Electrodeposited iridium oxide (IrOx) pH sensors are gaining interest in numerous biomedical applications, attributed to their superior accuracy, straightforward fabrication, and compatibility with other microfabrication processes. However, the pH sensing range and stability of these sensors have been long-entrenched challenges to control. In this study, we explored the sensor’s behavior under pH conditions beyond its typical sensing range, unearthing potential correlations between the sensing range and other influential factors. Based on these findings, we proposed and implemented a practical method to manipulate the sensing range to designated pH regions. This methodology significantly expanded the sensor’s applicability to encompass extreme acidic or basic environments. A series of investigations revealed that, under such severe conditions, the sensor’s stability - as quantified by drift - could be enhanced more than tenfold post-treatment. Moreover, other stability metrics, including linearity and hysteresis, exhibited notable improvements through this technique, as validated by titration tests in the extended pH sensing ranges. This research offers meaningful insights and a promising approach to enhance the sensing range and stability of electrodeposited IrOx pH sensors, thereby potentially progressing their practical application in biomedical domains.

Single-atom catalysts (SACs) offer efficient metal utilization and distinct reactivity compared to supported metal nanoparticles. Structure-function relationships for SACs often assume that active sites have uniform coordination environments at particular binding sites on support surfaces. Here, we investigate the distribution of coordination environments of Pt SAs dispersed on shape-controlled anatase TiO 2 supports specifically exposing (001) and (101) surfaces. Pt SAs on (101) are found on the surface, consistent with existing structural models, whereas those on (001) are beneath the surface after calcination. Pt SAs under (001) surfaces exhibit lower reactivity for CO oxidation than those on (101) surfaces due to their limited accessibility to gas phase species. Pt SAs deposited on commercial-TiO 2 are found both at the surface and in the bulk, posing challenges to structure-function relationship development. This study highlights heterogeneity in SA coordination environments on oxide supports, emphasizing a previously overlooked consideration in the design of SACs.

Electrodeposited amorphous hydrated iridium oxide (IrOx) is a promising material for pH sensing due to its high sensitivity and the ease of fabrication. However, durability and variability continue to restrict the sensor’s effectiveness. Variation in probe films can be seen in both performance and fabrication, but it has been found that performance variation can be controlled with potentiostatic conditioning (PC). To make proper use of this technique, the morphological and chemical changes affecting the conditioning process must be understood. Here, a thorough study of this material, after undergoing PC in a pH-sensing-relevant potential regime, was conducted by voltammetry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Fitting of XPS data was performed, guided by raw trends in survey scans, core orbitals, and valence spectra, both XPS and UPS. The findings indicate that the PC process can repeatably control and conform performance and surface bonding to desired calibrations and distributions, respectively; PC was able to reduce sensitivity and offset ranges to as low as ±0.7 mV/pH and ±0.008 V, respectively, and repeat bonding distributions over ~2 months of sample preparation. Both Ir/O atomic ratios (shifting from 4:1 to over 4.5:1) and fitted components assigned hydroxide or oxide states based on the literature (low-voltage spectra being almost entirely with suggested hydroxide components, and high-voltage spectra almost entirely with suggested oxide components) trend across the polarization range. Self-consistent valence, core orbital, and survey quantitative trends point to a likely mechanism of ligand conversion from hydroxide to oxide, suggesting that the conditioning process enforces specific state mixtures that include both theoretical Ir(III) and Ir(IV) species, and raising the conditioning potential alters the surface species from an assumed mixture of Ir species to more oxidized Ir species.

