Ultrathin Metal Oxide Research Articles

Composite membranes with ultrathin and conformal passivation for universal microfiltration compatible with organic solvents

The development of porous membranes with excellent solvent resistances is an important technical task in relation to environmental and energy issues, including chemical waste treatment in the semiconductor industry. However, common polymer membranes, except for Teflon-based membranes, do not guarantee universal resistance to various organic solvents. In this study, a universal strategy is proposed to improve the solvent resistances of polymeric porous membranes through conformal passivation of ultrathin metal oxide layers via atomic layer deposition. Conformal ZnO shells with a thickness below ∼62 nm can significantly improve the inherently low resistances of nitrocellulose, polyethersulfone, and polycarbonate membranes to organic solvents without significantly degrading the flux. Although the polymer core is gradually dissolved by the organic solvent penetrating through the native cracks of the ZnO shell as the time of exposure to the organic solvent increases, the rigid porous network composed of hollow ZnO can well maintain its monolithic form for more than 1 week. Thus, it is possible to effectively filter pollen grains and heavy metal microparticles dispersed in harsh organic solvents without expensive Teflon-based membranes. This study opens new opportunities for selection of organic solvent-compatible membranes for microfiltration.

Al alloy protection via ultra-thin ceramic coatings and different surface pretreatments

The efficiency of ultra-thin corrosion protection coatings for susceptible to local corrosion AA2024-T3 is demonstrated. For this purpose, with atomic layer deposition (ALD) 50 nm thick TiO2, Al2O3, an Al2O3-TiO2 mixture and Al2O3/TiO2 nanolaminate coatings are grown on AA2024-T3 substrates, receiving different pretreatments prior to ALD coating. One pretreatment involves polishing and cleaning with organic solvents, and the second utilizes an additional low-potential electrochemical treatment in sulfuric acid. The samples are characterized with scanning electron microscopy, scanning transmission electron microscopy, wavelength-dispersive X-ray fluorescence spectroscopy, X-ray diffraction and reflection, and nanoindentation and are tested using linear sweep voltammetry, electrochemical impedance spectroscopy, and immersion in a salt solution. The study shows that ultrathin metal oxide coatings applied via ALD can significantly enhance the corrosion resistance of Al alloys, and the performance depends on the pretreatment of the alloy, as well as on the material/structure of the coating. The best ultrathin coating with less than 100 nm thickness withstood well 1000 h immersion test.

Ultrathin 2D nanosheets of transition metal (hydro)oxides as prospective materials for energy storage devices: A short review

The ultrathin two-dimensional (2D) transition metal oxides and hydroxides (TMO and TMH) nanosheets are attractive for creating high-performance energy storage devices due to a set of unique physical and chemical properties. Flat 2D structure of such materials provides a sufficient number of active adsorption centers, and the ultra-small thickness, on the order of several nanometers, provides fast charge transfer, which significantly improves electronic conductivity. This brief review summarizes recent progress in the synthesis of materials based on ultrathin 2D nanosheets for energy storage applications, including pseudocapacitors, lithium-ion batteries, and other rechargeable devices. The review also presents examples of representative work on the synthesis of ultrathin 2D nanomaterials based on TMO and TMH for various power sources. In conclusion, the article discusses possible prospects and directions for further development of methods and routes for the synthesis of ultrathin two-dimensional transition metal oxides and hydroxides.

Growing 3D-nanostructured carbon allotropes from CO2 at room temperature under the dynamic CO2 electrochemical reduction environment

