Symposium NT2 : Oxide and Chalcogenide-Based Thin Films and Nanostructures for Electronics and Energy Applications

Innovative routes for the fabrication of semiconductors that are both stable and able to efficiently convert solar energy to molecular Hydrogen under the conditions required for the safe and operation of a solar-fuel generator will be imperative to the development of the next generation of such devices. Layered Transition-Metal Dichalcogenides (TMDCs) are suitable candidates for efficient and stable photoelectrosynthetic hydrogen generation in acidic and alkaline media, with inherent stability in harsh electrochemical environments. Furthermore, TMDCs are capable of dual functionality in such devices, having demonstrated both efficient photoconversion and electrocatalysis. Tungsten diselenide (WSe₂) is an attractive TMDC material for this purpose, owning to its high light absorption coeffiecnt, 1.2 eV band gap and anisotropic properties. We investigate fundamental insights into the factors that determine the photoelectrical properties of WSe₂. The present work focuses on nano- topographical, electrical and mechanical studies of the WSe₂ in the absence and presence of catalysts, including the density of edge sites, surface potential profile, local current mapping and the nano lithography. These studies were performed mostly based on the PeakForce-based tapping AFM mode. Our studies show that the surface work functions and stiffness of the edge sites are different from the basal plan. The edge sites are also more conductive while showing inhomogeneous conductivity. Hence, WSe₂ displays distinct chemical and physical differences when terrace and edge sites are compared. Therefore, performance optimization of the WSe₂ photoelectrode requires the control of both density and electrical/chemical properties of the edge sites.

Acknowledgement: This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993. Research was in part carried out at the Molecular Materials Research Center of the Beckman Institute of the California Institute of Technology.

Reference:
[1] Velazquez et. al. “A Scanning Probe Investigation of the Role of Surface Motifs in the Behavior of p-WSe₂ Photocathodes”, Energy & Environmental Science, 2015, (Accepted)

Semiconductor oxide nanosheets derived from layered metal oxides have attracted attention in various fields due to their unique structural properties. The anisotropic feature of nanosheets, which have a thickness of ~1 nm and lateral dimensions ranging from several hundred nanometers to a few micrometer, seems to be advantageous for heterogeneous photocatalysts, as the diffusion length of photogenerated electrons and holes to the surface is shortened, resulting in higher activity.
For application in photocatalysis, the composition of a metal oxide determines the band structure of the material, which has a strong impact on photocatalytic activity, as the reactivity of electrons and holes for surface redox reactions is determined by the band-edge potentials. Therefore, precise control of energy band structure is essential to designing a highly efficient photocatalyst. To date, however, such band-structure-controlled nanosheets have not been devised, and how the band gap structure affect photocatalytic activity remains little explored. We prepared perovskite nanosheets of HCa_2–xSr_xNb_3–yTa_yO₁₀, and found that the conduction band-edge potentials were successfully controlled by cationic substitution. These nanosheets functioned as highly efficient heterogeneous photocatalysts for H₂ evolution from an aqueous methanol solution under band-gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa₂Nb₂TaO₁₀ nanosheets, giving an apparent quantum yield (AQY) of ~80% at 300 nm. This is the highest value among nanosheet-based photocatalysts reported so far.
It is also known that certain nanoparticulate metals or metal oxides on a semiconductor photocatalyst work as cocatalysts to promote water reduction and/or oxidation. In heterogeneous photocatalysis, the effect of cocatalyst size on the water-splitting performance had not been examined at sizes smaller than 1 nm due to the lack of an effective preparation method and a suitable photocatalyst. We have very recently demonstrated that metal nanoclusters (such as Pt) of <1 nm in size could be deposited on the interlayer nanospace of KCa₂Nb₃O₁₀ using the electrostatic attraction between a cationic metal complex and a negatively charged Ca₂Nb₃O₁₀^– sheet, without the aid of any additional reagent. The material obtained exhibited 8 times greater photocatalytic activity for overall water splitting under band-gap irradiation than the previously reported analog using a RuO₂ promoter. This study highlighted the superior functionality of < 1 nm Pt nanoclusters for photocatalytic overall water splitting

