In-Situ Fabrication of a Self-Aligned Selective Emitter Silicon Solar Cell Using the Gold Top Contacts To Facilitate the Synthesis of a Nanostructured Black Silicon Antireflective Layer Instead of an External Metal Nanoparticle Catalyst

Silicon solar cells with nanopore-type black silicon (b-Si) antireflection (AR) layers and self-aligned selective emitter (SE) are reported in which the b-Si structure is prepared without the traditional addition of a nanoparticle (NP) catalyst. The contact-assisted chemical etching (CACE) method is reported here for the first time, in which the metal top contacts on silicon solar cell surfaces function as the catalysts for b-Si fabrication and the whole etching process can be done in minutes at room temperature. The CACE method is based on the metal-assisted chemical etching (MACE) solution but without or metal precursor in the Si etchant (HF:H₂O₂:H₂O), and the Au top contacts, or catalysts, are not removed from the solar cell surface after the etching. The effects of etching time, HF and H₂O₂ concentration, and the HF:H₂O₂ ratio on the b-Si morphology, surface reflectivity, and solar cell efficiency have been investigated. Higher [HF] and [H₂O₂] with longer etching time cause collapse of the b-Si nanoporous structure and penetration of the p–n junctions, which are detrimental to the solar cell efficiency. The b-Si solar cell fabricated with the HF:H₂O₂:H₂O volume ratio of 3:3:20 and a 3 min etch time shows the highest efficiency 8.99% along with a decrease of reflectivity from 36.1% to 12.6% compared to that of the nonetched Si solar cell

Carbon Dioxide Absorption by Polyethylenimine-Functionalized Nanocarbons: A Kinetic Study

The kinetics of absorption of dry and wet CO₂ of polyethylenimine-functionalized single walled carbon nanotube (PEI-SWNT), graphite oxide (PEI-GO), and fullerene C₆₀ (PEI-C₆₀) were analyzed in detail using six different kinetic models: Elovich, pseudo-1st-order, pseudo-2nd-order, pseudo-<i>n</i>^th-order, modified Avrami, and extended model. It is found that PEI-SWNT follows a pseudo-2nd-order kinetics both in dry and wet CO₂, whereas PEI-GO follows a modified Avrami kinetics with values of the parameter <i>m</i> close to 1, being this a simple correction to a pure pseudo-1st-order kinetics. The kinetics of PEI-C₆₀ appears to be more complex and slower due to gas diffusion limitations. A comparison of the kinetics of PEI-GO and PEI-SWNT supports the hypothesis that carbon scaffolds of higher curvature can activate and enhance the CO₂ absorption capability of PEI

CO₂ Capture Partner Molecules in Highly Loaded PEI Sorbents

Decoupling amine loading from diffusion resistance is one of the main challenges in the development of immobilized amine CO₂ sorbents. Water has been reported to serve this goal, alleviating CO₂ diffusional hindrance in highly loaded amine sorbents. Acting as a mass transport facilitator, water is not the only partner molecule able to enhance bulk CO₂ diffusion. Herein, we show that the enhancing effect of methanol is comparable to that of water in polyethylenimine-based sorbents. Other molecules, such as ethanol, isopropanol, and chloroform, were also examined but did not appear to facilitate CO₂ transport and uptake. Based on a comparison of the Hansen solubility parameters of these molecules, it appears that polarity plays a crucial role in enhancing CO₂ diffusion together with molecular hindrance and hydrogen bonding to a lesser extent

Branched Hydrocarbon Low Surface Energy Materials for Superhydrophobic Nanoparticle Derived Surfaces

We present a new class of superhydrophobic surfaces created from low-cost and easily synthesized aluminum oxide nanoparticles functionalized carboxylic acids having highly branched hydrocarbon (HC) chains. These branched chains are new low surface energy materials (LSEMs) which can replace environmentally hazardous and expensive fluorocarbons (FCs). Regardless of coating method and curing temperature, the resulting textured surfaces develop water contact angles (θ) of ∼155° and root-mean-square roughnesses (<i>R</i>_q) ≈ 85 nm, being comparable with equivalent FC functionalized surfaces (θ = 157° and <i>R</i>_q = 100 nm). The functionalized nanoparticles may be coated onto a variety of substrates to generate different superhydrophobic materials

Silica Nanoparticle Enhancement in the Efficiency of Surfactant Flooding of Heavy Oil in a Glass Micromodel

