Supercritical Fluid Deposition Research Articles

Experimental and theoretical investigation on pre-deposited precursor as growth sites for monolayer MoS2 growth by supercritical fluid deposition

The assistance of promoters, Molybdates or alkali metal salts can regulate the nucleation site of MoS2 in chemical vapor deposition. However, the introduction of impurities inevitably reduces the MoS2 quality. Here, we present the growth of monolayer MoS2 by supercritical fluid deposition to pre-deposit Mo(CO)6 precursors as nucleation sites, replacing difficult to clean conventional promoters. Domain-limited homogeneous and heterogeneous reactions occur at nucleation sites, and through optimization of sulfurization parameters, high-quality monolayer MoS2 with about 20 μm domain size can be obtained by growing at 760 °C and 40 sccm argon for 20 min. Monolayer MoS2 coverage ranged from 2.6 % to 39.1 % by regulating dissolution mass in range of 100–400 mg, utilizing the highly soluble Mo(CO)6 in supercritical CO2. Density functional theory calculations show that decarbonylated Mo(CO)6 possesses higher sulfide reactivity. This study provides new insights into the impurity-free controlled site growth of two-dimensional materials.

Prediction of ionic liquid solubilities in supercritical CO2 + co-solvent systems using Peng–Robinson equation of state with accurate critical temperature

Supercritical fluid deposition (SCFD) using supercritical carbon dioxide (CO2) to impregnate substrates with ionic liquids (ILs) has attracted considerable attention for the preparation of porous-supported ILs. Because ILs are generally insoluble in pure CO2, it is essential to estimate the solubilities of ILs in supercritical CO2 + co-solvent systems for developing the SCFD process. In this study, a methodology was developed to predict the solubility of ILs in supercritical CO2 with co-solvents using the corresponding binary phase-equilibrium data. For the prediction, the binary interaction parameters for the Peng–Robinson equation of state (PR-EoS) were determined using the correlations to each phase-equilibrium system (i.e., IL + CO2, CO2 + co-solvent, and IL + co-solvent). The predictive model using the PR-EoS, which included a re-determined critical temperature as a pure component parameter of ILs, could reproduce the effects of the temperature, pressure, and co-solvent concentration on the IL solubilities in the supercritical fluid phase without any fitting parameters for the ternary systems. The average absolute relative deviations based on the logarithmic scale for the predictions were <23 %, indicating that the methodology was acceptable for describing the IL solubilities in the supercritical CO2 + co-solvent system.

Preparation of graphene oxide-doped silica aerogel using supercritical method for efficient removal of emerging pollutants from wastewater

Emerging pollutants and a large volume of unused dyes from the textile industry have been contaminating water bodies. This work introduces a scalable approach to purifying water by the adsorption of Acid green 25 (AG), Crystal Violet (CV), and Sulfamethoxazole (SMA) from an aqueous solution by graphene oxide (GO) doped modified silica aerogel (GO-SA) with supercritical fluid deposition (SFD) method. Characterization of GO-SA using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), high-resolution scanning electron microscopy (HR-SEM), thermogravimetric analysis (TGA), and Brunauer–Emmett–Teller (BET) adsorption isotherms revealed the improvement in the adsorbent surface area, and its textural properties. The high removal percentages observed in most of the experimental runs provide evidence of the excellent performance of the adsorbent towards the anionic and cationic dyes along with the antibiotic. The adsorption isotherm and kinetics showed that the Langmuir isotherm and pseudo-second-order kinetic models could explain adsorption. The adsorbent holds a higher adsorption capacity for SMA (67.07 mg g−1) than for CV (41.46 mg g−1) and AG (20.56 mg g−1) due to the higher hydrophobicity that interacts with the hydrophobic adsorbent. The GO-SA successfully removed AG, CV, and SMA with removal percentages of 98.23%, 98.71%, and 94.46%, respectively. The parameters were optimized using Central Composite Design (RSM-CCD). The prepared aerogel showed excellent reusability with a removal efficiency of > 85% even after 5 cycles. This study shows the potential of GO-SA adsorbent in textile and other wastewater purification.

