be repeated for at least hundreds of read/set/read/erase cycles. A fluorene͑Fl͒/TiO 2 junction can be switched to a higher conductance "set" state by a positive voltage pulse and brought back to the lower conductance "erased" state by a negative pulse. Similar conductance changes occurred following exposure to H 2 or UV radiation, and the conductance change is completely inhibited in a dry atmosphere. The bias-induced conductance switching of TiO 2 junctions is consistent with electrochemical reduction of Ti IV oxide to the much more conductive Ti III oxide, analogous to a solid-state redox reaction. The observations are consistent with reduction of hydroxylated TiO 2 sites to a much more conductive Ti III oxide by the electrons injected into the TiO 2 during a positive voltage pulse. If Fl is replaced by aminodecane ͑C 10 H 21 N͒ or nitroazobenzene, the conductance switching is modified slightly, consistent with TiO 2 being the active switching component. The results bear directly on the origin of the hysteresis observed in TiO 2-based junctions and also on their suitability as examples of recently reported "memristors" ͓J.