Nanoporous Silica Films Research Articles

Nanoporous Silica–Alumina Films Fabricated on Silicon Photonic Chips for Selective Ammonia Sensing

Surface-modified nanoporous silica films offer attractive features for analyte-specific gas detection applications. Here we demonstrate the integration of highly porous silica–alumina films on silicon nanophotonic chips and their performance in selective NH3 detection. Prototype sensors with microporous as well as mesoporous silica films were assembled. The incorporation of aluminum in trace amount needed to generate acid sites was achieved during film deposition or using postsynthesis atomic layer deposition. Silicon photonic micro-ring resonators functionalized with both techniques demonstrated a selective response to NH3 relative to CO2. Furthermore, the response was rapid and reversible. The role of preadsorbed water vapor on the reversible nature of the sensor is also investigated. Experimental observations indicate that water vapor preadsorbed on the films leads to fast sensor recovery while maintaining selectivity toward NH3. This could be attributed to the relatively less strong and still selective binding of NH3 on protonated water molecules preadsorbed on the surface acid sites. The potential of modified nanoporous films for portable and low-cost NH3 sensing on optical chips demonstrated here can be exploited in health care as well as industrial applications.

On‐Demand Circularly Polarized Room‐Temperature Phosphorescence in Chiral Nematic Nanoporous Silica Films

AbstractCircularly polarized room‐temperature phosphorescence (CPRTP) is of paramount importance for applications in information encoding, bioimaging, and optoelectronic devices, however, it remains challenging to organize highly efficient CPRTP materials with controlled handedness and long afterglow. Herein, as an outstanding novel finding, it is diclosed that ultralong CPRTP with on‐demand chiroptical properties is simple to realize using the earth‐abundant cellulose and silica. It is shown that left‐handed chiral nematic nanoporous silica films featuring defect photoluminescence enable ultralong CPRTP with an unprecedented control of the handedness. This work presents that the chiral nematic nanoporous silica films display right‐handed CPRTP and left‐handed CPRTP with the phosphorescence dissymmetry factors up to −0.130 and 0.093 as well as the afterglow lifetimes of 1.094 and 0.949 s, respectively. The potential of the transparent chiral nematic nanoporous silica films, capable of ultralong CPRTP and selective reflection of left‐handed circularly polarized light, for the development of anticounterfeiting optical labels is showcased. This work presents a versatile strategy and a step toward the development of ultralong CPRTP materials with on‐demand chiroptical properties from readily available defect phosphors for photonic applications.

Effect of nanoporous thin silica films on interface microstructure and bond strength of cementitious grouts

As in many interfaces found within cement-based materials and structural systems, the interface between field-cast grout connections and prefabricated bridge elements (PBE) is a region susceptible to accelerated deterioration. Ensuring a good quality grout-concrete interface is crucial in reducing maintenance costs of the PBE connections and enhancing the service life of the overall structural system. This study continued previous work on the effect of nanoporous thin films (NPTF) of colloidal silica as surface preparation technique to increase the bond strength between the precast components and the field-cast grout. The new work focuses on the influence of the NPTF on the microstructure of the interface between a cementitious grout and a conventional concrete. Important variables of the NPTF, such as application rate and dosage, were evaluated. Changes in the interface microstructure induced by the presence of NPTF were studied through a quantitative image analysis (QIA) and correlated with improvements in the bond strength measured using “pull-off” bond tests. The formation of calcium silicate hydrate (C-S-H) triggered by the reaction of the NPTF with portlandite at the interface was identified as the main contributor to the increase in bond strength. The benefits in the bond strength of the interface rendered by the presence of NPTF were compared against other common surface preparation techniques, such as sandblasting or exposed aggregates via in-form retarders. The results showed that it might be possible to reduce the need for these costly and cumbersome surface preparation techniques.