Complex concentrated oxides (CCOs) are an emerging material class that includes high-entropy oxide (HEOs) and entropy-stabilized oxides (ESOs), whose unprecedented properties stem from disorder-induced distributions in electronic structure and chemistry caused by stabilizing many-cation (typically > 5) solid solutions [1-2]. Integrating these materials into composites with nanoscale tunability will enable tailored (multi)functionality beyond what is possible in a single phase [3]. Here, we demonstrate a novel, highly extensible approach, exsolution self-assembly (ESA), to realize CCO-based nanocomposite thin films with intricate multi-element nanostructures [4]. Using pulsed-laser deposition (PLD), we selectively reduce cations in a 6-cation model perovskite CCO, inducing defect-interaction-driven exsolution and simultaneous self-assembly of metal nanorods and metal-oxide core-shell nanoparticles, depending on film growth conditions. ESA using a CCO precursor enables novel multielement composite morphologies with finely tuned sub-phase chemical composition that are not possible by conventional exsolution or self-assembly independently. By mapping the distribution of distinct sub-phase structure and chemistry with sub-nanoscale electron microscopy and atom probes, we show that Ellingham's model of cation reducibility predicts nanostructure chemical composition while mass transport via extended defects governs local morphology formation. Importantly for various applications, the electronic conductivity of our nanocomposites exceeds 0.1 S/cm at room temperature, with transport correlated to nanostructure formation primarily through B-site cation mixed valency and site occupancy in the CCO matrix. A correlated analysis using aberration-corrected scanning transmission electron microscopy (STEM) imaging, energy dispersive X-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS), geometric phase analysis (GPA) strain mapping, atom probe tomography (APT) with 3D mass spectrometry, and X-ray photoemission spectroscopy (XPS) was performed to elucidate the fundamental nanostructure formation mechanisms underlying the highly tailorable synthesis approach. We show that the method achieves uniform bulk exsolution with short process time and is manipulated by tuning readily accessible PLD conditions. Detailed characterization resolved the impact of increasing oxygen vacancy concentration and varying cation reducibility on nanostructure self-assembly. For example, metal nanorods grow from the bottom of the thin film to the top surface, with growth restricted by compressive stress exerted by the matrix in the in-plane direction. Metal-oxide core-shell nanoparticles embedded in the matrix form via seed growth effect triggered by metal nanoparticle formation followed by subsequent reduction of additional mobile cations. This work demonstrates a route towards vast morphological tunability and compositional complexity through nanocomposite design of CCO thin films. Given CCOs’ vast combinatorial space, ESA is expected to be highly extensible via application to novel compositions and crystal structures, with the understanding presented here enabling one to take advantage of chemical complexity in a rational way. References [1] M. Brahlek...A. Yamamoto (2022) APL Materials. [2] H. Guo...W. J. Bowman (2022) Journal of Materials Research. [3] S. Misra and H. Wang (2021) Materials Horizons. [4] H. Guo...W. J. Bowman (2023) Submitted. Acknowledgements H.G., M.B., H.G., X.W., J.D.S. and W.J.B. acknowledge support from American Chemical Society's Petroleum Research Fund (Grant PRF# 61961-DNI), National Science Foundation (NSF) CAREER DMR-2042638, NSF Materials Research Science and Engineering Center (MRSEC DMR-2011967) and UC Irvine Materials Research Institute (IMRI). L.L. and C.M. acknowledge the NSF MRSEC (DMR-1720139), the Northwestern University Center for Atom-Probe Tomography (NUCAPT via NSF-MRI DMR-0420532 and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs), the SHyNE Resource (NSF ECCS-2025633), and the Initiative for Sustainability and Energy (ISEN).

We have characterized soot particles measured in situ in a laminar co-flow ethylene-air diffusion flame using small-angle X-ray scattering (SAXS). The analysis includes temperature measurements made with coherent anti-Stokes Raman spectroscopy (CARS) and complements soot volume-fraction and maturity measurements made with laser-induced incandescence (LII). We compared the results of fits to the SAXS measurements using a unified model and a fractal core-shell model. Power-law parameters yielded by the unified model indicate that aggregates of primary particles are in the mass-fractal regime, whereas the primary particles are in the surface-fractal regime in the middle of the flame. Higher and lower in the flame, the primary-particle power-law parameter approaches 4, suggesting smooth primary particles. These trends are consistent with fits using the fractal core-shell model, which indicate that particles have an established core-shell structure in the middle of the flame and are internally homogeneous at higher and lower heights in the flame. Primary-particle size distributions derived using the fractal core-shell model demonstrate excellent agreement with distributions inferred from transmission electron microscopy (TEM) images in the middle of the flame. Higher in the flame, a second small mode appears in the size distributions, suggesting particle fragmentation during oxidation. Surface oxidation would explain (1) aggregate fragmentation and (2) loss of core-shell structure leading to smoother primary-particle surfaces by removal of carbon overlayers. SAXS measurements are much more sensitive to incipient and young soot particles than LII and demonstrate significant volume fraction from particles low in the flame where the LII signal is negligible.