Solvent selection in physical supercritical fluid deposition

Physical supercritical fluid deposition is a method of thin film deposition that does not rely on in-situ chemical reactions. Instead, the technique relies on a maximum in the isobaric saturation solubility to deposit material onto a heated substrate. As such, a strong understanding of the saturation solubility is crucial to designing a successful deposition procedure. Here, we report the first study on the problem of solvent selection in physical supercritical fluid deposition, developing qualitative categories of a solvent-solute systems by studying poly(3-hexyl thiophene) (P3HT) solutions at supercritical pressures. Overpowered solvents exhibit high saturation solubilities and do not exhibit a maximum in the isobaric solubility as exemplified by pentane. Underpowered solvents exhibit extremely low saturation solubilities and as such are not suitable for this deposition technique as demonstrated by a fluorocarbon solvent. Optimum solvents are characterized by moderate saturation solubilities, showing a maximum in the isobaric saturation solubility with respect to temperature. This behavior is demonstrated with a chlorofluorolefin solvent. We also demonstrate the properties of mixtures formed by these three classes of solvents. The mixture of an optimum solvent with an overpowered solvent appears effective for physical supercritical fluid deposition, whereas the mixture of an overpowered and underpowered solvent does not. In sum, the work demonstrates guidelines to solvent selection that will allow for the deposition of a wider range of materials.

Synthesis of high temperature resistant ZrO2-SiO2 composite aerogels via supercritical fluid deposition

A series of ZrO2-SiO2 composite aerogels have been synthesized by adjusting the volume of tetraethoxysilane (TEOS) during supercritical fluid drying process. ZrO2 aerogels have been prepared via the sol-gel reaction using ZrOCl2ꞏ8H2O and the gel initiator malic acid as raw materials. Then the ZrO2-SiO2 composite aerogels have been obtained by supercritical fluid deposition (SCFD). The samples have been characterized applying XRD, FTIR, SEM, TEM, TGA and Nitrogen adsorption-desorption. The introduction of SiO2 can effectively restrain the growth of the nanoparticles and the transformation of crystalline phase, and reduce the linear shrinkage of the aerogels. It indicates the improvement of the thermostability of the ZrO2-SiO2 composite aerogels.

Catalytic conversion of CO2 by supported ionic liquid prepared with supercritical fluid deposition in a continuous fixed-bed reactor

The capture and fixation of CO2 can effectively alleviate greenhouse effect. Propylene carbonate is a highly efficient solvent, which can be used as an excellent medium for lithium battery. It is of great significance to convert CO2 into propylene carbonate. In this paper, ionic liquid BMImBr was immobilized into the pores of SBA-15 by supercritical deposition, and BMImBr@SBA-15 was obtained for catalytic conversion of CO2. The catalytic properties of BMImBr and BMImBr@SBA-15 were investigated by batch method. Then the catalytic performance of supported catalyst BMImBr@SBA-15 was investigated by continuous method, which was carried out in a fixed-bed reactor. The continuous experiments controlled the CO2 pressure change (0.5–2.0 MPa) when the inlet liquid flow rate was 0.02 mL/min and 0.04 mL/min. Under the conditions of 0.04 mL/min and 1.0 MPa, the yield was the highest (66.14 %). In this paper, through the calculation of the reaction rate constant, it can be concluded that the continuous method can avoid the repeated packing of the catalyst and obtain a higher yield in a short time. It has a broad prospect of industrialization.

Supercritical fluid deposition for conformal Cu film formation on sub-millimeter-scale structures used to fabricate terahertz waveguides

A small-volume hot-wall batch reactor with excess precursor loading was proposed for supercritical fluid deposition of Cu during the fabrication of sub-millimeter-scale, metal-coated terahertz (THz) wave devices. Conformal film formation was experimentally demonstrated, validating our method. Our method enables a much higher precursor concentration (at least 20 mol m−3) than the conventional method (below 2 mol m−3), facilitating conformal film formation on sub-millimeter structures. Kinetic analysis revealed that our proposed method was applicable for fabricating rectangular metal-coated THz waveguides; furthermore, it was promising for monolithically integrated THz wave devices.

