Dominik Eder

Application and Future Challenges of Functional Nanocarbon Hybrids

Hybridizing nanocarbons, such as carbon nanotubes (CNTs) or graphene, with an active material is a powerful strategy towards designing next-generation functional materials for environmental and sustainable energy applications. While research on nanocomposites, created by dispersing the nanocarbon into polymer or ceramic matrices, began almost immediately after the popularization of CNTs and graphene in 1991 and 2004, respectively, nanocarbon hybrids are a relatively recent addition to the family of composite materials. In contrast to nanocomposites, which typically combine the intrinsic properties of both compounds, nanocarbon hybrids additionally provide access to both a large surface area required for gas/liquid-solid interactions and an extended interface, through which charge and energy transfer processes create synergistic effects that result in unique properties and superior performance. This progress report looks at the history of research on nanocarbons (fullerenes, CNTs and graphene) and their composites and hybrids, presents the origin of synergistic effects, reviews the most intriguing results on nanocarbon hybrid performance in heterogeneous catalysis, electrocatalysis, photocatalysis, batteries, supercapacitors, photovoltaics and sensors, and discusses remaining challenges and future research directions.

Interface engineering in nanocarbon–Ta2O5 hybrid photocatalysts

Hybridizing inorganic nanomaterials with carbon nanotubes and graphene constitutes a powerful approach towards creating new functional materials for environmental and sustainable energy applications. Their superior performance originates from synergistic effects based on charge and energy transfer processes at the hybrids interfaces. However, only few studies have been devoted so far towards rationally designing these hybrids. In this work we demonstrate that engineering interfaces as well as the morphology of the functional inorganic compound can maximise the synergistic effects in hybrids, thus further enhancing the hybrids photocatalytic properties. Particularly, we have stimulated the growth of ultra-thin single-crystalline layers of tantalum (V) oxide (Ta2O5) with preferred orientation at substantially reduced crystallisation temperatures, by utilising the graphitic CNT surfaces as seed crystals through heterogeneous nucleation. The resulting hybrids possess outstanding activities for the evolution of hydrogen via sacrificial water splitting that are about 35 times higher than those of comparable materials such as tantalates. The additional improvements in this hybrid are attributed to the single-crystalline nature of the coating, which alleviates transport of electrons to the interface, as well as the formation of a Schottky-type junction between the metallic nanocarbon and the semiconducting metal oxide, which facilitates charge transfer and thus charge separation at the interface.

Atomic Layer Deposition of TiO2 for a High-Efficiency Hole-Blocking Layer in Hole-Conductor-Free Perovskite Solar Cells Processed in Ambient Air

In this study we design and construct high-efficiency, low-cost, highly stable, hole-conductor-free, solid-state perovskite solar cells, with TiO2 as the electron transport layer (ETL) and carbon as the hole collection layer, in ambient air. First, uniform, pinhole-free TiO2 films of various thicknesses were deposited on fluorine-doped tin oxide (FTO) electrodes by atomic layer deposition (ALD) technology. Based on these TiO2 films, a series of hole-conductor-free perovskite solar cells (PSCs) with carbon as the counter electrode were fabricated in ambient air, and the effect of thickness of TiO2 compact film on the device performance was investigated in detail. It was found that the performance of PSCs depends on the thickness of the compact layer due to the difference in surface roughness, transmittance, charge transport resistance, electron-hole recombination rate, and the charge lifetime. The best-performance devices based on optimized TiO2 compact film (by 2000 cycles ALD) can achieve power conversion efficiencies (PCEs) of as high as 7.82%. Furthermore, they can maintain over 96% of their initial PCE after 651 h (about 1 month) storage in ambient air, thus exhibiting excellent long-term stability.