Unlike artificial intelligent systems based on computers, which must be programmed for specific tasks, the human brain can learn in real-time to create new tactics and adapt to complex, unpredictable environments. Computers embedded in artificial intelligent systems can execute arbitrary inference algorithms capable of outperforming humans at specific tasks. However, without real-time self-programming functionality, they must be preprogrammed by humans and will likely to fail in unpredictable environments beyond their preprogrammed domains. In this work, a Si-based synaptic resistor (synstor) was developed by integrating Al2Ox/TaOy materials to emulate biological synapses. The synstors were characterized, and their operation mechanism based on the charge stored in the oxygen vacancies in the Al2Ox material was simulated and analyzed, to understand the inference, learning, and memory functions of the synstors. A self-programming neuromorphic integrated circuit (SNIC) based on synstors was fabricated to execute inference and learning algorithms concurrently in real-time with an energy efficiency more than six-orders of magnitudes higher than those of standard digital computers. The SNIC dynamically modified its algorithm in a real-time learning process to control a morphing wing, thus successfully improving its lift-to-drag force ratio and recovering the wing from stall in complex aerodynamic environments. The synaptic resistor circuits can potentially circumvent the fundamental limitations of computers, thus providing a platform analogous to neurobiological network with real-time self-programming functionality for artificial intelligent systems.

We have developed a strategy for distinguishing between small-angle X-ray scattering (SAXS) from gas-phase species and newly formed nanoparticles in mixed gas- and particle-phase reacting flows. This methodology explicitly accounts for temperature-dependent scattering from gases. We measured SAXS in situ in a sooting linear laminar partially premixed co-flow ethylene/air diffusion flame. The scattering signal demonstrates a downward curvature as a function of the momentum transfer (q) at q values of 0.2-0.57 Å-1. The q-dependent curvature is consistent with the Debye equation and the independent-atom model for gas-phase scattering. This behavior can also be modeled using the Guinier approximation and could be characterized as a Guinier knee for gas-phase scattering. The Guinier functional form can be fit to the scattering signal in this q range without a priori knowledge of the gas-phase composition, enabling estimation of the gas-phase contribution to the scattering signal while accounting for changes in the gas-phase composition and temperature. We coupled the SAXS measurements with in situ temperature measurements using coherent anti-Stokes Raman spectroscopy. This approach to characterizing the gas-phase SAXS signal provides a physical basis for distinguishing among the contributions to the scattering signal from the instrument function, flame gases, and nanoparticles. The results are particularly important for the analysis of the SAXS signal in the q range associated with particles in the size range of 1-6 nm.

The presence of molecular monolayers on semiconductor surfaces can improve the stability of semiconductor interfaces by inhibiting the growth of native oxides and defects which affect the materials’ electronic properties. The development of catalytically active passivated interfaces on semiconductor materials presents a useful material design for value‐added product conversion. Herein, an iron‐based catalyst covalently attached to silicon (Si) is reported for the investigation of activity and electrochemical decomposition pathways of diiron hydrogenase enzyme mimics. The employed catalyst, Fe 2 (CO) 6 (µ‐S‐C 6 H 4 ‐p‐OH) 2 ([FeFe]), mimics the active sites of these enzymes. Surface modification using this catalyst passivates the interface, hindering the formation of native SiO 2 for more than 300 h. [FeFe] modification improves the overpotential required to produce 10 mA cm –2 by 100 mV, with a hydrogen evolution rate of 2.31 × 10 –5 mol h –1 cm –2 (−0.78 V versus RHE). However, structural rearrangement transpires within 1 h of electrolysis, where Fe‐S bond dissociates at the catalytic center, resulting in an aromatic linkage modified Si interface. While semiconductor−catalyst interfaces have often been reported in the literature, their decomposition pathways have received limited discussion. Herein, this Si−[FeFe] interface is used as a tool for understanding the activity and decomposition mechanisms of the attached molecular catalyst.