Nonlinear Optics to Glucose Sensing: Multifunctional Nitrogen and Boron Doped Carbon Dots with Solid‐State Fluorescence in Nanoporous Silica Films

AbstractMultifunctional triple color photoluminescent (PL) nitrogen–boron doped carbon quantum dots (CQDs) with high quantum yield (QY) of 58% are fabricated by one step femtosecond pulsed laser irradiation of a single precursor (2‐aminopyrimidine‐5‐boronic acid) in solution. In situ generated non‐linear and linear emissions are used to monitor CQDs formation which results in enhanced second harmonic generation, two photon absorption (2PA), and linear fluorescence; implying triple mode emission. These CQDs present blue, green, and possible red color rendering which are mostly independent to the respective excitation wavelengths (λ) with large stokes shift of 100 nm. Solid‐state photoluminescence with QY of 46% is achieved by incorporating CQDs into thin transparent nanoporous silica (pSiO2) films (thickness 50 µm) to form a CQDs‐pSiO2 composite which exhibits reverse saturable absorption at λ = 800 nm with 2PA coefficient and excited state absorption cross‐section of 4.94 × 10−10 m W−1 and 6.23 × 10−17 cm2, respectively. CQDs‐pSiO2 is also sensitive to glucose concentration down to 1.0 mg dL−1 in a wide linear range up to 100 mg dL−1. This work therefore demonstrates facile, controllable, and up‐scalable bottom‐up fabrication of CQDs forming multifunctional solid‐state CQDs‐pSiO2 with proven application in optical limiting and glucose sensing.

On the Properties of Nanoporous SiO2 Films for Single Layer Antireflection Coating

Single layer antireflection coatings (SLAR) consisting of nanoporous silica (NP SiO2) films are developed by selective chemical etching of atomic layer deposited (ALD) Al2O3:SiO2 composite films. The reflective index of the final NP SiO2 film is finely adjusted from 1.132 to 1.400 at 600 nm wavelength by applying an appropriate ratio in the composite. To meet the requirements of the SLAR coatings from the deep UV (DUV) to the near IR (NIR) spectral range, the film thickness is controlled with nanometer precision by the ALD process. The SLAR are simultaneously applied on both sides of flat or highly curved substrates. Transmittance values above 99.4% are achieved even at a wavelength of 193 nm on fused silica substrates. Various characterization methods demonstrate the advantages of these SLAR with regard to impurities, optical losses, laser induced damage threshold (LIDT) properties, and surface super‐hydrophilicity. The absorption losses at 193 nm wavelength as determined by laser induced deflection measurements amount to approximately 200 ppm, and to approximately 2 ppm at a wavelength of 1064 nm, while the scattering losses are around 30 ppm at 532 nm wavelength for quarter‐wave layers. The LIDT values at 1064 nm are in the range of 93 J cm−2 being close to the values measured on the uncoated substrate.

Highly Organized Ferrocene-Functionalized Nanoporous Silica Films with an Extremely Fast Electron-Transfer Rate for an Intrinsically Nonconducting Oxide-Modified Electrode

Modified electrodes from functionalized nanoporous xerogel coatings, prepared by using the electrochemically assisted self-assembly (EASA) method, containing ferrocene moieties grafted onto vertically oriented pores perpendicular to an indium-tin oxide (ITO) electrode, have been prepared and studied. The electrochemical behavior of these new materials exhibits the fastest charge transfer ever observed in oxide-modified electrodes. The cyclic voltammetry curves allow us, however, to discriminate two types of ferrocene species inside the coatings and to determine the rate constants for the electron exchange between the ITO electrode and the coatings, with the help of simulations and an advanced software package1. Fig. 1: Scheme depicting the electrode building. While it is not clear why the charge transfer is so fast, some hypotheses will be presented along. 1 P. Audebert, N. Vila, C. Allain, F. Maisonneuve, A. Walcarius, and P. Hapiot ChemElectroChem 2015, 2,1695. Figure 1

Nanoporous SiO2 thin films made by atomic layer deposition and atomic etching

A new route to prepare nanoporous SiO2 films by mixing atomic-layer-deposited alumina and silica in an Å-scale is presented. The selective removal of Al2O3 from the composites using wet chemical etching with phosphoric acid resulted in nanoporous thin SiO2 layers. A diffusion-controlled dissolution mechanism is identified whereby an interesting reorganization of the residual SiO2 is observed. The atomic scale oxide mixing is decisive in attaining and tailoring the film porosity. The porosity and the refractive index of nanoporous silica films were tailored from 9% to 69% and from 1.40 to 1.13, respectively. The nanoporous silica was successfully employed as antireflection coatings and as diffusion membranes to encapsulate nanostructures.