Synthesis of nanostructured carbon allotropes using CO2 as a cheap carbon source is challenging and usually limited by high temperature activation. Herein, formation of 3D nanostructured carbon allotropes including nano-graphene and single crystalline nanodiamond films is feasible at room temperature on various single crystal metals (e.g., Bi, Ag, Zn, and Co) that were formed under the dynamic CO2 electrochemical reduction reaction environment at relatively low applied potential (−1.1 to −1.6 V vs. Ag/AgCl). The nanocrystalline carbon was obtained as the major products from CO2 (≥96% selectivity, ∼1 μm thickness) without any liquid products. Upon applying the negative potential, nanoclustering of the self-limiting ultrathin metal oxide layers of metal particles on the highly conductive substrate could lead to formation of negatively charged metal clusters, which were well stabilized by the ternary electrolyte system containing [BMIm]+[BF4]-/propylene carbonate/water. This system allows the reduction of CO2 into single atoms C∗ and the subsequently electrocrystallization of C∗ into carbon allotropes on the crystallographic planes of the single crystal metals formed as the building blocks. The CO2-derived Ag–C/epoxy composites show promising thermal conductivity. The results present a breakthrough advancement in the growth of nanostructured carbon allotropes from CO2 by the viable negative CO2 emission approach.

Tunable atomic level surface functionalization of a multi-layered graphene oxide membrane to break the permeability-selectivity trade-off in salt removal of brackish water

To enhance graphene oxide (GO) membrane performance, atomic level surface functionalization was applied via plasma-enhanced atomic layer deposition (ALD) to a GO membrane. Unlike conventional ALD approaches, to functionalize the surface without increasing the membrane thickness, we conducted only a few (3–9) ALD cycles, which allowed for the formation of a tunable ultra-thin (1.44 nm) and uniform hydrophilic metal oxide (Al2O3) layer on the multi-layered membrane. The ALD-treated GO membrane exhibited enhanced water permeability (from 32.9 to 68.0 LMH/bar) and NaCl rejection (from 46.6 up to 63.8%), successfully overcoming the typical trade-off between permeability and rejection efficiency in briskish water desalination. The formed atomic level Al2O3 layer was characterized via Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and d-spacing measurements. The results revealed that this enhanced water permeability can be mainly attributed to the increase in the surface hydrophilicity achieved without narrowing the GO membrane nanochannel structure (dry state 7.5 ~ 7.7 Å). Moreover, healed defects on the two-dimensional GO and improved electrostatic interaction (induced by the ALD treatment) resulted in improved salt rejection. Therefore, the distinctive features of ALD-treated GO membrane may contribute to enhancing its application in brackish water desalination.

Flexible and Ultrathin Metal-Oxide Films for Multiresonance-Based Sensors in Plastic Optical Fibers

We have exploited a laser-based integration process of ultrathin metal-oxide (MO) films to improve the plasmonic effect in sensors based on D-shaped plastic optical fibers (POFs). More specifically, using ultrathin MO films, the performances of the surface plasmon resonance (SPR) phenomenon improve and a lossy mode resonance (LMR) can occur. Although the role of this kind of materials has been already presented, when they are deposited as overlayer (upside the thin metal film), we have used a different approach by depositing MOs, especially zirconium oxide (ZrO2) and titanium oxide (TiO2), as flexible intermediate layers between the exposed core of POFs and the gold film. The MO layer is prepared from sol–gel solution, and deep-UV laser curing allows us to densify the thin film and tune the refractive index, with a room-temperature process fully compatible with the flexible polymer substrates. In a preliminary step, we have carried out numerical results, based on transfer matrix formalism, to predict the SPR response. Subsequently, we have experimentally characterized the developed sensor configurations. Numerical and experimental results have shown above all an enhancement of the sensor performances, in terms of SPR sensitivity, with respect to a reference sensor based on a polymer instead of MOs. Moreover, in some proposed sensor configurations, together with the SPR phenomenon, an LMR phenomenon was observed. It occurred in a different wavelength range, for a typical refractive index range present when considering receptors for biochemical sensing applications. Therefore, both resonances (SPR and LMR) could be used in several application fields.