Multi-functional membranes with high permeance and selectivity that can mimic nature's designs have tremendous industrial and bio-medical applications.

We report on the oxygen vacancy effects controlling oxygen evolution activity of Sr2-xCaxFe2O6-delta perovskites. This system shows a tunable surface reactivity toward oxygen evolution reaction (OER) through changing the A-site composition (A = Sr2-xCax) and the oxygen vacancy content, delta. We use solid-state synthesis to control the structural ordering and the content of oxygen vacancy. Samples with chemical compositions of x >= 0.75 crystallize in the brownmillerite-type structure, while those with low calcium content (x < 0.75) crystallize in either perovskite or brownmillerite structure. Our electrochemical analysis shows that compounds with mixed A-site compositions outperform Ca2Fe2O6-delta and Sr2Fe2O6-delta and reveal several compounds with higher activity than LaCoO3-delta (measured at 2 mA cm(-2)). The oxygen vacancy content (delta) of the compounds was quantified using X-ray photoelectron spectroscopy. The delta vs A-site composition trend shows a transition point where the structural transition was also observed, i.e., x similar or equal to 0.75. Interestingly, electrochemical activity correlates with delta, and compounds with delta -> 1 tend to show higher activity. Our results show how short-range and long-range structural ordering and oxygen vacancy content can be employed to tune catalytic properties of Fe-based perovskites.

Catalytic substrate, which is devoid of expensive noble metals and enzymes for hydrogen peroxide (H2O2), reduction reactions can be obtained via nitrogen doping of graphite. Here, we report a facile fabrication method for obtaining such nitrogen doped graphitized carbon using polyacrylonitrile (PAN) mats and its use in H2O2 sensing. A high degree of graphitization was obtained with a mechanical treatment of the PAN fibers embedded with carbon nanotubes (CNT) prior to the pyrolysis step. The electrochemical testing showed a limit of detection (LOD) 0.609 µM and sensitivity of 2.54 µA cm−2 mM−1. The promising sensing performance of the developed carbon electrodes can be attributed to the presence of high content of pyridinic and graphitic nitrogens in the pyrolytic carbons, as confirmed by X-ray photoelectron spectroscopy. The reported results suggest that, despite their simple fabrication, the hydrogen peroxide sensors developed from pyrolytic carbon nanofibers are comparable with their sophisticated nitrogen-doped graphene counterparts.

N-type electrically conductive ultrananocrystalline diamond (UNCD) films were deposited using the hot filament chemical vapor deposition technique with a gas mixture of H2, CH4 and NH3. Depending on the deposition temperature and ammonia feed gas concentration, which serves as a nitrogen source, room temperature electrical conductivities in the order of 10−2 to 5 × 101 S/cm and activation energies in the meV range were achieved. In order to understand the origin of the enhanced electrical conductivity and clarify the role of ammonia addition to the process gas, a set of UNCD films was grown by systematically varying the ammonia gas phase concentration. These samples were analyzed with respect to their morphology and electrical properties as well as their carbon and nitrogen bonding environments. Temperature dependent electrical conductivity measurements (300–1200 K) show that the electrical conductivity of the samples increases with temperature. The near edge x-ray absorption fine structure measurements reveal that the electrical conductivity of the UNCD films does not correlate directly with ammonia addition, but depends on the total amount of sp2 bonded carbon in the deposited films.

Development of next generation red phosphors for commercial lighting requires understanding of how increased luminescence is achieved by various treatment strategies. In this work, we compare co-doping with Nb to NH3 treatment of CaTiO3:Pr phosphors to reveal a general mechanism responsible for the increased luminescence. The phosphors were synthesized using standard solid-state synthesis techniques and the fluorescence was characterized for potential use in fluorescent lighting, with 254 nm excitation. The lifetime of the fluorescence was determined and used to identify a change in a trap state by the co-doping of Nb5+ in the phosphor. The oxidation state of the Pr was probed by NEXAFS and revealed that both Nb5+ co-doping and NH3 treatment reduced the number of non-fluorescing Pr4+ centers. Calculations were performed to determine the energetically favorable defects. Vacuum annealing was also used to further probe the nature of the trap state. It was determined that NH3 treatments reduce the number of Pr4+ non-fluorescing centers, while Nb5+ co-doping additionally reduces the number of excess oxygen trap states that quench the fluorescence. (C) 2016 Elsevier B.V. All rights reserved.