Currently, hydrocarbon reformation accounts for 96% of commercially available hydrogen, usually with high concentrations of carbon-rich molecules. This poses a problem for achieving carbon neutrality in the plethora of hydrogen-dependent processes. Transition metal chalcogenides (TMCs) (e.g., MoS₂, WS₂, and WSe₂) have been investigated as earth abundant catalyst for hydrogen gas evolution to replace precious Pt-based catalyst.
In this work, hierarchically core-shell MoO₂-MoS₂ nanofibers with controlled morphology and composition were fabricated by combining various fabrication techniques including electrospinning, rapid thermal annealing, and CVD sulfurization to enhance HER properties. Design of Experiment (DOE) and dimensionless analysis will be utilized during electrospinning to optimize nanofiber diameter and morphology utilizing physical properties of both precursor solutions and electrospinning parameters. Several complementary techniques, including FE-SEM, HR-TEM, XRD, nitrogen physisorption isotherm and XPS were used to systemically evaluate the material and physical properties of the nanofibers. Lastly, the HER experiments were conducted under different pH (0, 1, 3 and 5) conditions to determine surface overpotential and Tafel slope.

Nanocrystalline zinc oxide thin films deposited on glass substrates by sputtering technique were subjected to γ- irradiation with an equivalent intensity of 1.404 kG/hour for extended periods of time (40 to 160 min). The irradiated samples were characterized to understand the changes in structural, morphological, optical and electrical properties. XRD patterns showed that the intensity of orientation peak (002) enhanced with the increase of gamma irradiation doses and the grain size increased marginally. The micro strain (ε) and dislocation density (δ) values decreased. The bond length (L) as well as the texture coefficient TC (hkl) values exhibited an increasing trend signifying plenty of grains in a given (hkl) direction. AFM studies displayed spherical nature of the crystallites and showed an increase in their size with the increase of irradiation doses. As the gamma irradiation dose increased, the transmittance decreased in the visible range. In addition, the transmittance curve showed creeping behavior towards the red wavelength indicating the decrease in energy gap. These results corroborated well with the XRD as well as AFM studies. We noticed that resistance decreased substantially when measured just after irradiation. Aging effect was distinct which showed gradual increasing trend. The results have been explained keeping in view of employing the tailored films with gamma irradiation for possible optoelectronic applications.

SiO₂ is the most widely used dielectric material in the semiconductor industry and the continuous miniaturization of features requires ultrathin SiO₂ films which are difficult to produce by traditional fabrication techniques such as chemical vapor deposition or thermal oxidation. Atomic layer deposition (ALD) is capable of accurately controlling the thickness of thin films at the atomic scale. However, ALD SiO₂ growth usually requires catalysts, high temperatures, and/or a large precursor flux. Recently, aminosilanes were demonstrated to be promising precursors for low temperature SiO₂ growth which was free of catalysts or corrosive by-products. The amino ligand catalyzes the reaction, so is described as a “self-catalysed” reaction. We have examined a new group of volatile aminosilanes, cyclic azasilanes, to grow thin SiO₂ through ring-opening reaction under a wide range of temperatures. The unstable Si-N bonding makes this class of molecules chemically reactive with hydroxyl surfaces to form a monolayer. Subsequent oxidation with O₃ afford silanol groups, which are amenable to further reaction with cyclic azasilanes. The growth rates obtained are 0.6-1.2 Å/cycle for various silanes under different ALD conditions, due to side chain structure variation of silane precursors. This offers a novel group of chemicals for the preparation of ALD SiO₂ and also the interfacial functionality, enables detailed study of the adsorption of silanes and surface oxidation mechanism, and may be helpful for the design of ALD precursors for SiO₂ and hybrid molecular layer deposited films.