Synthesis of Cu-based nanoparticulated electrocatalysts for CO2 electroreduction by supercritical fluid deposition

This work has focused on the synthesis of Cu nanostructured electrocatalysts for CO2 electroreduction by supercritical fluid deposition (SFD) technique. Different synthesis strategies such as the suppression of cosolvent (methanol) and/or reducing agent (hydrogen) have been studied. Good copper deposition yields (around 85%) have been attained in the absence of hydrogen. This material (C-200Me) has been tested as catalyst in the electrochemical reduction of CO2 in gas phase and the results obtained have been compared with those achieved with a catalyst synthesised using H2 as reducing agent (C-200H2Me). CO and formic acid have been the main reduction products with both catalysts. CO2 conversion rate has been almost six times higher with C-200H2Me, what may be probably attributed to a higher proportion of Cu0/Cu+ species in this catalyst. These results indicate the higher catalytic activity of Cu0 (than that of Cu2O) in the electrocatalytic reduction of CO2 in gas phase.

Preparation of highly dispersed Pt/Hβ catalyst via supercritical fluid deposition and its catalytic performance for combustion of toluene

The emission of volatile organic compounds (VOCs) causes serious atmospheric environmental problems and catalytic combustion is one of the most promising methods in the treatment of VOCs. In this work, the Pt/Hβ catalyst was fabricated via supercritical fluid deposition (SFD) for the first time and its activity was evaluated for toluene catalytic combustion. The preparation conditions during the SFD process were optimized. Na2PtCl6 was selected as the precursor and 450 °C as the reduction temperature. The catalysts prepared by the SFD and impregnation (IM) were compared. The Pt/Hβ catalyst prepared by SFD had smaller size and higher dispersion and its T90 was 25 °C lower than that prepared by IM, which showed obvious size effect. For the similar activity, the Pt loading on catalyst prepared by SFD is only 50% of that on catalyst prepared by IM, showing obvious reduction effect for noble metal.

Self-assembly of PBTTT–C14 thin films in supercritical fluids

In order to develop more atom-economical deposition methods for polymer semiconductors, we investigated physical supercritical fluid deposition (p-SFD) to form thin films of a popular bithiophene semiconducting polymer (PBTTT).

Drop-In Test Structure to Evaluate Residual Stress in Conformally Grown Films

We propose and demonstrate a drop-in test structure to visualize and measure the residual stress in the conformally deposited film. For reliable device process of microsystems (such as 3-D MEMS), the residual stress must be controlled through quantitative evaluation at each deposition step. A small drop-in test structure placed near the main sample is suitable for monitoring film characteristics. We developed a free-standing rotating beam stress sensor as the drop-in test structure to visualize and measure the residual stress in conformally deposited films with no additional processes. The dimensions of the developed drop-in test structure chips were 5 mm ×10 mm. For a demonstration, supercritical fluid deposition (SCFD) was performed over the test structure chips under a couple of conditions. The residual stresses in SCFD films were successfully extracted by equation-based analysis.

Simultaneously deposited Pt-alloy nanoparticles over graphene nanoplatelets via supercritical carbon dioxide deposition for PEM fuel cells

