Porous Thin Films Research Articles

Layered Au-Pd-Au nanorod catalysts: Pd-layer thickness effects on catalyst performance

Au/Pd/Au and Au/Pd layered nanorods (NR) with different Pd thickness, down to 15 nm, are electrodeposited in Si-supported porous anodized aluminum thin film templates. These structures are, in contrast to Au-Pd core-shells, mechanically relaxed, so that chemical and electronic interaction effects between Au and Pd are expected to predominate in controlling their performance in the electrooxidation of formic acid. The XRD results are conclusive about the formation of pure metal layers, but a corrugated Au/Pd interface is revealed by scanning electron microscopy. The CV behavior of the different structures shows that the PdO reduction peak is shifted towards more noble values as the Pd thickness decreases. Regarding the electrocatalytic oxidation behavior there is a substantial increase in performance with decreasing Pd-layer thickness. It is further shown that the Au/Pd/Au nanostructures perform better than the Au/Pd bilayer structure for the same Pd layer thickness. Also the long term behavior is improved. Both CV and electrocatalytic behaviors are discussed in terms of interfacial intermixing between Pd and Au at the nanoscale and a higher interface contribution for thinner Pd layers.

Small Molecule Adsorption on Colloidal Alumina and Zirconia Abrasives Used in Chemical-Mechanical Planarization (CMP) Slurries

Small molecule adsorption on CMP slurry abrasive particles has been investigated previously for silica (1) and ceria (2, 3) abrasives using fluorescence correlation spectroscopy (FCS) and attenuated total reflectance – Fourier transform infrared spectroscopy (ATR-FTIR). The need for further characterization of such adsorptive interactions is dictated by the increasing use of abrasive particles in CMP slurries with minimized hydrodynamic diameters (≤ 10 nm) as a means to reduce surface defects created during CMP processes. Incorporation of ever-smaller abrasive particles in CMP slurries will, however, promote the adsorption of chemical additives on a slurry’s abrasive particles due to the increase in the total abrasive particle surface area. In the present study, FCS and ATR-FTIR are employed in the analysis of small molecule adsorption on colloidal alumina and zirconia abrasive particles. This work is motivated by the widespread commercial use of alumina-based slurries in CMP processes for planarization of deposited copper films on dielectric layers and the proposed use (4) of zirconia abrasives in CMP slurries for metal film removal on dielectric metal oxide materials. The reported FCS studies use fluorescent dyes as probes for adsorption sites on alumina and zirconia colloids dispersed in aqueous solution. Described ATR-FTIR analyses summarize the characterization of molecular interactions driving glycine and picolinic acid adsorption on porous thin films of colloidal alumina and zirconia, respectively. The potential for employing these methods in studies modelling the adsorption of CMP slurry additives on a metal film deposited on a silicon wafer surface is also discussed.

Fabrication and Characterization of Porous Thin Film Cathodes Prepared By RF Sputtering for Li-Ion Microbatteries

Li-ion batteries play an important role in powering the electronic devices and in storing energy due to its high energy and power density, which are expected to be a solution for the future energy storage requirements. Due to the lack of suitable on-board power sources, the advances in the miniaturization of microelectronics is growing which then open up the opportunities to explore the both cathode and anode materials [1]. Olivine-type LiFePO4 becomes a popular cathode owing to its thermally stable, low cost, more abundant, and less toxic [2]. The strong Fe-O covalent bonds in LiFePO4 cathodes has such a special characteristic compared to layered LiCoO2-type materials. This covalent bond greatly improve the stability of O in the lattice, thus increasing the safety of the materials [2]. In the field of thin-film microbatteries, a study of the thin film cathodes is essential for the fundamental research and application since neither binder nor conductive carbon additive are utilized. It means a good contact between the active material and the current collector could be greatly established [3]. By using the thin film cathodes, the intrinsic drawbacks such as a low electronic conductivity and low Li+ diffusion mobility can also be suppressed because the thickness of the cathode material has been reduced. It would also help facilitating the transport and diffusion of electrons and ions through the thin electrodes compared to their bulk counterparts [4]. Similar to olivine-type LiFePO4, NASICON type-Li3Fe2(PO4)3 showed their own advantages such as low toxicity, low cost, a good ionic conductor and naturally abundant [5]. In addition, the synthesis of Li3Fe2(PO4)3 is easier because the Li3Fe2(PO4)3 cathode can be prepared directly in air without avoiding the oxidation of Fe2+. In the present work, radio frequency (RF) sputtering is utilized to fabricate the thin film cathodes. The results show that the annealing temperatures at 500 °C after deposition is found to be an optimal condition to obtain a porous olivine type-LiFePO4 film (Figure 1a). By further increasing the annealing temperature up to 700 °C , a complete transformation from porous olivine type-LiFePO4 to porous NASICON-type Li3Fe2(PO4)3 can be observed (Figure 1b); this film provides better cycling stability. Porous NASICON-type Li3Fe2(PO4)3 film attains a good capacity of ~103 mAh g-1 (~32 µAh cm-2 µm-1) at C/10 rate with an excellent cycling performance over 110 cycles. In this work, the effect of various annealing temperatures on morphology and electrochemical performances of the thin film cathodes will be discussed.