Multifunctional Metal‐oxide Integrated Monolayer Graphene Heterostructures for Planar, Flexible, and Skin‐mountable Device Applications

The adoption of nanostructured metal-oxides integrated graphene monolayers-based heterostructures appears to be a promising approach for enhancing the performance of various devices. However, precisely controlled growth of such unique heterostructures without disturbing the monolayer graphene characteristics remains a challenging task especially over a large area with good uniformity. Herein, ultrathin metal-oxide (p-Co 3 O 4 and n-ZnO) nanostructures (MONSs) integrated graphene monolayer (GML) heterostructures are carefully developed by fascinating the graphene native defects while nucleation and growth of MONSs. Metal-oxides integrated graphene monolayers with lower material densities (≤ 30 μg/cm 2 ) significantly enhanced the quality (2D/G ~5–9) and reduced the electrical resistance (11–17 Ω/sq.) of graphene layers, whereas the heterostructures developed with higher densities possess predominant water-oxidation characteristics than that of their individual components. Further, the Co 3 O 4 /GML heterostructures-based micro-supercapacitors, fabricated over 25 µm polyimide sheets, showed excellent mechanical stability and flexibility with a volumetric and specific capacitance of 7.76 F/cm 3 and 1.27 F/g, respectively. The ZnO/GML heterostructures designed over micron thick parylene film displayed exciting photoresistor characteristics with photosensitivity of ~1.54 and superb flexibility and skin-mountability. Synergistic multifunctional characteristics of these ultrathin heterostructures offer the possibility to realize various eco-friendly ultrathin as well as skin-mountable energy and health monitoring devices. Ultrathin metal-oxide(s) precisely integrated single-layer graphene heterostructures, developed by low-temperature methods, exhibit high conductivity and multifunctional characteristics. As a catalyst, the heterostructures show excellent electrocatalytic performance, and whereas, the active electrodes defined over flexible and skin-mountable surfaces exhibit significant energy storage and photoresistive characteristics. • Ultrathin metal-oxides nanostructures integrated graphene monolayers developed. • The heterostructures grown under optimum conditions possess high conductivity. • These heterostructures exhibited synergistic OER electrocatalytic performance. • Supercapacitors on flexible sheets showed good mechanical stability and durability. • The structures developed on ultrathin sheets are skin-mountable and integratable.

Structural, electronic, and optical properties of two-dimensional hafnium monoxide nanosheets

Ultrathin two-dimensional transition metal oxides are highly desirable as promising materials for energy storage, gas sensors, photonic and electronic devices. In this work, we studied the structural stability, electronic structure and optical properties of two-dimensional hafnium monoxide with hexagonal and tetragonal lattice by the first principles calculations. Phonon dispersion reveals the lattice dynamic stability of hexagonal structures. Results of band structure showed that both hexagonal and tetragonal HfO nanosheets had highly anisotropic metallic behavior. The optical properties showed strong light absorption and anisotropy.

Instilling Monovalent Selectivity in Cation Exchange Membranes By Molecular Layer Deposition

Desalination is becoming a vital part of the solution to water scarcity with a current worldwide desalination capacity of approx. 100 million m3/day, which is expected to grow by 40% by 2030. In 2019, 29% of the global desalination capacity was of brackish and river water (500-20000 ppm TDS). Most (98%) of the brackish water nowadays is desalinated by reverse osmosis (RO) which completely deprives the permeate of Mg2+ and Ca2+. Both are critical nutrients needed to sustain human health and agriculture. Replenishing these divalent ions to the desalinated water increases the cost and complexity of the overall process and decreases its sustainability and economic attractiveness. Electrodialysis is energetically more efficient than RO for desalination of brackish water, and using monovalent selective membranes—membranes that enable monovalent ions passage, but retains divalent ions—offer a potential solution to preserve the nutrients in the permeate. However, current methods for instilling ion selectivity uses large macromolecular building blocks to form the selective layer, which increases ion transport resistance and hence the energy demand and cost of selective electrodialysis. To disrupt the resistance-selectivity trade-off, a method for uniformly depositing an ultrathin and highly charged selective layer is needed.While low-temperature atomic layer deposition is a suitable method for depositing ultrathin metal oxides on membranes, the elastic mismatch between metal oxides and polymers hinders the stability of the selective layer. Instead, we used molecular layer deposition of alucones to resolve the elastic mismatch. We show that the electrostatic charge of alucone surface leads to monovalent selectivity values on par with commercial membranes, but with only a marginal increase in resistivity.

