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The length, spacing and morphology of vertical aligned nanostructural RuO2 one-dimensional (1D) arrays are varied independently to investigate their effect on the field-emission properties. It is shown that, in general, the lowest turn-on field, highest emission current density and apparent field enhancement factor can be achieved in nanocrystals (NCs) with the smallest emitter radius for the tips and the highest aspect ratio. Moreover, the lower-density NCs exhibit better field-emission characteristics, which are related to the screening effect. The results could be valuable for the application of field-emission-based devices using RuO2 NCs arrays as cathode materials.

We review the results of the synthesis of IrO2 nanocrystals (NCs) on different substrates via metal-organic chemical vapour deposition (MOCVD) using (MeCp)(COD)Ir as the source reagent. The surface morphology, structural and spectroscopic properties of the as-deposited NCs were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffractometry (XRD), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The roles of different substrates for the formation of various textures of nanocrystalline IrO2 are studied. Several one-dimensional (1D) nanostructures have evolved by decreasing the degree of interface instability. The morphological evolution occurs from triangular/wedged nanorods via incomplete/scrolled nanotubes to square nanotubes and square nanorods (NRs), with increasing morphological stability. The results show that the three-dimensional (3D) grains composing traditional film belong to the most stable form as compared to all the 1D NCs, and the sequential shape evolution has been found to be highly correlated to a morphological phase diagram based on the growth kinetics. In addition, area selective growth of IrO2 NRs has been demonstrated on sapphire(012) and sapphire(100) substrates which consist of patterned SiO2 as the nongrowth surface. The initial growth of IrO2 nuclei is studied. Selectivity, rod orientation, and other morphological features of the nanorod forest can find their origins in the nucleation behaviour during initial growth. XPS analyses show the coexistence of higher oxidation states of iridium in the as-grown IrO2 NCs. The usefulness of the experimental Raman scattering together with the modified spatial correlation (MSC) model analysis as a residual stress and structural characterization technique for 1D IrO2 NCs has been demonstrated. The field emission properties of the vertically aligned IrO2 NRs are studied and demonstrated as a high-performance and robust field emitter material owing to its low work function, low resistivity and excellent stability against oxygen.

The effects of post-thermal treatment and rod diameter on the field emission (FE) properties of RuO2 nanorod films are reported. The FE properties of RuO2 nanorods with pyramidal tips have been studied on samples of two average rod diameters, 48 and 35 nm. Both of them exhibit a transient behaviour in emission current under a fixed electric field. Thermal annealing at 400 and 500 °C generally improves the FE characteristics in lowering the emission barriers of nanorods, as manifested by a decrease in the turn-on field, the threshold field, the slope of the Fowler–Nordheim plot, and the time-span of the transient period. Reduction in emission barrier appears to be particularly evident for the 35 nm nanorods. Yet 500 °C annealing also degrades the emission current stability of 35 nm nanorods. The standard deviation of emission current density of 35 nm nanorods after 500 °C annealing is around 44%. On the other hand, both specimens after 400 °C annealing display a much more stable emission current density, whose fluctuations are 26% of their average values. The transient period is regarded as a consequence of gas molecules being desorbed from the RuO2 tips during emission, resulting in a reduction of the emission barrier. Thermal annealing weakens the bonding between the adsorbed gas and the tip surface, and facilitates the gas desorption and electron tunnelling processes.

In this paper, we report on a direct observation of the morphological evolution of one dimensional (1D) nanocrystals via a systematic metal?organic chemical vapour deposition (MOCVD) study. Several IrO2 1D nanostructures have emerged as a result of a decrease in the degree of interface instability. The morphological evolution occurs from triangular/wedged nanorods via incomplete/scrolled nanotubes to square nanotubes and square nanorods according to the morphological stability. The results show that the polycrystalline films composed of continuous 3D grains belong to the most stable form as compared to the 1D nanocrystals. The shape evolution has been found to highly correlate to a morphological phase diagram based on the growth kinetics.

Capacity degradation and ion insertion of a miniaturized electrochemical capacitor are studied using ionic liquid [EMI] [TFSI] as the electrolyte. This capacitor is featured with two comb-like electrodes of vertical carbon nanotubes, ∼70 μm in height and 20 μm in interelectrode gap. We quantify the levels of ion insertion damage with Raman spectroscopy after the electrode experiences 120 consecutive voltammetric cycles to various potential limits. Distinct structural damage emerges due to [EMI] when the negative potential reaches -1.7 V, and those due to [TFSI] arise when the positive potential reaches 1.7 V vs. RHE. Judging from the peak broadenings, [EMI] is more detrimental than [TFSI]. When the voltage window ΔU is set as less than or equal to 2.8 V, both electrode potentials are within the two intercalation limits, little or no decay is observed in 10(4) charge/discharge cycles. When ΔU is 3.4 V, the positive potential exceeds the upper limit, but the negative potential stays within the lower limit, the cell capacitance decreases moderately. When ΔU increases to 3.8 V, both electrodes suffer from damages because of exceeding the intercalation limits. And the cell capacitance decreases substantially, even leading to a premature failure.

