Nanoporous Films Research Articles

Guided Wrinkling of Hierarchically Structured Nanoporous Gold Films for Improved Surface‐Enhanced Raman Scattering Performance (Adv. Mater. Interfaces 18/2023)

Guided Wrinkling of Hierarchically Structured Nanoporous Gold Films for Improved Surface‐Enhanced Raman Scattering Performance (Adv. Mater. Interfaces 18/2023)

Guided Wrinkling of Hierarchically Structured Nanoporous Gold Films for Improved Surface‐Enhanced Raman Scattering Performance

AbstractPlasmonic nanostructured metals have many advantages for applications in high‐performance surface‐enhanced Raman scattering (SERS) spectroscopy. In particular, unique designing nanostructures with bicontinuous ligaments surrounded by cylindrical voids with tunable dense pores from a few to hundreds of nanometers can be utilized for the high‐performance SERS‐active substrate. Here, a fabrication strategy is reported to prepare hierarchically arranged micro/nanostructures of wrinkled nanoporous gold (WNPG) films, which involves laminating of the dealloyed Au film on the heat‐shrinkable shape‐memory polymer film and geometrical modulation of the substrate. As a result, the various types of WNPG films are crafted with a remarkable density of cracks in the structured surface area. Specifically, the WNPG films consisting of multilayered overlapping features are explored and used as the SERS‐active substrate. This dual porosity coupled with localized surface plasmon resonance estimated by numerical simulation in a suitable model of bicontinuous ligaments is found to be the core mechanism for the enhancement of SERS sensitivity, which quantitatively characterizes the “hot spots” from the surface to interlayers. These suggested characteristic features are fully assessed by applying a series of dye molecules and DNA strands on the prepared SERS substrate, demonstrating the enhanced intensity of the Raman scattering signals on the optimized WNPG surface.

Elucidating Mass Transport within Nanoporous Au for CO2 Electroreduction

The electrochemical reduction of CO2 to value-added chemicals renewable electricity is a promising and ecofriendly strategy to achieve the national strategic goal of “carbon peak and carbon neutrality” and solve the greenhouse effect. Due to the variety of products in CO2 electroreduction (CO2ER), catalytic selectivity has become a key factor in the design of electrode structure. Herein, a systematic investigation of CO2ER on the nanoporous gold films with different thicknesses prepared by the self-deposition method developed by ourselves. Mass transfer effects are found to play an important role in determining product selectivity and activity. The specific activity for CO evolution (jCO) with exponential declination has more dramatic tendency than the specific activity for hydrogen evolution (jH2) with linear decay with increasing nanoporous gold film thickness. Different from the behaviors within the mesoporous structures in previous studies, the retarded transport of HCO3− ions within the nanoscale pores is more sensitive than that of protons. This phenomenon implies the necessity of considering mass transfer effects in the design of outstanding electrocatalysts for CO2ER as well as for understanding the geometrical infrastructure-performance relationships.

Effect of Functional Nanoporous TiO2 Film Obtained on Ti6Al4V Implant Alloy to Improve Resistance in Biological Solution for Inflammatory Conditions.

The metallic titanium-based biomaterials are sensitive to corrosion-induced degradation in biological fluids in the presence of inflammatory conditions containing reactive oxygen species (ROS). Excess ROS induces oxidative modification of cellular macromolecules, inhibits protein function, and promotes cell death. In addition, ROS could promote implant degradation by accelerating the corrosive attack of biological fluids. The functional nanoporous titanium oxide film is obtained on titanium alloy to study the effect on implant reactivity in biological fluid with reactive oxygen species such as hydrogen peroxide, which are present in inflammations. The TiO2 nanoporous film is obtained by electrochemical oxidation at high potential. The untreated Ti6Al4V implant alloy and nanoporous titanium oxide film are comparatively evaluated for corrosion resistance in biological solution by Hank's and Hank's doped with hydrogen peroxide by electrochemical methods. The results showed that the presence of the anodic layer significantly improved the resistance of the titanium alloy to corrosion-induced degradation in biological solutions under inflammatory conditions.