Solution processable transition metal oxide ultra-thin films as alternative electron transport and hole blocking layers in dye sensitized solar cells

An electron transporting and hole-blocking layer (ETL) between the fluorine-doped tin oxide (FTO) and mesoporous titanium dioxide interface of photoanode is essential in dye-sensitized solar cells (DSCs) for improving their photovoltaic performance. In the present work, we have examined the potential for further improving device performance by using alternative materials for ETLs. Accordingly, the effect of replacing TiO2 based ETLs with spin coated ultra-thin transition metal oxides (TMOs), molybdenum trioxide (MoO3) and tungsten trioxide (WO3) films has been studied. TMO films of 10 nm thickness have been found to be optimal for obtaining good transparency and charge transport properties, without compromising device performance. Further, devices fabricated with surfactant assisted (SA-) MoO3 ETLs have shown efficiency ∼4%, which is comparable with devices based on conventionally prepared TiO2 ETLs. In addition, a significant improvement in efficiency (15 %) has been observed for DSCs prepared using SA-WO3 films. The overall improvement in device efficiency is attributed to the increase in short-circuit current density without loss of open circuit potential and fill factor. Detailed analyses of results suggest that in addition to surface morphology, transparency and charge transport properties; oxygen vacancies in sub-stoichiometric TMOs also plays an important role in determining the device performance. The findings reveal that TMOs have significant potential to replace TiO2 ETL in DSCs and other third generation solar cell, viz. perovskite and polymer solar cells.

Feasibility of a Full Chalcopyrite Tandem Solar Cell: A Quantitative Numerical Approach

The potential of tandem solar cells combining two chalcopyrite absorbers is evaluated using numerical modeling based on an exhaustive set of experimental parameters, offering a high degree of confidence in the numerical values reported herein. The simple yet reliable approach used here combines a transfer matrix‐based optical model of the wide bandgap CIGSe top subcell used as input for the 1D electrical modeling of a reference narrow bandgap CIGSe bottom cell. Various optical optimizations to the top subcell are investigated, with the aim to increase the bottom subcell current and reduce the efficiency threshold needed at the top subcell for the tandem device to beat the current single junction efficiency record. The results here suggest that significant progress compared with the state of the art can be made using a pure CuGaSe2 absorber combined with an optimized back contact with an ultrathin transition metal oxide interlayer. With a bottom subcell current more than doubled in the optimum top subcell configuration, a challenging yet clear pathway for the future realization of tandem solar cells based on chalcopyrite absorbers is offered.

Ultrathin ternary metal oxide Bi2MoO6 nanosheets for high performance asymmetric supercapacitor and gas sensor applications

Two dimensional (2-D), unstacked Bi2MoO6 nanosheets are directly grown on nickel foam and prepared as powder via easy, template-free hydrothermal route. The prepared material was systematically characterized using techniques viz XRD, Cs-TEM, HRTEM, FESEM, XPS, elemental mapping, EDS. Bi2MoO6 nanosheets grown on Nickel foam employed for supercapacitor application, whereas powder form was utilized to fabricate ethanol sensor. The electrochemical characteristics viz cyclic voltammetry (CV), galvanic charging-discharging (GCD), and electrochemical impedance spectroscopy (EIS) were investigated for Bi2MoO6 nanosheets grown nickel foam in 1 M KOH aqueous solution within −0.2 to 0.8 V potential window. The areal capacitance of 655.5 mF/cm2 was achieved at 1 mA/cm2 current density with the energy density of 22.76 × 10−3 Wh/cm2 and 347.18 × 10−3 W/cm2 power density. The 5000 charge–discharge cycles were performed with excellent capacitance retention. The Bi2MoO6 sensor was tested for 10–100 ppm ethanol. A high response of 82% was noted for 100 ppm ethanol concentration at 275 °C. The dynamic resistance response, selectivity, and sensor’s stability were tested and analysed.