Quantitative image analysis for evaluating the abrasion resistance of nanoporous silica films on glass.

The abrasion resistance of coated glass surfaces is an important parameter for judging lifetime performance, but practical testing procedures remain overly simplistic and do often not allow for direct conclusions on real-world degradation. Here, we combine quantitative two-dimensional image analysis and mechanical abrasion into a facile tool for probing the abrasion resistance of anti-reflective (AR) coatings. We determine variations in the average coated area, during and after controlled abrasion. Through comparison with other experimental techniques, we show that this method provides a practical, rapid and versatile tool for the evaluation of the abrasion resistance of sol-gel-derived thin films on glass. The method yields informative data, which correlates with measurements of diffuse reflectance and is further supported by qualitative investigations through scanning electron microscopy. In particular, the method directly addresses degradation of coating performance, i.e., the gradual areal loss of antireflective functionality. As an exemplary subject, we studied the abrasion resistance of state-of-the-art nanoporous SiO2 thin films which were derived from 5–6 wt% aqueous solutions of potassium silicates, or from colloidal suspensions of SiO2 nanoparticles. It is shown how abrasion resistance is governed by coating density and film adhesion, defining the trade-off between optimal AR performance and acceptable mechanical performance.

Hierarchical Nanoporous Silica Films for Wear Resistant Antireflection Coatings.

High-performance antireflection (AR) layers were prepared by depositing hierarchical nanoporous silica films on glass substrates. We designed a composite layer consisting of mesoporous silica nanoparticles (MSNs) and a mesoporous silica matrix. The introduction of bimodal nanoporous structures, i.e., independent nanopore formation within the MSN and within the matrix, was achieved by using surface-protected MSNs and a polymeric nonionic surfactant template during the fabrication process. A porosity of more than 40% was achieved for composite AR materials. The protrusion of MSNs from the matrix led to spontaneous formation of nanoscale roughness on the surface of the coatings, which enhanced the AR properties. The solid bonding of the MSNs to the nanoporous matrices played an important role in the achievement of high mechanical durability. The optimal nanoporous coating, which contained ca. 50 wt % MSN, exhibited high transparency (91.5-97.5%) and low reflectance (<2.2%), over the whole range of visible light wavelengths, and sufficient wear resistance.

Highly Organized Ferrocene‐Functionalized Nanoporous Silica Films with an Extremely Fast Electron‐Transfer Rate for an Intrinsically Nonconducting Oxide‐Modified Electrode

AbstractModified electrodes from functionalized nanoporous xerogel coatings, prepared by using the electrochemically assisted self‐assembly (EASA) method, containing ferrocene moieties grafted onto vertically oriented pores perpendicular to an indium tin oxide (ITO) electrode, have been prepared and studied. The electrochemical behavior of these new materials exhibits the fastest charge transfer ever observed in oxide‐modified electrodes. The cyclic voltammetry curves allow us, however, to discriminate two types of ferrocene species inside the coatings and to determine the rate constants for the electron exchange between the ITO electrode and the coatings, with the help of simulations and an advanced software package.

In-situ ellipsometric characterization of the growth of porous anisotropic nanocrystalline ZnO layers

ZnO films have increasingly been in the spotlight due to their largely varied electro-physical and optical properties. For several applications, porous anisotropic nanocrystalline layers are especially interesting. To study the growth kinetics of such films during different fabrication processes, a powerful non-destructive in-situ technique is required. In this work, both ex-situ and in-situ spectroscopic ellipsometry are used along with advanced modelling techniques that are able to take both the anisotropy and the porosity of the films into account. Scanning electron microscopy, along with nitrogen absorption methods for measuring porosity, validated the ellipsometric data and proposed model. The film, grown by chemical bath deposition, was monitored from around 700 to 1800 nm in thickness. This same principle can now be used to monitor any other porous and/or anisotropic structure in an effective in-situ manner, e.g., growth of porous anodic aluminium oxides, nano-porous silica films, etc.