Nanoporous thin films of hydrophobic block copolymers enabled by selective swelling for membrane distillation

Membrane distillation (MD) is receiving growing interest because of its >99.9% salt rejection even with low-grade waste heat as the energy supply. However, MD membranes are prone to be wetted and usually require complicated modification strategies to gain sufficient hydrophobicity. In this work, we report a new type of MD membranes with hydrophobic films carrying pores in the ultrafiltration range as the separation layers. Such membranes are prepared by coating an A-B-A triblock copolymer polystyrene-block-polydimethylsiloxane-block-polystyrene (PS-b-PDMS-b-PS, further abbreviated as SDS) on the macroporous PVDF substrate, followed by soaking in alkanes to cavitate the SDS coating layers following the mechanism of selective swelling. The PDMS microdomains in the SDS layers are converted to interconnected nanopores, and the PDMS blocks are enriched along the pore walls and membrane surface, endowing the membranes with intrinsic hydrophobicity. Thus-prepared SDS membranes exhibit stable rejection to NaCl (>99.99%) and high permeate flux up to 32.8 kg m−2 h−1, better than most MD membranes with large pores in the microfiltration range. Moreover, the SDS membranes display excellent wetting resistance, good heat resistance, and long-term stability. This work provides a new strategy to prepare MD membranes enabled by selective swelling of hydrophobic block copolymers, and demonstrates the great potential of hydrophobic ultrafiltration membranes in the long-term use in membrane distillation.

Nanoscale morphology tailoring in plasma deposited CN x layers

Magnetron discharge plasma was applied for the synthesis of CN x thin layers using methane and nitrogen gas precursors. The incorporation of nitrogen in the carbon network resulted in the dramatic evolution of growth morphology: from a ‘buried’ porous layer observed at low nitrogen incorporation to aligned bundles of nanorods grown perpendicular to the substrate surface at maximum discharge power and nitrogen flow. The films deposited at the low discharge power and high nitrogen incorporation exhibited a mesoporous sponge-like morphology after vacuum annealing. Relevant physical mechanisms responsible for the formation of nano- and mesoshaped morphology are discussed in terms of the effects of internal mechanical stresses and plasma etching. In addition, the sensing properties of the sponge-like layer were preliminarily examined in water vapor and ammonia ambients. The CN x films showed enhanced sensitivity to ammonia and reverse electrical response to moisture in comparison with a nitrogen-free nanoporous carbon film, which were assigned to modification of the electronic properties of the nitridated surface.

Self-Supporting Nanoporous Copper Film with High Porosity and Broadband Light Absorption for Efficient Solar Steam Generation

HighlightsSelf-supporting Cu film with high porosity was obtained by dealloying of Al98Cu2.Nanoporous Cu (NP-Cu) film shows good hydrophilicity and strong broadband light absorption.NP-Cu film exhibits outstanding solar steam generation and desalination performance.

Mechanistic Insights on Permeation of Water over Iron Cations in Nanoporous Silicon Oxide Films for Selective H2 and O2 Evolution

Electrocatalysts encapsulated by an ultrathin and semipermeable oxide layer offer a promising avenue for efficient, selective, and cost-effective production of hydrogen through photoelectrochemical water splitting. This architecture is especially attractive for Z-scheme water splitting, for which a nanoporous oxide film can be leveraged to mitigate undesired, yet kinetically facile, reactions involving redox shuttles, such as aqueous iron cations, by limiting transport of these species to catalytically active sites. In this work, molecular dynamics simulations were combined with electrochemical measurements to provide a mechanistic understanding of permeation of water and Fe(III)/Fe(II) redox shuttles through nanoporous SiO2 films. It is shown that even for SiO2 pores with a width as small as 0.8 nm, water does not experience any energy barrier for permeating into the pores due to a favorable interaction with hydrophilic silanol groups on the oxide surface. In contrast, permeation of Fe(III) and Fe(II) into microporous SiO2 pores is limited due to high energy barriers, which stem from a combination of distortion and dehydration of the second and third ion solvation shells. Our simulations and experimental results show that SiO2 coatings can effectively mitigate undesired Fe(III)/Fe(II) redox reactions at underlying electrodes by attenuating permeation of iron cations, while allowing water to permeate and thus participate in water splitting reactions. In a broader context, our study demonstrates that selectivity of solvated cations can be manipulated by controlling the pore size and surface chemistry of oxide films.