Alternately aligned 2D heterostructures enabled by d-spacing accessible, highly periodic accordion-like graphene oxide frameworks

Two-dimensional (2D) heterostructures hold great promise in designing integrated materials, while the current synthesis strategies still confront challenges for multilayer heterostructure construction and scale-up production. Here we report a generalized host-guest strategy based on non-exfoliated layered graphene oxide (LGO) to construct graphene-based heterostructures that consist of multilayered, alternately aligned graphene and metal oxide nanosheets. The 2D-aligned GOs and open interlayer spaces make LGO an ideal platform to create periodic 2D host frameworks. Polyetheramine oligomers covalently bond the adjacent GOs. The extended chain conformation endows the resulting accordionlike GO frameworks with high structural stability, periodicity and enlarged interlayer space. Owing to the high affinity of the open and well-arranged 2D channels toward guest precursors, a variety of high-quality heterostructures can be synthesized. Furthermore, a variety of exfoliated, ultrathin metal oxide nanosheets can also be prepared by removing the graphene skeleton. The flexible interlayer chemistry presented in this study paves a way toward the synthesis of a large family of graphene-inorganic/organic 2D heterostructures.

Synthesis of ultrathin metal oxide and hydroxide nanosheets using formamide in water at room temperature

Here, 6 different ultrathin (<5 nm) 2D metal oxides and hydroxides have been successfully synthesized via a simple precipitation route in formamide aqueous solution at room temperature.

Nanoengineered Organic Electrodes for Highly Durable and Ultrafast Cycling of Organic Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have become increasingly important as next-generation energy storage systems for application in large-scale energy storage. It is very crucial to develop an eco-friendly and green SIB technique with superior performance for sustainable future use. Replacing the conventional inorganic electrode materials with green and safe organic electrodes will be a promising approach. However, the poor electrochemical kinetics, unstable electrode-electrolyte interface, high solubility of the electrodes in the electrolyte, and large amount of conductive carbon present great challenges for organic SIBs. In this study, the issues of organic electrodes are addressed through atomic-level manipulation of these organic molecules using a series of ultrathin (Å-level) metal oxide coatings (Al2 O3 , ZnO, and TiO2 ). Uniform and precise coatings on the perylene-3,4,9,10-tetracarboxylicacid dianhydride by gas-phase atomic layer deposition technique shows a stable interphase, enhanced electrochemical kinetics (71C, 10 A g-1 ), and excellent stability (89%-500 cycles) compared to conventional organic electrode (70%-200 cycles). Further studies reveal that the chemical stability of the metal oxide coating layer plays a critical role in influencing the redox behavior, and improving kinetics of organic electrodes. This study opens a new avenue for developing high-energy organic SIBs with performance equivalent to inorganic counterparts.

Enhanced electrical properties of ZrO2-TiN based capacitors by introducing ultrathin metal oxides

Owing to the scaling down of dynamic random-access-memory, the development of new high-k dielectrics as well as the reduction in the equivalent-oxide-thickness value using ZrO2 and TiN electrode-based capacitors have become crucial. Because the unwanted interfacial layer between ZrO2 and TiN electrodes can induce the degradation of electrical properties, we propose a new approach for improving capacitor properties by introducing ultrathin metal oxides as the buffers. Each ultrathin TiO2, Ta2O5, and ZnO is inserted between ZrO2 thin films and the top or bottom electrodes, and the variations in their electrical properties are investigated. We discovered that the electrical properties of ZrO2-based capacitors, such as the dielectric constant and leakage current density, can be improved by introducing certain types of buffers without requiring a noble metal electrode and higher-k dielectrics. Furthermore, we discovered that the EOT scaling of approximately 0.15 nm is achievable only through the introduction of the appropriate buffer.

Ultrathin Ion-Sensitive Field-Effect Transistor Chips with Bending-Induced Performance Enhancement.