Kesan Ketebalan Filem Terhadap Fotoelektrokimia Titania Dioksida (TiO2 ) Yang Disediakan Melalui Kaedah Pemendapan Bantuan Aerosol Wap Kimia (AACVD)

Titanium dioxide (TiO2) is a wide band gap semiconductor, which owing to its photoconversion properties in UV spectrum range shows various useful applications. In this study, aerosol-assisted chemical vapor deposition (AACVD) technique was employed to deposite TiO2 thin films with different thickness based on different deposition time (3, 5 and 7 mins). The films were deposited at temperature of 450oC. Using AACVD technique, deposited TiO2 layer on (Fluorin doped tin oxide) FTO glass became very thin layer with compact structure. Titanium diisopropoxide bis(acetylacetonate) and ethanol were used to prepare the AACVD precursor. TiO2 thin film samples were characterized on their morphological and roughness using atomic force microscopy (AFM) which showed a porous morphology structure of the thin films. The optical properties were studied using UV-Vis transmittance spectra and showed the film are highly transparent in the visible region with suitable bandgap were obtained. The performances of the thin films were characterized using linear sweep voltammetry (LSV) and electrochemical spectroscopy (EIS). TiO2 -5 min obtained an average thickness of 61 nm showed the best J-V performance of 6.30 × 10-4 A/cm-2 with lowest charge resistant. In addition, the deposition film obtained also showed excellent adherence to the substrate, reproducible and uniform thin film. Based on the results obtained, the prepared TiO2 using AACVD is very suitable to be used in many applications such as third generation solar cell which require low thickness, porous, highly transparent, photoactive and stable thin film to be applied.

Compensating porosity gradient to produce flat, micromachined porous silicon structures

Abstract Obtaining flat micromachined porous silicon structures is extremely challenging due to the myriad of factors affecting film stress in such structures. In this work, the relationships between the current control during anodization, average porosity, and residual stress within porous silicon thin films and micro-fabricated structures were investigated. Using a combination of electron microscopy, surface profilometry and reflectance spectroscopy, the optimum conditions to produce near zero residual stress in thin films and micro-fabricated structures were determined. The residual stress was adjusted by a continuous variation of the anodization current in order to achieve flat structures. The flattest released porous silicon microbeams were 2.3 μm thick with a peak to valley height variation of only 72 nm over a length of 150 μm. These were achieved by using an initial current density of 20 mA/cm2 that was reduced down to 8 mA/cm2 during anodization. By using this method of reducing the current during anodization, the inherent high porosity at the porous-silicon/silicon interface was reduced, which also enabled a 36% increase in the film thickness before film delamination compared with using a constant current. These results provide a pathway to fabricate thick, optically flat micromachined multi-layer filters using a single base material (silicon) for the structural and released layers.

Carrier conduction mechanisms of mesoporous titania thin films assessed by impedance spectroscopy

The electrical characterization of porous titania thin films remain a subject of study due to the material interest in widespread device applications. Highly ordered mesoporous titania thin films prepared by sol-gel method under different post-synthesis annealing temperatures were prepared and characterized by impedance spectroscopy at different humidity conditions. Slow ionic and fast electronic processes have different responses to the humidity and are carefully studied by assessing their frequency dependence. Specific ionic conduction regimes are identified for different water content that strongly depend on the structural properties of the material such as porosity and pore size. Also, the analysis of the Warburg coefficient describes the response of the mass transport for the different materials characteristics. Highly porous materials obtained for elevated annealing temperatures do not saturate with water even at high humidities and present inferior ionic transport mechanism. By analyzing the impedance response at different bias voltage it was possible to discriminate the electronic conduction for the mesoporous layer and observe the electronic defects by reproducing a space charge limited current behavior.

