Chemical Bath Deposition of Aluminum Oxide Buffer on Curved Surfaces for Growing Aligned Carbon Nanotube Arrays

Direct growth of vertically aligned carbon nanotube (CNT) arrays on substrates requires the deposition of an aluminum oxide buffer (AOB) layer to prevent the diffusion and coalescence of catalyst nanoparticles. Although AOB layers can be readily created on flat substrates using a variety of physical and chemical methods, the preparation of AOB layers on substrates with highly curved surfaces remains challenging. Here, we report a new solution-based method for preparing uniform layers of AOB on highly curved surfaces by the chemical bath deposition of basic aluminum sulfate and annealing. We show that the thickness of AOB layer can be increased by extending the immersion time of a substrate in the chemical bath, following the classical Johnson–Mehl–Avrami–Kolmogorov crystallization kinetics. The increase of AOB thickness in turn leads to the increase of CNT length and the reduction of CNT curviness. Using this method, we have successfully synthesized dense aligned CNT arrays of micrometers in length on substrates with highly curved surfaces including glass fibers, stainless steel mesh, and porous ceramic foam

Photoinduced Crystallization and Activation of Amorphous Titanium Dioxide

Titanium dioxide (TiO₂) is one of the most common photosensitive materials used in photocatalysis, solar cells, self-cleaning coatings, and sunscreens. Although the crystalline TiO₂ phases such as anatase and rutile are well-known to be photoactive, whether amorphous TiO₂ is active in photocatalytic reactions is still controversial. Here we show that amorphous TiO₂ prepared by the commonly used sol–gel method of tetrabutyl titanate hydrolysis is active in photocatalytic water reduction and methylene blue oxidation under the irradiation of a xenon lamp. The amorphous TiO₂ gains photoactivity after an induction period of approximately an hour, suggesting that phase transition is involved. Using an extensive series of microscopic and spectroscopic analyses, we further show that the photoinduced crystallization by amorphous TiO₂ forms a nanometer-thin layer of rutile nanocrystallites under the irradiation in the middle ultraviolet range. The resulting core–shell nanoparticles have a bandgap of 3.3 eV and are enriched with surface-active sites including reduced titanium and oxygen vacancies. The revelation of photoinduced crystallization raises the possibility of preparing photosensitive TiO₂ using low-temperature radiation techniques that can not only save energy but also incorporate heat-sensitive components into manufacturing

Isokinetic Temperature and Size-Controlled Activation of Ruthenium-Catalyzed Ammonia Borane Hydrolysis

Metal-catalyzed hydrolysis is an important reaction for releasing hydrogen stored in ammonia borane, a promising fuel form for the future hydrogen economy, under ambient conditions. A variety of catalysts made of different transition metals have been investigated to improve the efficiency of hydrogen generation; however, little attention has been given to the possible influence of the compensation effect on catalyst design. Using face-centered cubic (FCC) packed ruthenium (Ru) nanoparticles supported on layered double oxide nanodisks, we show that the compensation effect produces an isokinetic temperature at <i>T</i>_i = 17.5(±1.6) °C within the operational range of hydrogen generation. We further show that the turnover frequency (TOF) of the reaction can be maximized for operations performed below <i>T</i>_i by reducing the size of Ru-FCC nanoparticles, which increases the fraction of edge and corner atoms and lowers the activation energy. At 15 °C, TOF can reach more than 90% of the theoretical maximum (0.72 mol m^–2 h^–1) using Ru nanoparticles having an average diameter of 2 nm and giving an activation energy of 17.7(±0.7) kJ mol^–1. To generate hydrogen above <i>T</i>_i, TOF is maximized by using enlarged Ru nanoparticles with a diameter of 3.8 nm, giving an activation energy of 87.3(±5.8) kJ mol^–1. At 25 °C, these nanoparticles produce a TOF of 1.8(±0.3) mol m^–2 h^–1, representing at least an 81% increase in comparison to the highest TOF reported for elemental catalysts. Our results suggest that controlling the reaction activation energy by adjusting nanoparticle size represents a viable strategy for designing catalysts that can maximize TOF for ammonia borane hydrolysis operated both below and above the isokinetic temperature