We report in detail the synthesis and characterization of V-shaped IrO(2) nanowedges (NWs) with an angle of 110° between the two arms. The NWs were grown on top of rutile (R) phase TiO(2) nanorods (NRs) sitting on a sapphire (SA)(100) substrate via metal-organic chemical vapor deposition (MOCVD) by using (C(6)H(7))(C(8)H(12))Ir and titanium-tetraisopropoxide (TTIP, Ti[OCH(CH(3))(2)](4)) as the source reagents. The surface morphology, structural, and spectroscopic properties of the as-deposited nanocrystals (NCs) were characterized by field emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), micro-Raman spectroscopy, transmission electron microscopy (TEM), and selected-area electron diffractometry (SAED). The FESEM images and XRD patterns indicated growth of V-shaped IrO(2)(101) NWs on top of R-TiO(2) NRs. The Raman spectrum showed the nanosize induced redshift and peak broadening of the IrO(2) and rutile phase of TiO(2) signatures with respect to that of the bulk counterparts. TEM and SAED characterizations of IrO(2) NCs showed that the nanowedges were crystalline IrO(2) with a twin plane of (101) and twin direction of [Formula: see text] at the V-junction. The probable mechanisms for the formation of well-aligned IrO(2) NWs are discussed.

IrO(x) nanofoils (IrO(x)NF) of high surface area are sputtered on multi-wall carbon nanotubes (CNT) in the preparation of a structured electrode on a stainless steel (SUS) substrate for supercapacitor applications. This IrO(x)/CNT/SUS electrode is featured with intriguing IrO(x) curved foils of 2-3 nm in thickness and 400-500 nm in height, grown on top of the vertically aligned CNT film with a tube diameter of ∼ 40 nm. These nanofoils are moderately oxidized during reactive sputtering and appeared translucent under the electron microscope. Detailed structural analysis shows that they are comprised of contiguous grains of iridium metal, iridium dioxide, and glassy iridium oxide. Considerable Raman line broadening is also evidenced for the attributed nanosized iridium oxides. Two capacitive properties of the electrode are significantly enhanced with addition of the curved IrO(x) foils. First, IrO(x)NF reduces the electrode Ohmic resistance, which was measured at 3.5 Ω cm(2) for the CNT/SUS and 2.5 Ω cm(2) for IrO(x)NF/CNT/SUS using impedance spectroscopy. Second, IrO(x)NF raises the electrode capacitance from 17.7 F g(-1) (CNT/SUS) to 317 F g(-1) (IrO(x)/CNT/SUS), measured with cyclic voltammetry. This notable increase is further confirmed by the galvanostatic charge/discharge experiment, measuring 370 F g(-1) after 2000 uninterrupted cycles between - 1.0 and 0.0 V (versus Ag/AgCl).

A thin film of novel hierarchical structure, suitable for supercapacitor applications, has been developed through combining conductive multi-wall carbon nanotubes (MWCNTs) and square IrO(2) nanotubes (IrO(2)NT) of nanometer size. Synthesis of this hierarchical structure with open porosity is performed by depositing IrO(2) short tubes densely along the long wires of carbon nanotube on a substrate of stainless steel. A IrO(2) tube of rutile structure grows in the [001] direction, with an opening at its top, surrounded by very thin walls. The IrO(2) addition on the MWCNT template increases the capacitance of the CNT thin film effectively, because of pseudocapacitance of the IrO(2) surface. For this particular composite, featured with two tubular nanostructures, the specific capacitance increases from 15 F g(-1) (MWCNT) to 69 F g(-1) (IrO(2)NT/MWCNT), measured using the galvanostatic discharge experiment. Its property of fast retrieval of the stored charge is assured in the impedance measurement, showing that the internal resistance of the IrO(2)NT/MWCNT nanocomposite electrode is lower than that of the bare MWCNTs.

A symmetric ultracapacitor CNT_CNT and an asymmetric ultracapacitor CNT_hRuO(2) of mini size have been prepared with patterned carbon nanotubes (CNT) and hydrous ruthenium dioxide. Galvanostatic charge/discharge results indicate that CNT_hRuO(2) is the superior one in both power and energy densities. In a potential window 2.0 V, the CNT_hRuO(2) cell displays an energy density of 24.0 W h kg(-1) at a power density of 22.9 kW kg(-1). Its power density can be raised to 41.1 kW kg(-1) at the expense of the energy density, which drops to 6.8 W h kg(-1). On the other hand, CNT_CNT performs at a lower level, delivering 5.2 W h kg(-1) at 5.5 kW kg(-1). The favorable charge/discharge performance of CNT_hRuO(2) is attributed to hydrous RuO(2), whose pseudocapacitance drives the other electrode of the vertical CNT array to work harder and makes more use of its double-layer capacitance. The analysis of individual electrode capacitance indicates that the high capacitance of hRuO(2) also causes a disproportion in voltage partition, which restricts the low limit of cycling current in an extended potential window. On energy cycling, CNT_hRuO(2) demonstrates sufficient stability in 10,000 cycles, after an initial 13% drop in capacitance.

Well-aligned densely-packed rutile TiO(2) nanocrystals (NCs) have been grown on sapphire (SA) (100) and (012) substrates via metal-organic chemical vapor deposition (MOCVD), using titanium-tetraisopropoxide (TTIP, Ti(OC(3)H(7))(4)) as a source reagent. The surface morphology as well as structural and spectroscopic properties of the as-deposited NCs were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAED), x-ray diffraction (XRD) and micro-Raman spectroscopy. FESEM micrographs reveal that vertically aligned NCs were grown on SA(100), whereas the NCs on the SA(012) were grown with a tilt angle of ∼33° from the normal to substrates. TEM and SAED measurements showed that the TiO(2) NCs on SA(100) with square cross section have their long axis directed along the [001] direction. The XRD results reveal TiO(2) NCs with either (002) orientation on SA(100) substrate or (101) orientation on SA(012) substrate. A strong substrate effect on the alignment of the growth of TiO(2) NCs has been demonstrated and the probable mechanism for the formation of these NCs has been discussed.