Investigation of porous silicon thin films for electrochemical hydrogen storage

In this study, nanoporous silicon (PS) layers have been elaborated and used for hydrogen storage. The effect of the thickness, porosity and specific surface area of porous silicon on the amount of hydrogen chemically bound to the nanoporous silicon structures is studied by Infrared spectroscopy (FTIR), cyclic voltammetry (CV), contact angle and capacitance –voltage measurements. The electrochemical characterization and hydrogen storage were carried out in a three-electrode cell, using sulfuric acid 3 M H2SO4 as electrolyte by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge/discharge. The results indicate the presence of two oxidation peaks at 0.2 V and 0.4 V on the anodic side corresponding to hydrogen desorption and a reduction peak at −0.2 V on the cathodic side corresponding to the sorption of hydrogen. Moreover, the EIS studies performed on PS electrode in 3 M H2SO4 show that the hodograph contains a semicircle at high frequency region and a line in the lower frequency zone. An equivalent circuit has been proposed; the values of the equivalent circuit elements corresponding to the experimental impedance spectra have been determined and discussed. Finally, the highest hydrogen storage in PS was 86 mAh/g. This storage capacity decreases by only 7% of the initial capacity value, after 40 cycles.

Polarization-dependent ultrafast plasmon relaxation dynamics in nanoporous gold thin films and nanowires

Metal materials structured at nanoscale are extensively used in a variety of applications, including molecular sensing, nanoscale heaters for photothermal therapy, etc. These applications depend on the strong absorption and electric field enhancements associated with localized surface plasmon resonance (LSPR). Despite multiple investigations of LSPR relaxation dynamics in plasmonic metallic nanoparticles (i.e. nanospheres, nanocubes, etc), no detailed studies of porous thin films or nanowires have been reported so far. In this paper, LSPR relaxation dynamics of nanoporous gold thin film, gold nanowires, and nanoporous gold nanowires (structures with the potential for a vast range of sensing and optical applications) were studied by means of transient absorption spectroscopy (TAS). Au and Au/Cu thin films produced by magnetron sputtering deposition over a nanograted silicon substrate enabled the creation of highly organized nanowire arrays of centimetres length. Nanoporous structures were created by the dealloying of Au/Cu alloy. We found that formation of such a subwavelength period grating structure in gold increases the efficiency of hot electron relaxation compared to continuous film, while the porosity of gold nanostructures has the opposite effect. The results also expressed a clear dependence of the TAS signal on the polarization (i.e. orientation of the electric field with respect to the nanoporous gold nanowires) of the pump and probe pulses, compared to non-porous gold nanowires.

On the transformation of "zincone"-like into porous ZnO thin films from sub-saturated plasma enhanced atomic layer deposition.

The synthesis of nanoporous ZnO thin films is achieved through annealing of zinc-alkoxide (“zincone”-like) layers obtained by plasma-enhanced atomic layer deposition (PE-ALD). The zincone-like layers are deposited through sub-saturated PE-ALD adopting diethylzinc and O2 plasma with doses below self-limiting values. Nanoporous ZnO thin films were subsequently obtained by calcination of the zincone-like layers between 100–600 °C. Spectroscopic ellipsometry (SE) and X-ray diffraction (XRD) were adopted in situ during calcination to investigate the removal of carbon impurities, development of controlled porosity, and formation and growth of ZnO crystallites. The layers developed controlled nanoporosity in the range of 1–5%, with pore sizes between 0.27 and 2.00 nm as measured with ellipsometric porosimetry (EP), as a function of the plasma dose and post-annealing temperature. Moreover, the crystallinity and crystallite orientation could be tuned, ranging from a powder-like to a (100) preferential growth in the out-of-plane direction, as measured by synchrotron-radiation grazing incidence XRD. Calcination temperature ranges were identified in which pore formation and subsequent crystal growth occurred, giving insights in the manufacturing of nanoporous ZnO from Zn-based hybrid materials.