Oxide Film Formation on Stainless Steel 304L by Anodizing Method with Different Power Sources

In order to generate a nanoporous oxide film, stainless steel 304L is anodized using an alternating current (AC) and direct current (DC) source in an ethylene glycol solution containing ammonium fluoride, NH4F, and water, H2O. This study focuses on the differences between AC and DC anodizing in terms of current density versus time response, thickness, and depth of the oxide film formed on the anodized specimens, surface roughness, and element composition of the specimens because there is limited study on AC anodizing of stainless steel. In 0.5% 0.3M NH4F and 3% H2O in a glycol solution, the AC and DC anodizing is conducted for 30 minutes at voltages ranging from 30 V to 50 V and 21.2 V to 35.4 V, respectively. The current density versus time response revealed that AC anodizing produced more current density than DC anodizing. As for the thickness and depth of the generated oxide films, only the specimens that had been anodized in AC indicated an increase in these films, whereas specimens that had been anodized in DC showed the metal dissolving as the specimens degraded. The surface roughness of AC anodized specimens reduced with increasing anodizing voltage, whereas DC anodized specimens increased with increasing anodizing voltage. The element composition of the anodized specimens revealed that the mass percentage of oxygen, O element is higher for AC anodized specimens compared to DC specimens. This suggests that AC anodizing on stainless steel might offer important advantages and should be further researched.

Self-assembled biodegradable block copolymer precursors for the generation of nanoporous poly(trimethylene carbonate) thin films

Even though the domain of nanoporous template manufacturing from self-assembled block copolymers is in full expansion, challenging studies addressing innovative biodegradable block copolymer assemblies are still not reported. Herein, we address the usage of an original biodegradable precursor to prepare nanoporous thin films: the PTMC-b-PDLLA-b-PTMC triblock copolymer. The copolymer is designed to have a favorable interaction parameter for phase-separation, which was confirmed by differential scanning calorimetry by the appearance of two distinctive glass transition temperatures, corresponding to each of the blocks. Thin films are firstly produced by spin-coating and two methods are investigated for achieving organized self-assembly of the system: solvent evaporation and solvent vapor annealing. Well-organized bicontinuous nanoscopic phase-separated morphologies consisting of PTMC and PDLLA domains are achieved under precise solvent vapor annealing conditions for a particular triblock copolymer composition. Triblock copolymer self-assembly is compared to the one occurring in PTMC/PDLLA blends where no ordered phase-separation is observed. Moreover, thin films photo-crosslinking of previously methacrylated PTMC end-blocks, beneficial to the stability and cohesion of the PTMC phase, as well as mechanical strength of the porous matrix, proved to preserve the phase separation. From the optimal oriented block copolymer thin films, selective etching via PDLLA mild hydrolysis resulted in nanoporous PTMC with maintained long-range order and stability. Such approach to generate nanoporous PTMC could have great potential in both biomedical applications and the energy field.

Highly catalytic and durable nanocellulose fibers-based nanoporous membrane film for efficient organic pollutant degradation

Sulfate radical-based advanced oxidation processes (SR-AOPs) have received significant attention because of the efficient and non-selective degradation of organic pollutants during water purification. Metal-organic frameworks (MOFs) are considered a promising catalyst for efficiently activating the oxidant in SR-AOPs to generate radicals such as SO4·− and ·OH due to the large specific surface area and highly exposed active metal ions of MOFs. However, the natural characteristics, such as small-sized particles (micro or nano), brittleness, low density, and easy aggregation, limit using powdery MOFs in practical applications requiring recyclability. This study proposes a facile method to fabricate ZIF-67@cellulose nanofibril (CNF)-based catalytic hybrid membranes and their water purification by the peroxymonosulfate (PMS) activation. The chemical structure and surface morphology of membranes were investigated, indicating that CNFs immobilize the MOF nanoparticles to form a stable and porous two-dimensional membrane. Additionally, the catalytic membrane embedding ZIF-67 nanoparticles was reusable after wastewater treatments. The ZIF-67@CNFs hybrid membrane activated PMS to produce SO4·− and ·OH radicals. Thereafter, highly active radicals efficiently degraded approximately 98.7 % of the organic dye Rhodamine B within 10 min. These findings imply that MOFs can be hybridized with nanocellulose fibers for high-performance catalytic membranes for water remediation.