With the ever increasing demands on energy and environmental protection, there is an urgent need to develop multifunctional, high performance and durable materials for energy production and energy storage applications. Hydrogen generated from water splitting is an alternative and renewable energy source, and presently, platinum is one of the most effective catalyst for its generation. However, its wide application is limited due to its high cost and it is essential to develop low-cost and earth-abundant materials to replace precious-platinum based catalysts. Recently, chalcogenides have attracted considerable attention for these purposes where molybdenum disulfide shows some promise for hydrogen production but its charge storage capacity is low compared to metal oxides. In this work, we have developed a facile method for the synthesis of nanostructured cobalt sulfides which were then tested for supercapacitors as well as electro-catalyst for hydrogen generation, and their potential application for supercapacitors using cyclic voltammeter and galvanostatic charge discharge method. The maximum specific capacitance of 335 F/g was observed in 3 M NaOH electrolyte. A quasi-solid state supercapacitor device was fabricated by sandwiching two electrodes separated by an ion transporting layer, and the effect of temperature on the charge storage properties of the device was investigated for high temperature applications. The specific capacitance improved when operating temperature was raised from 10 to 70 ^oC. In addition to this, cobalt sulfides also showed a promising behavior as electro-catalyst for hydrogen generation, and their catalytic activity was observed to be significantly higher than that of molybdenum disulfide. Our results clearly suggest that these cobalt sulfides could be used as bifunctional material for energy generation and storage.

Solar-driven electrocatalytic water splitting holds great potential as a sustainable way of storing energy via the production of hydrogen (H₂) and oxygen (O₂), which can be subsequently recombined in a fuel cell with generation of energy on demand. Currently Pt-based electrodes are the most active hydrogen evolution reaction (HER) catalysts with negligible overpotentials and high stability. The main drawback is due by the high cost of the Pt which limits its utilization on industrial scale.¹ During the past 10 years numerous efforts have been devoted to the identification of active, stable and low-cost HER electrocatalysts able to efficiently work under harsh (acidic and/or basic) and/or neutral electrolytes. Particularly, first-row transition metal sulfides such as CoS₂, NiS₂, FeS₂, have recently emerged as a promising class of active HER electrocatalysts.² Among the above-mentioned compounds, CoS₂-based electrodes have proved to be extremely interesting HER catalysts because of their metal-like electrical conductivity, chemical stability in acid and base and low cost. In this work, a novel CoS₂-Ni₃S₂-Ni₁₇S₁₈/Ni foam composite material was synthesized through a simple method based on the thermal decomposition of a cobalt-thiourea molecular precursor onto the 3D metallic support. The obtained electrode exhibited outstanding activity toward the hydrogen evolution reaction, requiring small overpotentials of 152 mV and 183 mV at a current density of 10 mA cm^-2 in acidic (0.5 M H₂SO₄) and basic (1 M KOH) media, respectively. Remarkably the activity under alkaline medium is the highest ever reported among the CoS₂-based HER catalysts. The determination of the Tafel slopes gave insights into the mechanism involved during the HER, suggesting a Heyrovsky rate-determining step (RDS) in acidic and a Volmer-Heyrovsky RDS in basic electrolyte. The material was characterized through X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), linear sweep voltammetry (LSV), electrochemical impedence spectroscopy (EIS) and chronoamperometry analysis (CA). The origin of its high activity was mainly attributed to the low electrical resistance and the high surface area of the composite electrode as shown by EIS analysis, with efficient and rapid electron injection from the metallic Ni foam to the highly dispersed CoS₂ active sites. Indeed, the in-situ thermal generation of CoS₂ nanoparticles onto the Ni foam plays a fundamental role in having a low electrical resistance as a result of the strong binding between the two phases. A detailed characterization analysis was also performed on samples subjected to chronoamperometry tests in acidic and basic environments, evidencing the occurrence of some minor structural modifications on the electrode.