We report high efficiency luminescence with a manganese-doped aluminum nitride red-emitting phosphor under 254 nm excitation, as well as its excellent lumen maintenance in fluorescent lamp conditions, making it a candidate replacement for the widely deployed europium-doped yttria red phosphor. Solid-state reaction of aluminum nitride powders with manganese metal at 1900 °C, 10 atm N2 in a reducing environment results in nitrogen deficiency, as revealed by diffuse reflectance spectra. When these powders are subsequently annealed in flowing nitrogen at 1650 °C, higher nitrogen content is recovered, resulting in white powders. Silicon was added to samples as an oxygen getter to improve emission efficiency. NEXAFS spectra and DFT calculations indicate that the Mn dopant is divalent. From DFT calculations, the UV absorption band is proposed to be due to an aluminum vacancy coupled with oxygen impurity dopants, and Mn2+ is assumed to be closely associated with this site. In contrast with some previous reports, we find that the highest quantum efficiency with 254 nm excitation (Q.E. = 0.86±0.14) is obtained in aluminum nitride with a low manganese doping level of 0.06 mol%. The principal Mn2+ decay of 1.25 ms is assigned to non-interacting Mn sites, while additional components in the microsecond range appear with higher Mn doping, consistent with Mn clustering and resultant exchange coupling. Slower components are present in samples with low Mn doping, as well as strong afterglow, assigned to trapping on shallow traps followed by detrapping and subsequent trapping on Mn. Figure 1

Carbon nanotube porins (CNTPs), small segments of carbon nanotubes capable of forming defined pores in lipid membranes, are important future components for bionanoelectronic devices as they could provide a robust analog of biological membrane channels. In order to control the incorporation of these CNT channels into lipid bilayers, it is important to understand the structure of the CNTPs before and after insertion into the lipid bilayer as well as the impact of such insertion on the bilayer structure. Here we employed a noninvasive in situ probe, small-angle X-ray scattering, to study the integration of CNT porins into dioleoylphosphatidylcholine bilayers. Our results show that CNTPs in solution are stabilized by a monolayer of lipid molecules wrapped around their outer surface. We also demonstrate that insertion of CNTPs into the lipid bilayer results in decreased bilayer thickness with the magnitude of this effect increasing with the concentration of CNTPs.

Graphene/metal oxide (GMO) nanocomposites promise a broad range of utilities for lithium ion batteries (LIBs), pseudocapacitors, catalysts, and sensors. When applied as anodes for LIBs, GMOs often exhibit high capacity, improved rate capability and cycling performance. Numerous studies have attributed these favorable properties to a passive role played by the exceptional electronic and mechanical properties of graphene in enabling metal oxides (MOs) to achieve near-theoretical capacities. In contrast, the effects of MOs on the active lithium storage mechanisms of graphene remain enigmatic. Via a unique two-step solvent-directed sol-gel process, we have synthesized and directly compared the electrochemical performance of several representative GMOs, namely Fe2O3/graphene, SnO2/graphene, and TiO2/graphene. We observe that MOs can play an equally important role in empowering graphene to achieve large reversible lithium storage capacity. The magnitude of capacity improvement is found to scale roughly with the surface coverage of MOs, and depend sensitively on the type of MOs. We define a synergistic factor based on the capacity contributions. Our quantitative assessments indicate that the synergistic effect is most achievable in conversion-reaction GMOs (Fe2O3/graphene and SnO2/graphene) but not in intercalation-based TiO2/graphene. However, a long cycle stability up to 2000 cycles was observed in TiO2/graphene nanocomposites. We propose a surface coverage model to qualitatively rationalize the beneficial roles of MOs to graphene. Our first-principles calculations further suggest that the extra lithium storage sites could result from the formation of Li2O at the interface with graphene during the conversion-reaction. These results suggest an effective pathway for reversible lithium storage in graphene and shift design paradigms for graphene-based electrodes.