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The edge sites of molybdenum disulfide (MoS2) have been shown to be efficient electrocatalysts for the hydrogen evolution reaction (HER). To utilize these structures, two main strategies have been proposed. The first strategy is to use amorphous structures, which should be beneficial in maximizing the area of the edge-site moieties of MoS2. However, these structures experience structural instability during HER. The other strategy is nanostructuring, in which, to enhance the resulting HER performance, the exposed surfaces of MoS2cannot be inert basal planes. Therefore, MoS2may need critical nanocrystallinity to produce the desired facets. Here, we first describe that when atomic layer deposition (ALD) is applied to layered materials such as MoS2, MoS2exhibits the nonideal mode of ALD growth on planar surfaces. As a model system, the ALD of MoCl5and H2S was studied. This nonideality does not allow for the conventional linear relationship between the growth thickness and the number of cycles. Instead, it provides the ability to control the relative ratios of the edge sites and basal planes of MoS2to the exposed surfaces. The number of edge sites produced was carefully characterized in terms of the geometric surface area and effective work function and was correlated to the HER performance, including Tafel slopes and exchange current densities. We also discussed how, as a result of the low growth temperature, the incorporation of chlorine impurities affected the electron doping and formation of mixed 2H and 1T phases. Remarkably, the resulting 1T phase was stable even upon thermal annealing at 400 °C. With the simple, planar MoS2films, we monitored the resulting catalytic performance, finding current densities of up to 20 mA cm–2at −0.3 V versus the reversible hydrogen electrode (RHE), a Tafel slope of 50–60 mV/decade, and an onset potential of 143 mV versus RHE.

Cadmium sulfide (CdS) is a semiconducting absorber for photoelectrochemical (PEC) hydrogen production with suitable electronic band structures. However, it suffers from severe photocorrosion and rapid charge recombination during the desired PEC reactions. Herein, we describe the identification of the optimal junction thickness of CdS/MoS2core/sheath heterojunction nanostructures by employing atomic layer deposition (ALD) techniques. ALD-grown MoS2sheath layers with different thicknesses were realized on single-crystalline CdS nanorod (NR) arrays on transparent conducting oxide substrates. We further monitored the resulting solar H2evolution performance with our heterojunction photoanodes. The results showed that the junction thickness of MoS2plays a key role in the reduction of photocorrosion and the enhanced photocurrent density by optimizing the charge separation. A better saturation photocurrent (∼46%) was obtained with the 7 nm-thick MoS2@CdS NRs than that with the bare CdS NRs. Moreover, the external quantum efficiency was increased twofold over that of the pristine CdS NRs. The ALD-grown MoS2@CdS heterojunction structures provides an efficient and versatile platform for hydrogen production when combining ALD-grown MoS2with ideal semiconducting absorbers.

Atomic-layer deposition (ALD) is now being recognized as a powerful, general tool for modifying the surfaces of nanomaterials in applications for many energy conversion devices. However, ALD involves slow processes particularly when it is subjected to nanoporous media with high-aspect ratios. Predicting the exact experimental conditions of the desired reactions for coating inside deep pores by ALD is not available because of the lack of complete understanding of diffusion in nanoporous media. Here, we report a comparative study of the ALD coating onto two distinctive templates having nanopores, i.e., 2- and 3-dimensionally ordered media (DOM), of similar porosity and pore dimension. Self-supporting, crack-free templates were carefully prepared in centimeters for both 2- and 3-DOM and thus avoid any possible sources of uncontrollable diffusion of precursor gas molecules through unwanted microvoids and cracks. Comparison of the ALD coating profiles across the thickness of both templates reveals a fundamentally distinct coating mechanism. While a uniform growth zone develops along the pores of the 2-DOM (i.e., 1-D diffusion path), a gradual decrease in the deposition is observed in those of the 3-DOM (i.e., 3-D diffusion path) as ALD pulse time increases. This observation suggests an essential role of the pore connectivity, rather than individual pore sizes, in the gas diffusion dynamics inside nanoporous media. The present model can universally predict the ALD behaviors in nanoporous media even with different types of pore connectivity.

Single-crystalline gold nanowires (Au NWs) are directly synthesized by the photocatalytic reduction of an aqueous HAuCl4solution inside high-aspect-ratio TiO2nanotubes (NTs). Crystalline TiO2(anatase) NTs are prepared by the template-assisted atomic layer deposition technique with a subsequent annealing. Under the irradiation of ultraviolet light, photoexcited electrons are formed on the surfaces of TiO2NTs and could reduce Au ions to create nuclei without using any surfactant, reducing agent, and/or seed. Once nucleation occurred, high-aspect-ratio Au NWs are grown inside the TiO2NTs in a diffusion-controlled manner. As the solution pH increased, the nucleation/growth rate decreased and twin-free (or not observed), single-crystalline Au NWs are formed. At a pH above 6, the nucleation/growth rates increased and Au nanoparticles are observed both inside and outside of the TiO2NTs. The confined nanoscale geometries of the interior of the TiO2NTs are found to play a key role in the controlled diffusion of Au species and in determining the crystal morphology of the resulting Au NWs.