Biocompatibility of silver containing silica films on Bioverit® II middle ear prostheses in rabbits

For several centuries silver is known for its antibacterial effects. The middle ear is an interesting new scope for silver application since chronic inflammations combined with bacterial infection cause complete destruction of the fragile ossicle chain and tympanic membrane. The resulting conductive deafness requires tympanoplasty for reconstruction. Strategies to prevent bacterial growth on middle ear prostheses are highly recommended. In this study, rabbits were implanted with Bioverit® II middle ear prostheses functionalized with silver containing dense and nanoporous silica films which were compared with pure silica coatings as well as silver sulfadiazine cream applied on nanoporous silica coating. The health status of animals was continuously monitored; blood was examined before and after implantation. After 21 days, the middle ears were inspected; implants and mucosal samples were processed for electron microscopy. Autopsies were performed and systemic spreading of silver was chemically analyzed exemplarily in liver and kidneys. For verification of direct cytotoxicity, NIH 3T3 cells were cultured on similar silver containing silica coatings on glass up to 3 days. In vitro a reduced viability of fibroblasts adhering directly on the samples was detected compared to cells growing on the surrounding plastic of the same culture dish. In transmission electron microscopy, phagocytosed silver silica fragments, silver sulfadiazine cream as well as silver nanoparticles were noticed inside endosomes. In vivo, clinical and post mortem examinations were inconspicuous. Chemical analyses showed no increased silver content compared to controls. Mucosal coverages on almost all prostheses were found. But reduction of granulation tissue was only obvious around silver-coated implants. Single necroses and apoptosis in the mucosa were correlated by intracellular accumulation of metallic silver. For confirming supportive healing effects of middle ear implants, silver ion aggregates need to be tested in the future to optimize biocompatibility while assuring bactericidal effects in the middle ear.

Enlarged Pore Size in Mesoporous Silica Films Templated by Pluronic F127: Use of Poloxamer Mixtures and Increased Template/SiO2 Ratios in Materials Synthesized by Evaporation-Induced Self-Assembly

Although evaporation-induced self-assembly (EISA) has proven to be a convenient method for synthesizing nanoporous silica films (and particles), accessing material structures with pore sizes larger than ca. 10 nm remains experimentally inconvenient. The use of pore swelling agents (SAs), commonly used during the hydrothermal synthesis of mesoporous silicas, results in little or no pore size expansion due to evaporation or phase separation. Moreover, diblock copolymer templates can yield large pores but are quite expensive and generally require the addition of strong organic cosolvents. Here, we hypothesized that pores templated by the Pluronic triblock polymer F127 could be successfully enlarged, without phase separation, by using a chemically similar, nonvolatile, secondary Pluronic polymer (P103) as the SA. We find pore size increased up to 15 nm for a spherical pore morphology, with a phase transition to a multilamellar vesicle (MLV)-based nanostructure occurring as the P103/F127 ratio is further increased. This MLV phase produces even larger pore sizes due to the collapse of concentric silica shells upon template removal. Remarkably, F127 alone exhibits expansion of pore size (up to ca. 16 nm) as the template/silica ratio is increased. We find appearance of the MLV phase is due to geometric packing considerations, with expansion of F127 micelle size being a result of favorable intermolecular interactions driven by the large poly(ethylene oxide) content of F127. Other Pluronic polymers with this feature also exhibit variable pore size based on the template/silica ratio, enabling the synthesis of mesoporous films with 3D pore connectivity and truly variable pore size of ca. 4.5 to almost 20 nm.

Preparation of Nanocomposite Plasmonic Films Made from Cellulose Nanocrystals or Mesoporous Silica Decorated with Unidirectionally Aligned Gold Nanorods.