Flexible multifunctional sensors on skin or wearables are considered highly suitable for next-generation noninvasive health care devices. In this regard, the field-effect transistor (FET)-based chemical sensors such as ion-sensitive FETs (ISFETs) are attractive as, with the ultrathin complementary metal oxide semiconductor technology, they can enable a flexible or bendable sensor system. However, the bending-related stress or strain could change the output of devices on ultrathin chips (UTCs), and this has been argued as a major challenge hindering the advancement and use of this technology in applications such as wearables. This may not be always true, as with drift-free ISFETs, we show that bending could also enhance the performance of UTCs. Through fine control of bending radius in the micrometer scale, the mechanically flexible RuO2-based ISFETs on UTCs (44.76 μm thickness) are shown to reproducibly enhance the performance even after 1000 bending cycles. The 1.3 orders of magnitude improved stability (the drift rate changed from −557 nA/min to −28 ± 0.16 nA/min) is observed over a time period of 417.3 s (∼7 min) at fixed biasing and temperature conditions and under different pH conditions. Finally, a compact macromodel is developed to capture the bending-induced improvements in flexible ISFETs. The performance enhancement by controlled bending of devices could generally benefit the rapidly growing field of flexible electronics.

Self-Metalation of Porphyrins by Cobalt Oxide: Photoemission Spectroscopic Investigation

Understanding the electronic structure between porphyrins and ultrathin metal-oxide film interfaces is vital for the rational design of functional molecular architectures. Here, we have studied the...

Importance of Bi-O Bonds at the Cs2AgBiBr6 Double-Perovskite/Substrate Interface for Crystal Quality and Photoelectric Performance.

Interface interactions between perovskite materials and substrates are of great significance for the development of high-quality perovskite materials. Herein, we have successfully prepared Cs2AgBiBr6 double-perovskite films via a one-step spin-coating process and demonstrated a novel approach that modifies the surface of substrates with an ultrathin metal oxide (MOx) layer to promote the film quality and photoelectric performance. Characterization results strongly suggest that the improvement is attributed to the Bi-O interfacial interaction at substrate/perovskite interface. Benefiting from this interface interaction, the average grain size of Cs2AgBiBr6 films has remarkably risen up to ∼500 nm, which is nearly four times larger than the one directly deposited on a commercial fluorine-doped tin oxide substrate. Meanwhile, the pin hole surface area ratio has reduced from 2.61 to 0.60%. Furthermore, the corresponding photodetectors (PDs) have been fabricated and the performance has significantly improved owing to the enhanced Cs2AgBiBr6 film quality. The on-off ratio of the optimized PD has a boost of almost 10 times. In addition, the minimum detected irradiation has decreased from 9.7 × 10-8 to 1.9 × 10-9 W cm-2, as well as the maximum detectivity has increased from 3.3 × 1011 to 1.2 × 1013 Jones. These results suggest a feasible method for crystallization improvement of double-perovskite films and indicate promising promotion of photoelectric performance.

Thermal stability and reactivity of titanium halide precursors for the atomic layer deposition of TiO2 on a Pt (111) surface

The atomic layer deposition (ALD) of ultrathin metal-oxide film on metal surfaces is an effective technique for protecting supported metal catalyst against sintering, leaching and coking thereby improving catalyst stability and durability. Although numerous studies have demonstrated the advantages of TiO2 overcoats in catalysis, the reactivity and thermal stabilities of TiO2 ALD precursors on metal surfaces is still relatively unexplored. In this work, density functional theory (DFT) was employed to investigate the reactivity and thermal stability of titanium halide-based precursors with emphasis on homoleptic TiF4, TiCl4 and TiI4 precursors. In conjunction with DFT, the automated enumeration tool and the bond-ligand dissociation automation tool in Schrodinger material science suite were employed to investigate and compare the thermal stability and reactivity of the precursors. The surface reactivity of these precursors on a bare and hydroxylated Pt surface was also investigated. The results obtained from the simulations were compared and correlated with available literature. The results reveal that thermal stability of the precursors is represented as TiF4>TiCl4>TiI4 by whereas the opposite is true for the reactivity TiF4<TiCl4<TiI4. Moreover, the thermal stability of all heteroleptic precursors is dependent on the number of Ti-F bonds, while the reactivity depends on the number of Ti-I bonds. However, TiCl4 provides a better balance between the two extremes when taking both thermal stability and surface reactivity into consideration.