We describe the direct preparation of crystalline Ni3S2thin films via atomic layer deposition (ALD) techniques at temperatures as low as 250 °C without postthermal treatments. A new ALD chemistry is proposed using bis(1-dimethylamino-2-methyl-2-butoxy) nickel(II) [Ni(dmamb)2] and H2S as precursors. Homogeneous and conformal depositions of Ni3S2films were achieved on 4 in. wafers (both metal and oxide substrates, including Au and SiO2). The resulting crystalline Ni3S2layers exhibited highly efficient and stable performance as electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline solutions, with a low overpotential of 300 mV and a high turnover frequency for HER and an overpotential of 400 mV for OER (at a current density of 10 mA/cm2). Using our Ni3S2films as both the cathode and the anode, two-electrode full-cell electrolyzers were constructed, which showed stable operation for 100 h at a current density of 10 mA/cm2. The proposed ALD electrocatalysts on planar surfaces exhibited the best performance among Ni3S2materials for overall water splitting recorded to date.

Self-ordered porous anodic alumina (PAA) membranes have been widely employed as a scaffold for fabricating various nanomaterials and functional nanostructures with an excellent uniformity. The self-organization processes are only found in narrow experimental windows even in PAA, and their formation mechanisms have not been fully understood yet and might allow us to access a hint that generally extends into other material systems. Here, we revisit the self-organization process of PAA by experimentally observing its initial stage in great detail. Surface morphologies of PAA were carefully monitored which have been imprinted upon the first anodization in the solutions of oxalic acid around the inflection point in the current–time curves. The physical dimensions were analyzed by electron microscopy, and the degree of ordering was evaluated using the radial power spectral density method. We found that the inflection point reflects the occurrence of a uniform pore diameter as well as interpore distance which is crucial for the self-organization phenomena resulting from the minimization of surface free energy. The proposed model was further supported by electric field simulation near the inflection point.

Despite huge potential of nanostructured TiO2in many applications, the concrete understanding of optical properties associated with their defect structures remains elusive to date. Here, we present a systematic study on the photoluminescence (PL) characteristics of anatase TiO2nanotube (TNT) arrays as a function of wall thicknesses (twall) and thereby grain sizes. Highly uniform anatase TNTs were prepared by template-directed atomic layer deposition techniques, followed by thermal annealing. The relative amounts of surface/grain boundary (GB) defects to the bulk oxygen vacancies were tailored in a controlled manner by utilizing the crystallization kinetics between thin and thick wall layers during amorphous to anatase transformation. PL spectra of anatase TNT arrays were attained at variable temperatures from 300 down to 20 K. By changing twall, remarkably, the predominant PL emissions were likely discernible from either self-trapped excitons, oxygen vacancies, or surface/GB defects. Our results indicate that the properties of TNTs are surface-dominated when twall< ∼3 nm and bulk-dominated when twall> ∼3 nm by detecting surface-dangling bonds of TNTs located as deep traps with decreasing twall. Moreover, our TNT arrays with twall= 20 nm and highly crystalline anatase domains emit resonant PL that may suggest STEs coupled with a specific phonon mode. The present work provides us an emergent understanding on the optical properties of one-dimensional, wide-gap oxide semiconductor photoelectrodes in extremely uniform dimensions.

We describe a general strategy for fabricating multisegmented nanotubes and nanopores viasequential, surface-selective modification with organic and inorganic layers combined with in situformation of nanopores by electrochemical anodization. We found that cylindrical alumina nanopores can be continuously anodized upon coating thin organic and/or inorganic layers such as octadecyltrichlorosilane (OTS)-self-assembled monolayers (SAMs), and atomic layer deposition (ALD)-grown TiO2, ZnO, and ZrO2, allowing for three-dimensionally site-selective ALD. As model systems, we show that (1) isolated metal oxide nanotubes can be prepared as-grown with controlled opening at the distal ends of tubes, (2) inner surfaces of nanopores can be chemically and physically modified in a segmented manner, and (3) a nanotube core–multisegmented shell geometry can be achieved. We believe that the present approach will open up new avenues for realizing complex nanodevices such as nanofluidic diodes, photovoltaic junctions, and transistors by adding a degree of freedom in the synthesis of nanotubes/pores in their axis direction.