One of the main barriers to the commercialization of polymer electrolyte membrane fuel cell (PEMFC) systems is the cost, which is largely due to the need of platinum (Pt)-containing catalysts. In order to overcome this issue, several methods have been developed. One of them is the supported electrocatalysts prepared by combining a low cost second metal with Pt. Motivated by this idea, in this study, nano-sized Pt-M (M = Ni, Fe, Cu) binary electrocatalysts were synthesized on the graphene nanoplatelets (GNPs) via supercritical fluid deposition (SFD) technique in order to decrease the cost and increase the activity of the catalysts. Because, the unusual properties of supercritical fluids (SFs), including tunable solvent strength, high diffusivity, low viscosity, and low surface tension, offer significant advantages over conventional techniques for materials processing. In this regard, compared to the other SFs, carbon dioxide (CO2) has received much more attention due to its easy accessibility to the supercritical state, relative benignity to the environment, chemical inertness, low toxicity, low cost. The supercritical carbon dioxide (scCO2) deposition technique involves firstly the dissolution of the metal precursor onto the support material, and then conversion of the metal precursor to its metallic form by means of thermal or chemical conversion. Synthesis of supported bimetallic nanoparticles (NPs) via scCO2 can be sequentially or simultaneously. Herein, we investigated the development of Pt-M (PtCu, PtNi, PtFe) alloy NPs on GNPs by scCO2 deposition via simultaneous addition of metal precursors. Subsequently, these catalysts were characterized by employing various characterization techniques and systems such as X-Ray Diffaction (XRD), Thermogravimetric Analysis (TGA), Inductively Coupled Plasma Mass Spectrometer (ICP-MS), Transmission Electron Microscopy (TEM), Raman Spectroscopy, Cyclic Voltammetry (CV), Rotating Disk Electrode (RDE) and PEM Fuel Cell performance tests. The obtained results revealed the formation of bimetallic catalysts within the size ranges from 1.6 to 2.1 nm. Furthermore, CV results demonstrate that PtNi/GNPs catalyst exhibit significantly high catalytic activity toward the PEM fuel cell reaction in comparison with the PtFe/GNPs and PtCu/GNPs. Also, PtNi/GNPs catalyst show about 2–4 times higher durability than the PtFe/GNPs and PtCu/GNPs catalysts in the accelerated degradation tests (ADTs). All results suggested that the PtNi/GNPs catalyst has great potential for PEM fuel cell applications.

Estimation of the number of active sites through kinetic analysis on MWCNT‐supported nanocatalysts

AbstractA semiempirical approach was derived to obtain turnover frequency (TOF) and the number of active sites in the absence of chemisorption methods. First, a deductive kinetic analysis was carried out considering the overall reaction of the selected cross‐coupling model reaction. The catalytic rate constants at various temperatures were estimated, and they were used to calculate the activation energy barrier. Then, the obtained activation energy was substituted in an energy span model to interpret semiempirical TOF values that enable the prediction of the number of active sites of the heterogeneous catalyst. The multiwalled carbon nanotube (MWCNT) supported nanocatalyst was prepared via the supercritical carbon dioxide deposition method. The reasonable number of active sites showed that the MWCNT character and the supercritical fluid deposition method seem to be effective to obtain efficient catalysts. This semiempirical approach is expected to provide a practical way to understand the catalyst properties and useful input to further scale‐up catalyst design studies.

Preparation of highly dispersed Pt–Sn/Al2O3 catalysts via supercritical fluid deposition and their catalytic performance

Highly dispersed Pt–Sn/Al2O3, which shows excellent catalytic performance was produced by SFD and their particle size effect was studied.

Physical supercritical fluid deposition of polymer films: controlling the crystallinity with pressure

The self-assembly of isotactic polypropylene films from supercritical n-pentane was studied. The effects of solution thermodynamics and fluid mechanics were reported.

The role of solvent additive in polymer crystallinity during physical supercritical fluid deposition

The self-assembly of isotactic polypropylene as deposited from supercritical pentane/acetone solutions is studied using a combination of polarized optical microscopy (POM) and grazing incidence wide angle X-ray scattering (GIWAX).

