Determination of the surface hydrophobicity or wettability of nanomaterials and nanoparticles (NPs) is often challenged by the heterogeneous properties of NPs that vary with particle size, shape, surface charge, aggregation states, and surface sorption or coating. This study first summarized inherent limitations of the water contact angle, octanol–water partition coefficient ( K ow ) and surface adsorption of probe molecules in probing nanomaterial hydrophobicity. Then, we demonstrated the principle of a scanning probe method based on atomic force microscopy (AFM) for the local surface hydrophobicity measurement. Specifically, we measured the adhesion forces between functionalized AFM tips and self-assembled monolayers (SAMs) to establish a linear relationship between the adhesion forces and water contact angles based on the continuum thermodynamic approach (CTA). This relationship was used to determine the local surface hydrophobicity of seven different NPs ( i.e. , TiO 2 , ZnO, SiO 2 , CuO, CeO 2 , α-Fe 2 O 3 , and Ag), which agreed well with bulk contact angles of these NPs. Some discrepancies were observed for Fe 2 O 3 , CeO 2 and SiO 2 NPs, probably because of surface hydration and roughness effects. Moreover, the solution pH and ionic strength had negligible effects on the adhesion forces between the AFM tip and MWCNTs or C 60 , indicating that the hydrophobicity of carbonaceous nanomaterials is not influenced by pH or ionic strength (IS). By contrast, natural organic matter (NOM) appreciably decreased the hydrophobicity of MWCNTs and C 60 due to surface coating of hydrophilic NOM. This scanning probe method has been proved to be reliable and robust toward the accurate measurement of the nanoscale hydrophobicity of individual NPs or nanomaterials in liquid environments.
more »
« less
Inorganic salts and organic matter effects on nanorod, nanowire, and nanoplate MoO 3 aggregation, dissolution, and photocatalysis
Use of visible light photocatalytic nanomaterials in water treatment can be promising in treating contaminants. However, little research has been conducted examining the effects of more complex chemistries in the nanomaterial's performance. In this work, the effects of inorganic salts (NaCl and CaCl 2 ) and natural organic matter (NOM) such as humic acid (HA) and extracellular polymeric substances (EPS) on nanoparticle aggregation, dissolution, and ultimately on the photocatalytic properties of molybdenum trioxide (MoO 3 ), i.e. nanorods, nanowires, and nanoplates were examined. In the presence of NaCl, nanorod, nanowire, and nanoplate MoO 3 had similar critical coagulation concentrations, while the nanorods showed higher instability in CaCl 2 . Overall, the presence of inorganic salts caused high colloidal instability in the MoO 3 nanostructures in terms of aggregation behavior, but greatly aided in the reduction of MoO 3 dissolution. NOM presence decreased aggregation rates, albeit dissolution was not similarly affected in all three structures. Only the dissolution of the nanowire structures was reduced in the presence of HA or EPS. Furthermore, the photocatalytic activity of the nanowires and nanoplates was overall reduced when inorganic salts or natural organic matter were present. While the presence of natural organic matter alone did reduce photocatalytic effectiveness of the nanorod MoO 3 , the presence of salts seemed to negate the effects from the organic compounds. Furthermore, the presence of CaCl 2 resulted in a highly enhanced photocatalytic activity regardless of the presence of natural organic matter. The structural and chemical differences of the nanomaterials played a significant role in their aggregation, dissolution, and ability to photocatalytically degrade methylene blue in solution. This study demonstrates that a better understanding of water chemistry effects on nanomaterials is essential prior to their intended applications.