Binder-Free Carbon Nanotube Electrode for Electrochemical Removal of Chromium

Electrochemical treatment of chromium-containing wastewater has the advantage of simultaneously reducing hexavalent chromium (Cr^VI) and reversibly adsorbing the trivalent product (Cr^III), thereby minimizing the generation of waste for disposal and providing an opportunity for resource reuse. The application of electrochemical treatment of chromium is often limited by the available electrochemical surface area (ESA) of conventional electrodes with flat surfaces. Here, we report the preparation and evaluation of carbon nanotube (CNT) electrodes consisting of vertically aligned CNT arrays directly grown on stainless steel mesh (SSM). We show that the 3-D organization of CNT arrays increases ESA up to 13 times compared to SSM. The increase of ESA is correlated with the length of CNTs, consistent with a mechanism of roughness-induced ESA enhancement. The increase of ESA directly benefits Cr^VI reduction by proportionally accelerating reduction without compromising the electrode’s ability to adsorb Cr^III. Our results suggest that the rational design of electrodes with hierarchical structures represents a feasible approach to improve the performance of electrochemical treatment of contaminated water

Growth of Manganese Oxide Nanostructures Alters the Layout of Adhesion on a Carbonate Substrate

Nanostructures grown under natural conditions can modify the layout of adhesion on mineral surfaces. Using force-volume microscopy and a silicon-nitride probe, we measure changes in adhesion when a patchy overgrowth of manganese oxide nanostructures forms on the surface of rhodochrosite. For the most part, the observations show that the adhesive force to the nanostructures is dominated by van-der-Waals attraction. Measurements made across an area of the surface provide a frequency distribution of adhesive forces, and the mode of this distribution is 166 pN at pH 5.0, increasing to a maximum of 692 pN at pH 7.1, followed by a decrease to 275 pN at pH 9.7. At a few sampling locations over some nanostructures, electrostatic repulsion overtakes van-der-Waals attraction and thus results in negative adhesive forces (i.e., repulsion). Local roughness causes this effect. In comparison to the oxide nanostructures, the exposed rhodochrosite substrate has negligible adhesive force with the probe over the same pH range, suggesting both weak van-der-Waals attraction and weak electrostatic repulsion over this pH range. The quantitative mapping of adhesive force applied more generally to the study of other nanostructures can lead to an improved mechanistic understanding of how nanostructure growth influences contaminant immobilization and bacterial attachment

Mechanism and Kinetics of Cyanogen Chloride Formation from the Chlorination of Glycine

Glycine is an important precursor of cyanogen chloride (CNCl)a disinfection byproduct (DBP) found in chlorinated drinking water. To model CNCl formation from glycine during chlorination, the mechanism and kinetics of the reaction between glycine and free chlorine were investigated. Kinetic experiments indicated that CNCl formation was limited by either the decay rates of N,N-dichloroglycine or a proposed intermediate, N-chloroiminocarboxylate, ClNCHCO2-. Only the anionic form of N,N-dichloroglycine, NCl2CH2CO2-, however, decays to form CNCl, while the protonated neutral species forms N-chloromethylimine. At pH > 6, glycine-nitrogen is stoichiometrically converted to CNCl, while conversion decreases at lower pH due to the formation of N-chloromethylimine. Under conditions relevant to drinking water treatment, i.e., at pH 6 to 8 and with free chlorine in excess, a simplified rate expression for the concentration of glycine-nitrogen converted to CNCl, [CNCl]f, applies: where [Cl2-Gly]T,o is the initial concentration of total N,N-dichloroglycine, k2* is the first-order decay constant for ClNCHCO2-, k2*(s-1) = 1012(±4) exp(−1.0(±0.3) × 104/T), and T is the absolute temperature in K. Kinetic expressions for d[CNCl]/dt when free chlorine is in excess, however, must also account for the significant decay of CNCl by hypochlorite-catalyzed hydrolysis, which has been characterized in previous studies. Although CNCl formation is independent of the free chlorine concentration, higher chlorine concentrations promote its hydrolysis

Interfacial Forces are Modified by the Growth of Surface Nanostructures

Nanostructures formed by chemical reaction can modify the interfacial forces present in aqueous solution near a surface. This study uses force-volume microscopy to explore this phenomenon for the growth of manganese oxide nanostructures on rhodochrosite. The interfacial forces above the oxide nanostructures are dominated by electrostatic repulsion for probe−surface separations greater than ca. 2 nm but are overtaken by van der Waals attraction for shorter distances. Across the investigated pH range 5.0−9.7, the maximum repulsive force occurs 2.4 (±1.1) nm above the oxide nanostructures. The magnitude of the repulsive force decreases from pH 5.0 to 6.5, reaches its minimum at 6.5, and then increases steadily up to pH 9.7. Specifically, fmax(pN) = 23(±4)[6.8(±2.1)  pH] for pH fmax(pN) = 19(±2)[pH  6.1(±1.0)] for pH ≥ 6.5. This dependence indicates that oxide nanostructures have a point of zero charge in the pH range 6−7. In comparison to the nanostructures, the rhodochrosite substrate induces only small interfacial forces in the same pH range, suggesting a neutral or weakly charged surface. The quantitative mapping of interfacial forces, along with the associated influencing factors such as pH or growth of nanostructures, provides a basis for more sophisticated and accurate modeling of processes affecting contaminant immobilization and bacterial attachment on mineral surfaces under natural conditions