1. Q.Lu et al., Nature Communications, 2015, 6, 6567
2. D. Kong et al., Energy Environ. Sci., 2013, 6, 3553-3558.

Transition metal chalcogenides has been a heartthrob of solid state chemists owing to the richness of properties they exhibit as well as their structure-property correlation. In addition to their viable optoelectronic properties, it has been recently observed that some of these transition metal chalcogenide (especially selenides) show high electrocatalytic activities for oxygen evolution/reduction reactions (OER/ORR respectively) and hydrogen evolution reaction (HER). Recently we have formulated some new transition metal chalcogenide based compositions which show an enhanced catalytic activity for OER based on their d-electron occupancy and structural parameters. Among these the Ni-based selenide thin films and nanostructures show much better catalyst activity along with lower onset potential for O₂ evolution outperforming the state-of-the-art precious metal based catalysts. It was also observed that doping other transitional metals in the Nickel selenide matrix led to enhancing catalyst efficiency. The catalysts were synthesized mainly through electrodeposition on different substrates like Au-coated glass, Au-coated Si as well as glassy carbon (GC). The nanostructured catalysts were made through chemical vapor deposition (CVD) and solvothermal reaction. In a separate approach nanorod and nanotube arrays of these electrocatalysts were grown through electrodeposition on lithographically patterned nanoelectrodes. Nanostructuring typically led to much better efficiency with low catalyst volume, with the nanorod/nanotube pattern producing the highest activity with lowest electrode coverage with the active material. The catalytic activities for OER, ORR and HER were investigated through detailed electrochemical measurements including linear scale voltammetry, chronoamperometry, Tafel slope analysis as well as determination of Faradaic efficiency through rotating ring disk electrode studies. Interestingly some of these selenides showed bifunctional or trifunctional nature showing efficient catalytic activity for OER-HER and OER-HER-ORR processes, respectively. In this talk we will present a systematic study of the selenide based electrocatalysts including binary (Ni_xSe_y, Co_xSe_y) as well as ternary chalcogenides, Ni_xM_ySe_n (M = Fe, Co, Mn, Al) and investigating their catalytic activities for OER, HER, and ORR reactions. We will also discuss the effect of d-electron counts, electronic structure and morphology on the catalytic efficiency.

A process of growing large-area plasmonic-nano-particles-decorated ZnO nanorods on the polycarbonate optical disk substrate was developed, while a corresponding photocatalytic rotational reactor was fabricated. Hydrothermal process was adapted to grow ZnO nanorods perpendicular to the optical disk substrate at relatively lower temperature. The optical disk substrate has advantages of durable property in fast rotation and high impact-resistance. The plasmonic nano-particles, in this case, silver nano-particles, were deposed on the ZnO nanorods by direct sputtering. The morphology of ZnO nanorods and plasmonic nano-particles was investigated by Scanning Electron Microscope (SEM).The photocatalytic activity of the sample was evaluated by the degradation of methyl orange (MO) as a model compound in aqueous solution, and the decomposition rate of MO molecules is monitored by the optical spectroscopy measurements. In the optimized condition, less than 10% of the MO remained in the aqueous solution after a 20-minute treatment in the rotational reactor with our sample.

The search for high-efficiency and environmentally benign water splitting catalysts has been on the rise since this process is a source of renewable, clean energy. However the process is inherently slow, especially for the production of O₂ from H₂O (water oxidation) due to the high electron count and energy intensive bond formation. Hence the search for novel OER catalyst has led researchers to focus on various families of compounds including oxides and recently selenides. Multifunctional nanostructures containing the semiconductor electrocatalyst grafted onto an optically active component might boost the catalytic activity even further due to efficient charge injection. Magnetically active catalysts will also be lucrative since that might induce better adhesion of the oxygenated species at the catalytically active site. In this presentation we introduce multifunctional, magnetic Au₃Pd–CoSe nanostructures as high-efficiency OER electrocatalysts. These multifunctional nanostructures were synthesized in a horizontal tube furnace by a chemical vapor deposition (CVD) reaction with cobalt acetylacetonate and elemental selenium on Au-Pd sputter coated silica substrate at 800°C. These multifunctional nanostructures were further characterized by powder X-ray diffraction, scanning and transmission electron microscopy. The morphology was mostly bifunctional Janus-like nanoparticles, which showed soft ferromagnetic behavior. These bifunctional nanoparticles were coated on the anodes of a water oxidation cell and it was observed that these nanoparticles showed a higher OER activity with lower onset potential for O₂ evolution as compared to the conventional oxide-based OER electrocatalysts. In this presentation details about the catalytic activity studied through electrochemical characterization, mechanistic details along with synthesis and other properties will be discussed.