Nanoplasmonic response of porous Au-TiO2 thin films prepared by oblique angle deposition

In this work, a versatile method is proposed to increase the sensitivity of optical sensors based on the localized surface plasmon resonance (LSPR) phenomenon. It combines a physical deposition method with the oblique angle deposition technique, allowing the preparation of plasmonic thin films with tailored porosity. Thin films of Au–TiO2 were deposited by reactive magnetron sputtering in a 3D nanostructure (zigzag growth), at different incidence angles (0° ≤ α ≤ 80°), followed by in-air thermal annealing at 400 °C to induce the growth of the Au nanoparticles. The roughness and surface porosity suffered a gradual increment by increasing the incidence angle. The resulting porous zigzag nanostructures that were obtained also decreased the principal refractive indexes (RIs) of the matrix and favoured the diffusion of Au through grain boundaries, originating broader nanoparticle size distributions. The transmittance minimum of the LSPR band appeared at around 600 nm, leading to a red-shift to about 626 nm for the highest incidence angle α = 80°, due to the presence of larger (scattering) nanoparticles. It is demonstrated that zigzag nanostructures can enhance adsorption sites for LSPR sensing by tailoring the porosity of the thin films. Atmosphere controlled transmittance-LSPR measurements showed that the RI sensitivity of the films is improved for higher incidence angles.

High-performance of copper-doped vanadium pentoxide porous thin films cathode for lithium-ion batteries

In this work, porous Cu-doped V2O5·nH2O thin film electrodes have been directly synthesized via low-temperature annealing and simple drop-casting method from V2O5/H2O2 sol with various concentration copper ion (Cu2+). It is interesting to find that the mass fraction of Cu2+ has effects on the morphologies, valence state of vanadium, and electrochemical performance of Cu-doped V2O5·nH2O thin film. When the mass fraction of Cu2+ is 1 wt.%, the obtained Cu-doped V2O5·nH2O thin film electrode shows network porous nanostructure. Moreover, it still shows high discharge specific capacity of 344 mAh g−1 at a current density of 250 mA g−1. Excellent cycling stability with only 0.14% per cycle degradation during 104 cycles can be obtained even at a high current density of 550 mA g−1. Improvement of electrochemical performance is attributed to unique network porous nanostructure, accelerating the lithium-ion transfer rate in the de-intercalation or intercalation process.

Hierarchical porous ZnO films synthesized by sol–gel method using triethylenetetramine stabilizer

Hierarchically porous ZnO films are synthesized through a novel sol–gel method using triethylenetetramine (TETA) as a stabilizer. Highly crystalline hierarchical porous ZnO thin films with different pore sizes are synthesized in various reaction mediums by altering solvents, sol concentration, the molar ratio of TETA to zinc acetate dehydrate (ZAD), and withdrawal speed. FESEM images show the formation of a hierarchical porous film with spherical and agglomerated particles in the optimized condition of ethanol as the solvent, sol concentration of 0.2 M, the molar ratio of [TETA]:[ZAD] = 2, and withdrawal speed of 30 mm/min. Increasing the solvent chain length, increasing the sol concentration, decreasing the TETA content, or increasing the withdrawal speed increases the particle size and porosity. XRD data, UV–Vis, and PL spectroscopy indicate high crystallinity with appropriate optical properties for the optimized sample. The hierarchical porous ZnO film synthesized by this method can be utilized as a photoelectrochemical electrode in devices such as photocatalysis and photoelectrochemical water splitting.