Large area photoelectrodes based on hybrids of CNT fibres and ALD-grown TiO2

Hybridisation is a powerful strategy towards the synthesis of next generation multifunctional materials for environmental and sustainable energy applications. Here, we report a new inorganic/nanocarbon hybrid material prepared by atomically controlled deposition of a monocrystalline TiO2 layer that conformally coats a macroscopic carbon nanotube (CNT) fiber. Through X-ray diffraction, Raman spectroscopy and photoemission spectroscopy we detect the formation of a covalent Ti–O–C bond at the TiO2/CNT interface and a residual strain of approximately 0.7–2%, which is tensile in TiO2 and compressive in the CNT. It arises after deposition of the amorphous oxide onto the CNT surface previously functionalized by the oxygen plasma used in ALD. These features are observed in samples of different TiO2 thicknesses, in the range from 10 to 80 nm. Ultraviolet photoemission spectroscopy on a 20 nm-thick TiO2 coated sample gives a work function of 4.27 eV, between that of TiO2 (4.23 eV) and the CNT fiber (4.41 eV), and the presence of new interband gap states that shift the valence band maximum to 1.05 eV below the Fermi level. Photoelectrochemical measurements demonstrate electron transfer from TiO2 to the CNT fiber network under UV irradiation. Electrochemical impedance spectroscopy measurements reveal a low resistance for charge transfer and transport, as well as a large capacitance. Our results point to the fact that these hybrids, in which each phase has nanometric thickness and the “current collector” is integrated into the material, are very different from conventional electrodes and can provide a number of superior properties.

Near-Ambient-Pressure X-ray Photoelectron Spectroscopy Study of Methane-Induced Carbon Deposition on Clean and Copper-Modified Polycrystalline Nickel Materials

In order to simulate solid-oxide fuel cell (SOFC)-related coking mechanisms of Ni, methane-induced surface carbide and carbon growth was studied under close-to-real conditions by synchrotron-based near-ambient-pressure (NAP) X-ray photoelectron spectroscopy (XPS) in the temperature region between 250 and 600 °C. Two complementary polycrystalline Ni samples were used, namely, Ni foam—serving as a model structure for bulk Ni in cermet materials such as Ni/YSZ—and Ni foil. The growth mechanism of graphene/graphite species was found to be closely related to that previously described for ethylene-induced graphene growth on Ni(111). After a sufficiently long “incubation” period of the Ni foam in methane at 0.2 mbar and temperatures around 400 °C, cooling down to ∼250 °C, and keeping the sample at this temperature for 50–60 min, initial formation of a near-surface carbide phase was observed, which exhibited the same spectroscopic fingerprint as the C2H4 induced Ni2C phase on Ni(111). Only in the presence of this carbidic species, subsequent graphene/graphite nucleation and growth was observed. Vice versa, the absence of this species excluded further graphene/graphite formation. At temperatures above 400 °C, decomposition/bulk dissolution of the graphene/graphite phase was observed on the rather “open” surface of the Ni foam. In contrast, Ni foil showed—under otherwise identical conditions—predominant formation of unreactive amorphous carbon, which can only be removed at ≥500 °C by oxidative clean-off. Moreover, the complete suppression of carbide and subsequent graphene/graphite formation by Cu-alloying of the Ni foam and by addition of water to the methane atmosphere was verified.

Nontoxic (CH3NH3)3Bi2I9 perovskite solar cells free of hole conductors with an alternative architectural design and a solution-processable approach

Methylammonium iodide bismuthate ((CH3NH3)3Bi2I9) (MBI) perovskite was introduced as a new lead-free and air-stable absorber for hole conductor-free perovskite solar cells. The two-step soaking-assisted sequential solution (2-S) method was adopted to fabricate MBI films for the first time. We compared the formation processes and final morphologies of the MBI films fabricated using the 1-S and 2-S methods on planar and mesoporous TiO2 layers, respectively. We also investigated the effects of the morphologies of MBI films and device architectural design on device performance. Results showed that the MBI films fabricated using the 2-S method achieved a superior coverage both on the compact TiO2 and mesoporous TiO2 layers. The mesoporous structure devices presented higher power conversion efficiencies than the planar structure devices. In addition, all devices exhibited excellent thermal and long-term stabilities. The presented architectural design and solution-processable approach could inspire further research and practical applications on lead-free organic–inorganic hybrid perovskite solar cells.