Rational design of oriented glass fibers within nano-porous SiO2 films to improve evaporation-induced hydroelectric generation

Harvesting energy from the ubiquitous water evaporation process on earth has recently proved to be an effective solution for obtaining renewable energy. However, the electricity generated in this process is still difficult to meet practical use, and the energy conversion mechanism is controversial. Here, an enhanced output was generated by the rational design of oriented glass fibers within nano-porous SiO2 film. The ordered glass fibers could effectively improve the power generation performance by promoting the movement of water flow and carriers, achieving an induced open-circuit voltage of 1.4 V and an induced short-circuit current of 400 nA, and providing a performance improvement strategy for such evaporation-induced generators. And it is revealed that the generated voltage driven by evaporation is related to the humidity difference and water flow. The availability of raw materials and the simplicity of the fabrication process also enable the generators easy to scale up.

Using Applied Electrochemistry to Obtain Nanoporous TiO2 Films on Ti6Al4V Implant Alloys and Their Preclinical In Vitro Characterization in Biological Solutions

Nanoporous TiO2 film is deposited on grade 5 Ti6Al4V implant alloy by electrochemical oxidation. The nanopores of the film, as highlighted by electron microscopy, have a mean diameter of 58.6 nm, which is measured and calculated from an average value of 10 measurements. The increase in oxygen concentration compared to the untreated alloy, which indicates the oxidation of the titanium alloy surface, is visualized using X-ray spectroscopy coupled to an electron microscope. The beneficial effect of the oxidation and controlled formation of the TiO2 film on the implant alloy is proven by the comparative evaluation of degradation over time through the corrosion of both the untreated alloy and the alloy with an electrochemically formed and controlled TiO2 film in Hank’s solution, which simulates the most corrosive biological fluid, blood. The results show that the electrochemical modification of the grade 5 titanium alloy to form a nanoporous TiO2 surface film using the electrochemical oxidation method confirms the potential of improving the anticorrosive properties of titanium alloys used in implant applications.

Improved NO2 gas sensing performance of nanoporous conjugated polymer (CP) thin films by incorporating preformed CP nanowires

Conjugated polymer (CP)-based organic field-effect transistor (OFET) gas sensors can detect gas molecules at room temperature with high sensitivity and selectivity. However, their performance, particularly when detecting oxidizing gases, has fallen behind that of their metal oxide counterparts. Hence, improving their sensing performance as well as understanding the correlation between it and the structural and electrical properties of the sensing materials is necessary. Herein, we demonstrate that gas sensing performance of an OFET sensor based on nanoporous poly(3-hexylthiophene) (P3HT) films can be improved simply by incorporating preformed P3HT nanowires (NWs), which act as an efficient charge transport pathway. It detects nitrogen dioxide (NO2) molecules with high sensitivity due to the efficient charge transport pathway as well as ultrathin nanoporous structure of the P3HT films. Upon exposure to 10 ppm NO2 at room temperature, the OFET gas sensor based on the P3HT-NW embedded nanoporous P3HT film exhibits excellent responsivity (34%), fast response and recovery times (∼69 and ∼133 s, respectively), and a low limit of detection of under 0.1 ppm, which are comparable to those of conventional metal oxide-based gas sensors. Based on the correlation study between the morphology, charge transport ability, and sensing performance of the P3HT-NWs embedded nanoporous P3HT films, it is revealed that the efficient charge transport ability is an important factor for realizing high-performance CP-based OFET gas sensors, along with excellent morphological features such as thinness and high pore density.