The synergistic effects of fumed-Si nanoparticles (Si-NPs) in combination with sodium dodecyl sulfate (SDS) surfactant as suitable agents for oil displacing in enhanced oil recovery (EOR) are evaluated using a 5-spot glass micromodel. Optimum oil recovery (45%) is obtained for SDS near the critical micelle concentration; however, the addition of fumed silica nanoparticles (Si-NPs) enables a further 13% enhancement in oil recovery for the maximum concentration of the SDS/Si-NPs (2.2 wt %) as well as delaying the breakthrough point. The optimum mass ratio of SDS:Si-NP (1:11) suggests that the Si-NPs are aggregated by the SDS micelles, consistent with increased viscosity upon addition of Si-NPs. The presence of the Si-NPs also greatly increases the wettability on the glass surface with a decrease in the contact angle from 73° for SDS (1800 ppm) to 11° for SDS/Si-NPs (1800 ppm/2.0 wt %). The effective changes in the oil sweeping mechanism are directly observed in the glass micromodel and correlate to these physical measurements. The results demonstrated that addition of Si-NPs to SDS solutions made a significant improvement to oil recovery values and is potentially beneficial in EOR applications

Silica Decorated TiO₂ for Virus Inactivation in Drinking Water – Simple Synthesis Method and Mechanisms of Enhanced Inactivation Kinetics

A new method of modifying TiO₂ photocatalysts with SiO₂ is developed in which SiO₂ nanoparticles are simply mixed with TiO₂ in water under ambient conditions. This method does not require the use of toxic solvents or significant energy input. Although the SiO₂ modification slightly reduces hydroxyl free radical production, the composite SiO₂–TiO₂ nanomaterials have markedly higher photocatalytic inactivation rates for a common surrogate virus, bacteriophage MS2 (up to 270% compared to the unmodified TiO₂), due to the greatly improved adsorptive density and dark inactivation of MS2. The Langmuir isotherm describes the adsorption data well and shows that the TiO₂ modified with 5% SiO₂ has a maximum adsorption density <i>q</i>_<i>max</i> 37 times that of the unmodified TiO₂. The Langmuir–Hinshelwood model fits the photocatalytic inactivation kinetic data well. The SiO₂–TiO₂ material produces a greater maximum initial inactivation rate yet a lower intrinsic surface reaction rate constant, consistent with the reduced hydroxyl radical production and enhanced adsorption. These results suggest that modifying photocatalyst surface to increase contaminant adsorption is an important strategy to improve photocatalytic reaction efficiency. Simple and cheap synthesis methods such as that used in this study bring photocatalysis closer to being a viable water treatment option

Spatial and Contamination-Dependent Electrical Properties of Carbon Nanotubes

Two-point probe and Raman spectroscopy have been used to investigate the effects of vacuum annealing and argon bombardment on the conduction characteristics of multiwalled carbon nanotubes (MWCNTs). Surface contamination has a large effect on the two-point probe conductivity measurements which results in inconsistent and nonreproducible contacts. The electric field under the contacts is enhanced which results in overlapping depletion regions when probe separations are small (<4 μm) causing very high resistances. Annealing at 200 and 500 °C reduced the surface contamination on the MWCNT, but high resistance contacts still did not allow intrinsic conductivity measurements of the MWCNT. The high resistance measured due to the overlapping depletion regions was not observed after annealing to 500 °C. Argon bombardment reduced the surface contamination more than vacuum annealing at 500 °C but caused a slight increase in the defects concentration, enabling the resistivity of the MWCNT to be calculated, which is found to be dependent on the CNT diameter. The observations have significant implications for future CNT-based devices

Overcoming the “Coffee-Stain” Effect by Compositional Marangoni-Flow-Assisted Drop-Drying

Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the “coffee-stain” effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles

Overcoming the “Coffee-Stain” Effect by Compositional Marangoni-Flow-Assisted Drop-Drying

Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the “coffee-stain” effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles

Overcoming the “Coffee-Stain” Effect by Compositional Marangoni-Flow-Assisted Drop-Drying

Attempts at depositing uniform films of nanoparticles by drop-drying have been frustrated by the “coffee-stain” effect due to convective macroscopic flow into the contact line. Here, we show that uniform deposition of nanoparticles in aqueous suspensions can be attained easily by drying the droplet in an ethanol vapor atmosphere. This technique allows the particle-laden water droplets to spread on a variety of surfaces such as glass, silicon, mica, PDMS, and even Teflon. Visualization of droplet shape and internal flow shows initial droplet spreading and strong recirculating flow during spreading and shrinkage. The initial spreading is due to a diminishing contact angle from the absorption of ethanol from the vapor at the contact line. During the drying phase, the vapor is saturated in ethanol, leading to preferential evaporation of water at the contact line. This generates a surface tension gradient that drives a strong recirculating flow and homogenizes the nanoparticle concentration. We show that this method can be used for depositing catalyst nanoparticles for the growth of single-walled carbon nanotubes as well as to manufacture plasmonic films of well-spaced, unaggregated gold nanoparticles