Ordered gyroidal tantalum oxide photocatalysts: eliminating diffusion limitations and tuning surface barriers

In this work we synthesized well-ordered, Ta2O5 films with a 3D-interconnected gyroid mesopore architecture with large pore sizes beyond 30 nm and extended crystalline domains through self-assembly of tailor-made triblock-terpolymers. This has effectively eliminated diffusion limitations inherent to previously reported mesoporous photocatalysts and resulted in superior hydrogen evolution with apparent quantum yields of up to 4.6% in the absence of any cocatalyst. We further show that the injection barrier at the solid-liquid interface constitutes a key criterion for photocatalytic performance and can be modified by the choice of the carbon template. This work highlights pore and surface engineering as a promising tool towards high-performance mesoporous catalysts and electrodes for various energy-related applications.

Growth mechanism and electrochemical properties of hierarchical hollow SnO2 microspheres with a “chestnut” morphology

Hierarchical hollow microspheres (HHMSs) constitute a very popular class of materials for use as drug-delivery carriers, photocatalysts and electrode materials in batteries, owing to their large, porous surface area and mechanical integrity during intercalation reactions. Here, we used a template- and additive-free hydrothermal route to prepare an unusually shaped SnO2 material that comprises a hollow spherical morphology with uniform diameters and very thin petal-like nano-sheets grown perpendicularly on the spheres surface, resembling a “chestnut cupule”. We thoroughly investigated the formation mechanism by 119Sn Mossbauer spectroscopy, powder X-ray diffraction and X-ray photoelectron spectroscopy. Key to this process is the ultrasonic pre-treatment of an aqueous SnCl2 solution, followed by Ostwald “inside-out” ripening upon hydrothermal processing. This unique morphology has greatly improved the storage capacity and cycling performance of SnO2 as an anode material for lithium and sodium ion batteries compared with conventional SnO2 materials.

Growth, structure and stability of sputter-deposited MoS2 thin films

Molybdenum disulphide (MoS2) thin films have received increasing interest as device-active layers in low-dimensional electronics and also as novel catalysts in electrochemical processes such as the hydrogen evolution reaction (HER) in electrochemical water splitting. For both types of applications, industrially scalable fabrication methods with good control over the MoS2 film properties are crucial. Here, we investigate scalable physical vapour deposition (PVD) of MoS2 films by magnetron sputtering. MoS2 films with thicknesses from ≈10 to ≈1000 nm were deposited on SiO2/Si and reticulated vitreous carbon (RVC) substrates. Samples deposited at room temperature (RT) and at 400 °C were compared. The deposited MoS2 was characterized by macro- and microscopic X-ray, electron beam and light scattering, scanning and spectroscopic methods as well as electrical device characterization. We find that room-temperature-deposited MoS2 films are amorphous, of smooth surface morphology and easily degraded upon moderate laser-induced annealing in ambient conditions. In contrast, films deposited at 400 °C are nano-crystalline, show a nano-grained surface morphology and are comparatively stable against laser-induced degradation. Interestingly, results from electrical transport measurements indicate an unexpected metallic-like conduction character of the studied PVD MoS2 films, independent of deposition temperature. Possible reasons for these unusual electrical properties of our PVD MoS2 thin films are discussed. A potential application for such conductive nanostructured MoS2 films could be as catalytically active electrodes in (photo-)electrocatalysis and initial electrochemical measurements suggest directions for future work on our PVD MoS2 films.

Application of Functional Hybrids Incorporating Carbon Nanotubes or Graphene

Hybridizing nanocarbons, such as carbon nanotubes (CNTs) or graphene (G), with inorganic and organic compounds is a powerful strategy to designing next-generation functional materials for environmental and sustainable energy applications. While research on nanocomposites, created by dispersing the nancarbon into polymer and ceramic matrices, was started almost immediately after the popularization of CNTs and graphene in 1991 and 2004, respectively, nanocarbon hybrids are a relatively recent addition to the family of composite materials. In contrast to nanocomposites, which typically combine the intrinsic properties of both compounds, nanocarbon hybrids additionally provide access to both a large surface area required for gas–liquid–solid interactions and an extended interface, through which charge and energy transfer processes create synergistic effects that result in unique properties and superior performance. This chapter summarizes the current progress of nanocarbon hybrid materials and reviews the most intriguing results on their performance in heterogenous catalysis, electrocatalysis, photocatalysis, batteries, supercapacitors, photovoltaics and sensors.