Hierarchical Carbon Nanotube Membrane-Supported Gold Nanoparticles for Rapid Catalytic Reduction of <i>p</i>‑Nitrophenol

Gold nanoparticles (AuNPs) have attracted increasing attention as catalysts for pollutant degradation because of their unique reactivity. Direct use of gold nanoparticles in water treatment faces prohibitive challenges from nanoparticle aggregation and post-treatment separation. To prevent nanoparticles from aggregating and eliminate the need for separation, we affixed AuNPs on hierarchical carbon nanotube membrane (HCNM) that was approximately 50 μm thin with 10 μm × 10 μm openings as pores for water passage. HCNM was fabricated by growing vertically aligned carbon nanotube (CNT) arrays on stainless steel mesh. Using p-nitrophenol (PNP) as model pollutant, we showed that in batch experiments HCNM-supported AuNPs retained 78% of their catalytic capability compared to suspended AuNPs. The slight reduction in reactivity was attributed to the blockage of part of the gold surface at the AuNP–CNT juncture. When the membrane was used in continuous flow-through operation, HCNM-supported AuNPs achieved 71% of the maximum catalytic ability measured in batch. The rapid kinetics obtained with HCNM-supported AuNPs was in great contrast to the slow kinetics that one would expect for a rigid membrane of similar configuration. For a rigid membrane, water passing through microscopic pores was confined as laminar flow and thus would not mix well with catalysts affixed on pore walls. For HCNM, CNTs aligning pore walls were flexible so that they could move vigorously to create a chaotic mixing condition and promote AuNP-catalyzed PNP reduction

Surface-Potential Heterogeneity of Reacted Calcite and Rhodochrosite

Nanostructures can form on mineral surfaces through reactions with H2O or O2 in the natural environment. In this study, nanostructures on the (101̄4) surfaces of calcite and rhodochrosite are characterized by their surface potentials using Kelvin probe force microscopy. Water-induced nanostructures on calcite have a topographic height of 1.1 (±0.6) nm and an excess surface potential of 126 (±31) mV at 45% relative humidity. The corresponding values for oxygen-induced nanostructures on rhodochrosite at the same RH are 1.3 (±0.7) nm and 271 (±14) mV, respectively. For increasing relative humidity on calcite, the topographic height of the nanostructures increases while their excess surface potential remains unchanged. In comparison, on rhodochrosite the topographic height remains unchanged for increasing relative humidity but excess surface potential decreases. The nonzero excess surface potentials indicate that the nanostructures have compositions different from their parent substrates. The surface-potential heterogeneity associated with the distributed nanostructures has important implications for reactivity in both gaseous and aqueous environments. Taking into consideration such heterogeneities, which are not included in state-of-the-art models, should improve the accuracy of the predictions of contaminant fate and transport in natural environments

Surface-Potential Heterogeneity of Reacted Calcite and Rhodochrosite

Nanostructures can form on mineral surfaces through reactions with H2O or O2 in the natural environment. In this study, nanostructures on the (101̄4) surfaces of calcite and rhodochrosite are characterized by their surface potentials using Kelvin probe force microscopy. Water-induced nanostructures on calcite have a topographic height of 1.1 (±0.6) nm and an excess surface potential of 126 (±31) mV at 45% relative humidity. The corresponding values for oxygen-induced nanostructures on rhodochrosite at the same RH are 1.3 (±0.7) nm and 271 (±14) mV, respectively. For increasing relative humidity on calcite, the topographic height of the nanostructures increases while their excess surface potential remains unchanged. In comparison, on rhodochrosite the topographic height remains unchanged for increasing relative humidity but excess surface potential decreases. The nonzero excess surface potentials indicate that the nanostructures have compositions different from their parent substrates. The surface-potential heterogeneity associated with the distributed nanostructures has important implications for reactivity in both gaseous and aqueous environments. Taking into consideration such heterogeneities, which are not included in state-of-the-art models, should improve the accuracy of the predictions of contaminant fate and transport in natural environments