Enhancement of plasmonic transmittance of porous gold thin films via gold/metal oxide bi-layers for solar energy-saving applications

Thin films of noble metals, such as gold, have high visible transparency and infrared reflectance. The former is due to the presence of surface plasmons, and the latter is due to the large density of free electrons. Gold thin films have a transmittance peak at 520 nm, which is closely matched to the maximum emission of solar radiation. These properties have been utilized in several applications, including energy-saving transparent heat mirrors that consist of a bi-layer of a thin gold film over-coated with a dielectric layer. These coatings transmit light and reflect heat. For such an application, the transmittance of the thin gold film needs to be enhanced; this is achieved by the dielectric layer that functions as an antireflective coating. In this work, the dielectrics were metal oxides that are transparent in the visible range. The selection criteria based on the band gap, and refractive index were first imposed to identify potential metal oxides. This resulted in the choice of ten metal oxides (CeO2, HfO2, MoO3, Nb2O5, NiO, SnO2, Ta2O5, TiO2, WO3, and ZrO2). The bi-layer performance was then optimized theoretically. The gold-metal oxide layers were subsequently fabricated, and their optical spectra were measured over the ultraviolet–visible-near infrared ranges. The performance of the heat mirrors was assessed in terms of their visible and solar transmittance and reflectance. A figure of merit was defined to evaluate the suitability of the oxides. The optical properties suggest that tungsten oxide (WO3) is the best oxide; however, hafnium oxide (HfO2) has the best performance for solar energy applications.

Quantitative characterization of the interfacial morphology and bulk porosity of nanoporous cluster-assembled carbon thin films

The deposition of nanoporous carbon thin films with well-controlled morphology, high surface area and porosity is of great interest for different applications requiring autonomous and portable electrochemical devices for energy conversion and storage. The engineering of this class of devices and the evaluation of their performances require the measurement of the porosity and surface area of the active carbon layers. The quantitative characterization of these parameters by gas adsorption techniques is hampered by the small amount of material usually available on a thin film.Here we report a method based on the use of atomic force microscopy (AFM) and nitrogen adsorption technique for the quantitative characterization of the specific surface area and porosity of nanoporous carbon films. In particular, we apply it to nanostructured carbon films fabricated by supersonic cluster beam deposition (SCBD), studying the evolution of their structural properties with film thickness. We exploited the characteristics of SCBD in terms of high-throughput, stability, and reproducibility to fabricate films with controlled nano and microstructure over large substrates compatible with the requirements of gas adsorption measurements. Our approach demonstrates that the combination of AFM and nitrogen adsorption technique provides the possibility of undertaking systematic quantitative characterization of the nano- and mesostructure of cluster-assembled carbon films and of their evolution, which can be easily applied to porous thin films fabricated by many different techniques.

Enhanced Photoelectrochemical Performance of BiVO4 by a NiMoO4 Modification

In this work, porous BiVO4 thin films were deposited on the FTO glass through a spin-coating deposition method and their photoelectrochemical (PEC) properties were investigated. Further, the BiVO4 thin film was modified with a NiMoO4 thin layer for enhancing its PEC activity. It is demonstrated that the applied bias photo-to-current conversion efficiency is increased by 63% after a surface modification, which is ascribed to both the formation of a p–n junction and the suppressed carrier recombination rate by terms of the electrochemical impedance spectroscopy. Finally, a schematic band model is also proposed to clarify the charge carrier transfer mechanism which is responsible for the enhanced PEC performance.

NO2 sensing properties of porous Au-incorporated tungsten oxide thin films prepared by solution process

The use of chemoresistive gas sensors based on metal oxides has expanded to various fields such as medical diagnosis and air quality systems as well as gas leakage detectors with the development of the Internet of Things. Accordingly, sensitivity, selectivity, power consumption, and reproducibility become important factors in the development of gas sensors. Herein, we developed a facile method to fabricate a gas sensor based on porous Au-incorporated tungsten trioxide (WO3) thin films for highly sensitive and selective NO2 sensing. The mixed solution of ammonium tetrathiotungstate [(NH4)2WS4] and gold chloride (AuCl3) was transformed to Au-incorporated WO3 thin films through the spin-coating and annealing process. The gas sensors based on the Au-incorporated WO3 thin films exhibited improved sensitivity, selectivity, and response time upon exposure to NO2 with a significantly low theoretical detection limit of 28 ppt at 150 ℃ compared to gas sensors based on the pristine WO3 thin film. The high sensing properties are attributed to the porous structure and catalytic effects of Au nanoparticles. In addition to these remarkable NO2 sensing properties, the facile and cost-effective fabrication process enlarges the potential of the Au-incorporated WO3 thin films for practical and commercial gas sensing applications.