Size-Controlled Synthesis of CuO Nanoparticles by the Supercritical Antisolvent Method in SBA-15

Herein, we describe a facile method to control the size and dispersion of CuO nanoparticles (NPs) in SBA-15 by the supercritical antisolvent (SAS) method during the depressurization process of supercritical fluid deposition (SCFD), describing the mass transfer between supercritical carbon dioxide (scCO2) and a cosolvent. On reducing the concentration of the cupric nitrate precursor, the size of CuO NPs was decreased from 3.5 ± 1.0 to 1.0 ± 0.4 nm (Cu loading 14.2–12.1 wt %) under the depressurized rate of ∼20 MPa/min. Based on the size distribution and nucleation theory, the nucleation rate of the precursor could reach 1025 cm–3·s–1, and the calculated interface energy of cupric nitrate was less than 0.1 J/m2, indicating that scCO2 as an antisolvent caused the heterogeneous nucleation of the precursor on the surface of SBA-15 channels. Hence, the nucleation rate and growth of cupric nitrate were regulated under a given scCO2 depressurized rate, and then the CuO NPs’ size distribution and Cu loading were controlled subsequent to calcination.

Highly selective separation of CO2/N2 using [Emim][Tf2N] supported ionic liquid membranes prepared by supercritical fluid deposition

Supported ionic liquid membrane (SILM) using low surface tension ionic liquid [Emim] [Tf2N] and γ-Al2O3 is prepared with the supercritical fluid deposition (SCFD) method, and supercritical carbon dioxide (scCO2) shuttles and diffuses into the interior of the support to render the deposition of ionic liquid (IL) in its nano-micro pores. In this study, the influence of preparation parameters includes deposition pressure (9–18MPa), deposition time (0–24 h), ethanol amount (0–18.75 vol%) and IL amount (1.0–5 mg/g) were investigated. Besides, the performance of SILMs were evaluated by analyzing IL load, N2 and CO2 permeability, and ideal CO2/N2 selectivity. Under the optimum condition, the [Emim] [Tf2N]-SILMs exhibited separation performance with CO2 permeance of 22.6 GPU and CO2/N2 selectivity of 27.1. Furthermore, [Emim] [Tf2N]-SILM maintains proper pressure resistance and long-term stability, which is more suitable for CO2 separation, capture and storage in large-scale applications.

Synthesis and catalytic performance of highly dispersed platinum nanoparticles supported on alumina via supercritical fluid deposition

Highly dispersed platinum nanoparticles (NPs) supported on alumina were prepared by supercritical fluid deposition (SFD) with the aid of co-solvent ethanol using inorganometallic Na2PtCl6·6H2O as Pt precursor. The mean Pt particle size (1.5 nm) was obtained at the optimized deposition conditions of 40 °C and 110 bar and 3 h of deposition time. The catalysts were characterized by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), CO diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma optical emission spectrometry (ICP-OES) and hydrogen-oxygen titrations to investigate the structure-activity relationship. Compared with the catalyst prepared via an impregnation method, the Pt was highly dispersed to smaller particles for Pt/Al2O3 catalysts prepared by SFD and showed much higher catalytic activity of catalytic reforming of cyclohexane dehydrogenation.

Deposition of Cu on CNT to synthesize electrocatalysts for the electrochemical reduction of CO2: Advantages of supercritical fluid deposition technique

One of the most promising technologies for the mitigation of CO2 emissions involves its electrocatalytic reduction to valuable products. However, the development of effective catalysts is still a challenge. In this work, three different techniques have been compared for the synthesis of catalysts involving Cu-based nanoparticles deposited on carbon nanotubes (Cu/CNT): precipitation, microemulsion and supercritical fluid deposition (SFD). According to XRD, SFD allows obtaining mainly copper nanoparticles, while copper oxides are obtained by precipitation. Crystallite size of predominant Cu species of catalysts are between 7 (microemulsion) and 16 nm (SFD). The use of these catalysts in the electrochemical reduction of CO2 in gas phase produce carbon monoxide as main CO2 reduction product, followed by formic acid and methane. The catalyst obtained by microemulsion exhibited poor reproducibility. The catalyst synthesized by SFD achieved the largest CO2 conversion rate, a 30 % higher than that obtained with the one synthesized by precipitation.