Using liquid crystalline self-assembly of cellulose nanocrystals, we achieve long-range alignment of anisotropic metal nanoparticles in colloidal nanocrystal dispersions that are then used to deposit thin structured films with ordering features highly dependent on the deposition method. These hybrid films are comprised of gold nanorods unidirectionally aligned in a matrix that can be made of ordered cellulose nanocrystals or silica nanostructures obtained by using cellulose-based nanostructures as a replica. The ensuing long-range alignment of gold nanorods in both cellulose-based and nanoporous silica films results in a polarization-sensitive surface plasmon resonance. The demonstrated device-scale bulk nanoparticle alignment may enable engineering of new material properties arising from combining the orientational ordering of host nanostructures and properties of the anisotropic plasmonic metal nanoparticles. Our approach may also allow for scalable fabrication of plasmonic polarizers and nanoporous silica structures with orientationally ordered anisotropic plasmonic nanoinclusions.

Fabrication of mechanically robust films with high transmittance and durable superhydrophilicity by precursor-derived one-step growth and post-treatment

In this study, we developed a novel approach to the fabrication of mechanically robust films with high transmittance and durable superhydrophilicity. First, precursor-derived one-step growth (POG) was used to fabricate ordered nanoporous silica films, and the thin film surface was then coated with one layer of SiO2 nanoparticles and one layer of TiO2 nanoparticles by dip-coating. After calcination at 550 °C and hardening at 720 °C, the obtained films exhibited not only high transmittance and low reflectivity, but also durable superhydrophilicity and good mechanical robustness. The maximum transmittance of the coated substrates reached as high as 97.8%, while the control glass substrate achieved only 91.4%. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) were used to observe the morphology and structure of the nanoparticles and coating surfaces. Optical properties were characterized by a UV-visible-near IR spectrophotometer. Surface wettability was studied by a contact angle/interface system. The current approach may open a new avenue to practical applications in windshields, eyeglasses, swimming goggles, periscopes, lenses in laparoscopes, windows of high rise buildings and solar cells, etc.

Luminescence features of nanocomposites of silicon-organic polymer/porous SiO2 and TiO2 films

Photoluminescence properties of poly(di-n-hexylsilane) (PDHS) films as well as of PDHS adsorbed in nanoporous silica and titania films, and dispersed in films of silica nanoparticles have been investigated in a wide temperature range of 15–300K. Photoluminescence spectra of all compounds were found to be dominated by the aggregate band. PDHS photoluminescence has been quenched by about 5 times in nanoporous titania film in comparison with the silica film. Weak temperature dependences of PDHS photoluminescence intensities, lifetimes, band positions, and bandwidths have been observed in nanoporous silica and titania films in comparison with neat PDHS film and PDHS dispersed in films of silica nanoparticles. It has been interpreted as a consequence of slow energy migration and weak photoluminescence quenching in nanoporous films.

Nanoporous Silica Films as Novel Biomaterial: Applications in the Middle Ear

Nanoporous Silica Films as Novel Biomaterial: Applications in the Middle Ear

Sol-gel films with immobilized acid-base indicators

Nanoporous silica films containing a number of immobilized acid-base indicators have been obtained using the sol-gel method. The variation in their absorption spectra in the visible range depending on the hydrogen ion exponent (pH) has been characterized, and the durability of their application as pH sensors has been estimated.

Synthesis and characterization of nanoporous silica film via non-surfactant template sol–gel technique

Nanoporous silicon dioxide has received growing interests owing to their peculiar application potentials in conservation and storage energy. Therefore, the development of novel and simple techniques is required for raising these nanoporous materials to industrial level. In this research, we report novel strategy for the synthesis of nanoporous SiO2 via non-surfactant template sol–gel technique for the first time. The morphology and structure of the as prepared and annealed nanoporous silica films were studied using X-ray diffraction, scanning electron microscope, Fourier transform infrared spectroscopy and nitrogen absorption/desorption technique. The results showed that highly order nanoporous silica film has been obtained at annealing temperature 600°C with average diameter 5.1nm and average pore volume 3.6cm3g−1.

Facile preparation of oriented nanoporous silica films from solvent-free liquid-crystalline mixtures

Mixtures of an amphiphilic perylene bisimide derivative and tetramethoxysilane in the absence of solvents have been found to exhibit stable columnar liquid-crystalline phases which transform into macroscopically oriented nanoporous silica films as a result of simple mechanical shearing.