more »
« less
- Award ID(s):
- 1705511
- NSF-PAR ID:
- 10226836
- Date Published:
- Journal Name:
- Environmental Science: Nano
- Volume:
- 7
- Issue:
- 12
- ISSN:
- 2051-8153
- Page Range / eLocation ID:
- 3794 to 3804
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Lead( iv ) oxide PbO 2 is one dominant solid phase in lead corrosion scales of drinking water distribution systems. Understanding the colloidal dispersion of PbO 2 is important for lead control in drinking water, especially under scenarios of switching the residual disinfectant from chlorine to chloramine. This study investigated the changes in lead release and colloidal dispersion from PbO 2(s) associated with the presence of natural organic matter (NOM), the introduction of chloramine, and the addition of a phosphate corrosion inhibitor in drinking water distribution systems. Experimental data showed that when NOM was present, the surface charges of PbO 2 exhibited a prominent negative shift, leading to colloidal dispersion of Pb( iv ) particles. The presence of chloramine did not significantly change the detrimental effects of NOM on the colloidal behavior of PbO 2 . In contrast, the addition of phosphate greatly reduced colloidal lead release in the size range between 0.1 and 0.45 μm, and limited lead release with colloidal sizes less than 0.1 μm to below 15 μg L −1 , i.e. , the U.S. EPA regulatory standard. The beneficial effects of phosphate addition are mainly attributed to the suppression in colloidal dispersion of Pb( iv ) particles. Meanwhile, the presence of phosphate also limits the reductive dissolution of PbO 2 via the formation of hydroxypyromorphite Pb 5 (PO 4 ) 3 OH particles. Results from this study suggest that phosphate limits the dispersion of PbO 2(s) by NOM and prevented the release of Pb( iv ) colloids into drinking water.more » « less
-
Surface adsorption of two commonly detected emerging contaminants, amlodipine (AMP) and carbamazepine (CBZ), onto model colloidal microplastics, natural organic matter (NOM), and fullerene nanomaterials have been investigated. It is found that AMP accumulation at these colloidal–aqueous interfaces is markedly higher than that of CBZ. Measurements of surface excess and particle zeta potential, along with pH-dependent adsorption studies, reveal a distinct influence of colloidal functional group on the adsorption properties of these pharmaceuticals. AMP shows a clear preference for a surface containing carboxylic group compared to an amine modified surface. CBZ, in contrast, exhibit a pH-dependent surface proclivity for both of these microparticles. The type of interactions and molecular differences with respect to structural rigidity and charge properties explain these observed behaviors. In this work, we also demonstrate a facile approach in fabricating uniform microspheres coated with NOM and C 60 nanoclusters. Subsequent binding studies on these surfaces show considerable adsorption on the NOM surface but a minimal uptake of CBZ by C 60 . Adsorption induced colloidal aggregation was not observed. These findings map out the extent of contaminant removal by colloids of different surface properties available in the aquatic environment. The methodology developed for the adsorption study also opens up the possibility for further investigations into colloidal–contaminant interactions.more » « less
-
Silicon is an extremely important technological material, but the current industrial production of silicon by carbothermic reduction of SiO₂ is energy intensive and generates CO₂ emission. Here we developed a new and more sustainable method to produce silicon nanowires in bulk quantities via direct electrochemical reduction of CaSiO₃, an abundant and inexpensive silicon source soluble in molten salts, at a low temperature of 650 ⁰C by using low melting point ternary molten salts CaCl₂-MgCl₂-NaCl, which still retains high CaSiO₃ solubility, and a supporting electrolyte of CaO, which facilitates the transport of O²¯ anions, drastically improves the reaction kinetics and enables the electrolysis at low temperatures. The Si nanowire product can be used as high-capacity Li-ion battery anode materials with excellent cycling performance. This practical strategy at lower temperatures can be applied to other molten salt systems and also promising for waste glass and coal ash recycling.more » « less
-
The absorption spectra of molecular organic chromophores in aqueous media are of considerable importance in environmental chemistry. In this work, the UV-vis spectra of benzoic acid (BA), the simplest aromatic carboxylic acid, in aqueous solutions at varying pH and in the presence of salts are measured experimentally. The solutions of different pH provide insights into the contributions from both the non-dissociated acid molecule and the deprotonated anionic species. The microscopic interpretation of these spectra is then provided by quantum chemical calculations for small cluster models of benzoic species (benzoic acid and benzoate anion) with water molecules. Calculations of the UV-vis absorbance spectra are then carried out for different clusters such as C 6 H 5 COOH·(H 2 O) n and C 6 H 5 COO − ·(H 2 O) n , where n = 0–8. The following main conclusions from these calculations and the comparison to experimental results can be made: (i) the small water cluster yields good quantitative agreement with observed solution experiments; (ii) the main peak position is found to be very similar at different levels of theory and is in excellent agreement with the experimental value, however, a weaker feature about 1 eV to lower energy (red shift) of the main peak is correctly reproduced only by using high level of theory, such as Algebraic Diagrammatic Construction (ADC); (iii) dissociation of the BA into ions is found to occur with a minimum of water molecules of n = 8; (iv) the deprotonation of BA has an influence on the computed spectrum and the energetics of the lowest energy electronic transitions; (v) the effect of the water on the spectra is much larger for the deprotonated species than for the non-dissociated acid. It was found that to reproduce experimental spectrum at pH 8.0, additional continuum representation for the extended solvent environment must be included in combination with explicit solvent molecules ( n ≥ 3); (vi) salts (NaCl and CaCl 2 ) have minimal effect on the absorption spectrum and; (vii) experimental results showed that B-band of neutral BA is not sensitive to the solvent effects whereas the effect of the water on the C-band is significant. The water effects blue-shift this band up to ∼0.2 eV. Overall, the results demonstrate the ability to further our understanding of the microscopic interpretation of the electronic structure and absorption spectra of BA in aqueous media through calculations restricted to small cluster models.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0EN00708K