Insulating silicon dioxide (SiO₂) films can be produced by hydrolysis of metal alkoxide precursors using tetraethylorthosilicate (TEOS) in the presence of an acid catalyst in supercritical fluid carbon dioxide (sc-CO₂). In this study, silicon dioxide films are formed on different substrates using TEOS as a source of silicon, and acetic acid (HAc) as a catalyst. The chemical equation of the SiO₂ film formation can be expressed as Si(OCH₂CH₃)₄ + 2H₂O -- SiO₂ + 4CH₃CH₂OH. Water required for the hydrolysis reactions is from in situ generation of esterification and condensation reactions involving HAc and the alcohol produced. Without the catalyst, the hydrolysis of TEOS proceeds very slowly. The amount of the acid catalyst requires careful control to avoid excess acidity that may damage the nanoparticle layers. The acid catalyzed deposition reaction actually starts at room temperature but produces decent films in sc-CO₂ at moderately high temperatures (e.g., 50 ^oC).

Supercritical fluid CO₂ is known to have near zero surface tension and provides an ideal medium for fabrication of SiO₂ films. Formation of SiO₂ films via hydrolysis reaction in sc-CO₂ is more rapid compared to the traditional hydrolysis reaction at room temperature. In general, metal alkoxide hydrolysis reactions carried out in a closed sc-CO₂ system is not affected by moisture in air compared with traditional open-air hydrolysis systems. Using sc-CO₂ as a reaction medium also eliminates undesirable organic solvents utilized in traditional alkoxide hydrolysis reactions

The attenuated total reflectance-Fourier Transform infrared (ATR-FTIR), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electron diffraction (ED) measurements were conducted to characterize the films. XRD and ED measurements demonstrated the SiO₂ films produced were amorphous. EDS, ATR-FTIR and XPS spectra showed elemental composition of the films formed on the substrate surfaces to be SiO₂. The effects of pressure, amount of TEOS, reaction time and the amount of catalyst on the film quality and thickness are discussed.

Vanadium dioxide (VO₂) has been studied most intensively due to its near-room-temperature phase transition as well as its high phase stability. It undergoes a first-order metal-insulator transition (MIT) around 68^oC from a high-temperature metallic phase to a low-temperature insulating phase, which is accompanied by a structural phase transition from a high-temperature tetragonal (Rutile, R) structure to a low-temperature monoclinic (M1) structure. The crystallographic orientation of a VO₂ thin film depends strongly on the orientation of the sapphire substrate. It can have an application for thermochromic smart window and thermal switching devices as well as other electronic and optical applications.
VO₂ thin films with the thickness of 120 nm were grown on c-, r-, a-, and m-plane sapphire as well as SiO₂/Si substrates under identical conditions by RF reactive sputtering deposition from a VO₂ target. The sputtering pressure was set at 6 mTorr with 10 sccm flow of O₂ gas. All samples were grown 500^oC, and subsequently annealed with 1 sccm of O₂ gas flow rate for 20 min.
The structural and morphological properties of all the VO₂ thin films were studied using X-ray diffraction (XRD) and scanning electron microscopy. The electrical and optical properties of all the VO₂ thin films were studied using four point probe measurement, Raman spectroscopy, and spectroscopic ellipsometry. The grain sizes of VO₂ films were different depending on the substrate orientations. The MIT temperature (T_MI) varied with substrate orientations. The (011)-oriented VO₂ films on the r-plane sapphire showed the lowest T_MI of about 48.7^oC, while the (020)-VO₂ films on the c-plane sapphire displayed the highest T_MI of about 64.0^oC. The (002)-VO₂ films on the m-plane sapphire and the (101)-VO₂ films on a-plane sapphire exhibited T_MI of about 52.7^oC and 48.7^oC, respectively. The VO₂ films grown on SiO₂/Si substrate showed (132) and (204) phases with T_MI = 59.8^oC. The observed variations of T_MI with substrate orientations were due to the change in grain size and/or lattice strain. Raman spectroscopy showed that the MIT transitions were accompanied with M1-to-M2 phase transition rather than the direct M1 to tetragonal transition, for VO₂ films on c-plane substrate.[1] Using spectroscopic ellipsometry, the anisotropic dielectric functions of the VO₂ thin films grown on the sapphire and SiO₂/Si substrates were obtained near T_MI, although only isotropic dielectric functions were reported so far in literature.[2] The effect of MIT on the anisotropic