Hybrid Microporous Membranes Applied in Wastewater Treatment

In this work, membranes of pure polyamide 6 (PA6) and hybrids with 1, 3, and 5% in weight of organophilic montmorillonite (OMMT) clay in the form of thin porous films are obtained by the phase inversion technique. The membranes are characterized by X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), contact angle (CA), and flow measurements (FM). Through X‐ray diffraction, it is revealed that the hybrid membranes with OMMT have an exfoliated and/or partially exfoliated structure. From FTIR and DSC analyses of hybrid membranes with OMMT, it is found that the spectra and the crystalline melting temperature remained practically unchanged compared to PA6 membrane. From the SEM, it is verified that the addition of the clay caused an increase in the quantity and uniformity of pores distribution in the surface and cross section of these membranes. By MIP, it is observed that the presence of clay in the membrane caused an increase in the average pore diameters. By the contact angle, it is verified that the membrane with clay had a higher value of contact angle for the distilled water when compared to the angle obtained with the synthetic oil. The water flux of the membrane with clay is higher than that obtained by the membrane of pure PA6, due its surface structure with larger pore diameters, favoring its permeability. The obtained hybrid membrane presented adequate porosity morphology for wastewater treatment.

Porous Fe3O4 thin films by pulsed laser assisted chemical solution deposition at room temperature

Porous thin films showing improved physicochemical properties are desirable. Here, we developed a facile route, laser-assisted chemical solution deposition (LACSD), to deposit porous Fe3O4 thin films. The synthesized porous Fe3O4 thin films show excellent electrochemical performance as anodes in lithium ion batteries. The results will provide a facile route to synthesize porous large-area metal oxide thin films.

Preparation of a ZIF-67 Derived Thin Film Electrode via Electrophoretic Deposition for Efficient Electrocatalytic Oxidation of Vanillin.

The electrophoretic deposition method is employed to deposit uniform metal organic framework thin films. ZIF-67 particles dispersed in isopropanol move to a cathode as an electric field is applied on two conducting glass electrodes, the uniform ZIF-67 thin film can be formed on the conducting glass substrate. As the deposition time is fixed at 1 min, the prepared film thickness can be adjusted from 1.0 to 7.0 μm by applying different electric fields from 20 to 60 V·cm-1. The deposited ZIF-67 thin film is further converted into porous Co9S8 thin films by the vulcanization with S powder. The porous thin films vulcanized at different temperatures are characterized by the measurements of scanning electron microscope, X-ray photoelectron spectroscopy, X-ray powder diffraction, high resolution transmission electron microscopy, and Fourier transform infrared spectra. The prepared porous Co9S8 thin films are used as the thin film electrode to catalyze the degradation of vanillin. The Co9S8 thin film vulcanized at 500 °C shows better catalytic performance than the bare glassy carbon electrode and the electrodes vulcanized at other temperatures. The electrocatalytic degradation enhancement mechanism is analyzed by the measurements of Tafel curves and electrochemical impedance spectroscopies. It can be developed as a feasible method for the electrocatalytic detection of vanillin.

Novel controllable degradation behavior and biocompatibility of segmented poly–ε–caprolactone in rats

Bioresorbable polymers have multiple clinical applications; however, the reaction methods required for their synthesis generally require harsh reaction conditions and long reaction times. This study uses a new conceptual molecular control method that links poly–ε–caprolactone (PCL) diols of differing molar mass with an aliphatic diisocyanate as a coupling agent for polymerization to develop a new type of segmented polycaprolactone (SM-PCL) material. Relative to the polymerization of typical high molecular weight polyester materials, this polymerization process only requires ordinary pressure and low temperature to yield product with sufficiently high molecular weight. The results of in vitro cytotoxicity and in vivo implantation assays show that the newly synthesized SM-PCL exhibits excellent biocompatibility. In addition, in vivo degradation analyses demonstrate that the porous SM-A material was nearly completely degraded at 6 months after implantation in rat tissue (P < 0.01), whereas porous PCL homopolymer was only degraded to 53.25% of the initial molecular weight even after 12 months. The study indicated that the degradation rates of porous and solid thin film SM-PCL were higher than those of PCL homopolymer and that its degradation behavior could be controlled. Moreover, the material did not result in significant adverse pathological reactions and has good tissue compatibility. For clinical application, this material may therefore be suitable as a carrier for controlled drug release or as an artificial material for soft tissue repair.