Schottky Junction Made from a Nanoporous Au and TiO2 Film for Plasmonic Photodetectors

The generation, transfer, and collection of plasmon-derived hot electrons represent a distinctive pathway for the utilization of solar energy. Herein, we report the construction of a plasmonic photodetector from a metal/TiO2 Schottky junction, featuring using a solution-processed continuous nanoporous Au film (CNAuF) as the light absorption layer and an n-type TiO2 film as the electron acceptation layer. Because of its plasmon properties, the CNAuF can absorb broadband light in the visible region and in turn creates abundant hot electrons (via the decay of its plasmon). Using a CNAuF/TiO2 Schottky junction, these hot electrons can be collected into the TiO2 film to generate a steady-state photocurrent. As a result, the plasmonic photodetector constructed from the CNAuF/TiO2 Schottky junction delivers an appreciable photoresponse to visible light. Taking the photoresponse to 532 nm light as an example, this plasmonic photodetector delivers a high responsivity of 0.06 A/W, a specific detectivity of 3.9 × 109 Jones, and an external quantum efficiency of 13.8%, with a rise time and a decay time being 110 and 120 ms, respectively. The bottom-up nature of the solution-processed strategy offers great flexibility to tune the plasmonic nanostructure and in turn its optical properties, thereby creating many possibilities for the creation of next-generation plasmonic photoelectric devices and beyond.

TiO2 nanotube arrays grown in ethylene glycol-based media containing fluoride: Understanding the effect of early anodization stages on the morphology

In this work, we develop a phenomenological model describing the growth of TiO2 nanotube arrays (NTA-TiO2) by titanium anodization in ethylene glycol-based electrolytes containing fluoride. NTA-TiO2 formation is described in terms of the well-known High-Field mechanism (HFM) considering film thinning by dissolution, and the electronic current contribution according to the Oxygen Bubble Mold (OBM) model, as it has been previously reported. For the first time, the inclusion of the nucleation of oxygen bubbles as the rate determining step at early anodization times is considered. By simultaneous fitting of the potentiostatic j/t profiles and the time evolution of morphological features such as pore radius, layer thickness and density of pores, the behavior observed for several experimental conditions is accurately described, in contrast to previously reported Field Assisted Dissolution (FAD) and OBM models. Proposed model allows obtaining physical and phenomenological information concerning the formation and growth of nanoporous oxide films as well as their morphological properties. Results also indicate that fluoride species play a key role at the early anodization stages, determining the subsequent film morphology evolution. Also, conclusive evidence for a chemical-type instead of field-assisted film dissolution process is obtained.

Nanotubular TiO x N y -Supported Ir Single Atoms and Clusters as Thin-Film Electrocatalysts for Oxygen Evolution in Acid Media.

A versatile approach to the production of cluster- and single atom-based thin-film electrode composites is presented. The developed TiO x N y -Ir catalyst was prepared from sputtered Ti-Ir alloy constituted of 0.8 ± 0.2 at % Ir in α-Ti solid solution. The Ti-Ir solid solution on the Ti metal foil substrate was anodically oxidized to form amorphous TiO2-Ir and later subjected to heat treatment in air and in ammonia to prepare the final catalyst. Detailed morphological, structural, compositional, and electrochemical characterization revealed a nanoporous film with Ir single atoms and clusters that are present throughout the entire film thickness and concentrated at the Ti/TiO x N y -Ir interface as a result of the anodic oxidation mechanism. The developed TiO x N y -Ir catalyst exhibits very high oxygen evolution reaction activity in 0.1 M HClO4, reaching 1460 A g-1 Ir at 1.6 V vs reference hydrogen electrode. The new preparation concept of single atom- and cluster-based thin-film catalysts has wide potential applications in electrocatalysis and beyond. In the present paper, a detailed description of the new and unique method and a high-performance thin film catalyst are provided along with directions for the future development of high-performance cluster and single-atom catalysts prepared from solid solutions.