Abstract

Atomic layer deposition (ALD) not only presents a direct way to prepare nanomaterials when combined with templates, but also allows surface engineering to fine-tune the properties of the material. Here, we review recent progress in the field of nanostructured materials and devices that have been fabricated by ALD. Various materials, including semiconducting, magnetic, noble metallic, and insulating materials, can be used to form three-dimensional (3D), complex nanostructures with controlled composition and physical properties. We begin this review with ALD nanomaterials that can be prepared from porous templates with a 2D pore arrangement, such as anodic aluminum oxide, and advance toward opal structures with a 3D pore arrangement. We also discuss surface engineering by ALD on existing nanowires/nanotubes, devices, and chemical patterns that has the potential for application in high-performance transistors, sensors, and green energy conversion. Finally, we provide perspectives for future device applications that could arise from ALD nanomaterials.

Surface decoration strategy for one-dimensional nanostructures will improve their electrical, optical, mechanical, and electrochemical performances dramatically. Heterogeneous growth/deposition on surfaces, however, may create undesired junction interfaces in the system. Here we report a procedure during which amorphous titania nanotubes are readily self-branched with crystalline titanate nanorods at room temperature. The starting amorphous titania nanotubes were prepared by low-temperature atomic layer deposition combined with the template-directed approach. We routinely observed the self-branching phenomenon of crystalline titanate nanorods with a few nanometers in diameter onto the surfaces of the amorphous titania nanotubes in mild alkali solutions. The resulting structures were analyzed by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. The reactivity of the hierarchical titania nanotube arrays was observed to be improved as a Li secondary battery electrode. Upon complete consumption of the amorphous body of titania nanotubes, in addition, titanate nanosheets/layers consisting of single TiO2layers with unit-cell thickness were obtained, elucidating the formation mechanism of layered titanate materials by alkali treatment.

This study determined the aging characteristics of solution-processed amorphous In−Ga−Zn oxide (a-IGZO) layers as amorphous oxide semiconductors (AOSs) for transparent, flexible thin-film transistor (TFT) applications. The work function of a-IGZOs upon exposure to air immediately after vacuum annealing was monitored using Kelvin probe force microscopy (KPFM). An increase was observed in the work function with time. Additionally, X-ray photoelectron spectroscopy combined with the KPFM results revealed two competing factors responsible for the aging characteristics: adsorption of oxygen molecules on the nanopore surfaces within the films, and the creation of defects, including oxygen vacancies and zinc interstitials. Although the former was reversible by alteration of ambient conditions, the latter was irreversible. On the basis of modified band theory, we proposed an operative mechanism for solution-processed AOS TFTs based on both the nature of porous amorphous structures and the aging dynamics. The perspectives reported here may be useful in designing and fabricating advanced, flexible AOS TFTs for device operations in stable ambient conditions.

We generated a novel amorphous oxide semiconductor thin film transistor (AOS-TFT) that has exellent bias-stress stability using solution-processed gallium tin zinc oxide (GSZO) layers as the channel. The cause of the resulting stable operation against the gate bias-stress was studied by comparing the TFT characteristics of the GSZO layer with a tin-doped ZnO (ZTO) layer that lacks gallium. By photoluminescence, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, we found that the GSZO layer had a significantly lower oxygen vacancy, which act as trap sites, than did the ZTO film. The successful fabrication of a solution-processable GSZO layer reported here is the first step in realizing all-solution-processed transparent flexible transistors with air-stable, reproducible device characteristics.

Uniform-sized metal oxide nanocapsules, termed nanoampoules, have been fabricated. Hollow nanopipes of titania (TiO2) were first fabricated onto either a polymeric template or porous anodized aluminum oxides with monodisperse cylindrical holes by using atomic layer deposition (ALD) techniques. The inner volume of individual nanopipes can be tailored precisely by controlling the thickness of layers deposited with ALD. This encapsulation strategy should have potential for application in ideal drug delivery systems, gene therapy, cosmetics, inks, chemical sensors or pigments.

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Uniform-sized metal oxide nanocapsules, termed nanoampoules, have been fabricated. Hollow nanopipes of titania (TiO2) were first fabricated onto either a polymeric template or porous anodized aluminum oxides with monodisperse cylindrical holes by using atomic layer deposition (ALD) techniques. The inner volume of individual nanopipes can be tailored precisely by controlling the thickness of layers deposited with ALD. This encapsulation strategy should have potential for application in ideal drug delivery systems, gene therapy, cosmetics, inks, chemical sensors or pigments.