Enhanced Photothermal Steam Generation and Gold Using the Efficiency of Ultralight Gold Foam with Hierarchical Porosity

Controlling the optical properties of metal plasma nanomaterials through structure manipulation has attracted great attention for solar steam generation. However, realizing broadband solar absorption for high-efficiency vapor generation is still challenging. In this work, a free-standing ultralight gold film/foam with a hierarchical porous microstructure and high porosity is obtained through controllably etching a designed cold-rolled (NiCoFeCr)99Au1 high-entropy precursor alloy with a unique grain texture. During chemical dealloying, the high-entropy precursor went through anisotropic contraction, resulting in a larger surface area compared with that from the Cu99Au1 precursor although the volume shrinkage is similar (over 85%), which is beneficial for the photothermal conversion. The low Au content also results in a special hierarchical lamellar microstructure with both micropores and nanopores within each lamella, which significantly broadens the optical absorption range and makes the optical absorption of the porous film reach 71.1-94.6% between 250 and 2500 nm. In addition, the free-standing nanoporous gold film has excellent hydrophilicity, with the contact angle reaching zero within 2.2 s. Thus, the 28 h dealloyed nanoporous gold film (NPG-28) exhibits a rapid evaporation rate of seawater under 1 kW m-2 light intensity, reaching 1.53 kg m-2 h-1, and the photothermal conversion efficiency reaches 96.28%. This work demonstrates the enhanced noble metal gold using efficiency and solar thermal conversion efficiency by controlled anisotropic shrinkage and forming a hierarchical porous foam.

Preparation of FeNi-based nanoporous amorphous alloy films and their electrocatalytic oxygen evolution properties

The progress of this research is the preparation of FeNi alloy thin films by magnetron sputtering. Each step of the experimental process is based on the electrocatalytic performance of the sample, and characterized by many characterizations means such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-ray energy spectrometry (EDS) and step gauge thickness test for morphology, structure and elemental composition, etc. The analysis of the characterization results is used as a support for the experimental process. Adjustment of various preparation process parameters for material growth and subsequent processing include doping of non-metallic elements and construction of nanostructures. Doping of C elements can make FeNi based alloy films further amorphous. Zn element is used as a pore-forming agent. The two processes of doping and high-temperature vacuum dealloying can make the film obtain a nanoporous structure, which greatly increases the specific surface area. These two strategies reduce the overpotential (η10) of oxygen evolution reaction (OER) of FeNi alloy thin films to 393 mV and 314 mV, which are reduced by 47 mV and 79 mV step by step. The electrochemical properties of the finally obtained alloy film are: overpotential of 314 mV, Tafel slope of 61.8 mV/dec and the stability of only 10% decay at a current density of 10 mA/cm2 for 12 h. In this study, low-cost transition metals were used as the main materials to design OER catalysts, and the catalytic efficiency was comparable to that of commercial noble metal catalysts. The physical preparation methods made each sample have good reproducibility. It provides the experimental basis and theoretical basis for the design and synthesis of new catalytic materials at a higher level.

Highly enhanced photocatalytic activity of nanotubular Fe2O3/Fe2WO6 nanocomposite film formed by anodizing FeW alloy

This study demonstrates the highly enhanced photocatalytic activity of Fe2O3 nanotubes by making a heterojunction with Fe2WO6 and doping of W6+ ions into the α-Fe2O3 phase for water splitting and decomposition of an organic pollutant. Both nanotubular and compact Fe2O3/Fe2WO6 films are formed by anodizing the magnetron-sputtered Fe-9 at% W alloy in fluoride-containing ethylene glycol electrolyte and subsequent annealing at 450 °C. Their photocatalytic activities are compared with W-free nanoporous Fe2O3 film formed on high-purity Fe. The nanotubular Fe2O3/Fe2WO6 film exhibits a markedly enhanced photoelectrochemical oxygen evolution reaction and photocatalytic degradation of methylene blue compared with the nanoporous W-free Fe2O3 film. Because of the larger surface area, the nanotubular Fe2O3/Fe2WO6 film reveals further increased activity than the compact Fe2O3/Fe2WO6 film. The STEM study discloses the dispersion of Fe2WO6 nanoparticles in the α-Fe2O3 phase after annealing at 450 °C, and XRD analysis shows the possible doping of W species into α-Fe2O3. The formation of the W-doped α-Fe2O3/Fe2WO6 heterojunction likely promotes the separation of photogenerated electron-hole pairs, enhancing photocatalytic activity. Hence, the nanotubular Fe2O3/Fe2WO6 film is a possible candidate for visible-light-driven photocatalysts.