Publications

Abstract Efforts to expedite accurate identification of environmental plastics pollution have strong focus on machine learning (ML) techniques. We published two Raman spectroscopy datasets to support the development of next-generation ML methods. The Raman spectra for plastics identification (RaSPI) dataset presents 402 high-quality Raman spectra with <1 cm −1 resolution between 100 and 4000 cm −1. RaSPI spans 14 plastic types and has variability in (unknown) additives. The Raman maps for plastics identification dataset (RaMPI) contains 34 two-dimensional spectroscopic maps containing 33,119 spectra. RaMPI spectra offer <1 cm −1 resolution across the fingerprint region, with significant variability in signal:noise ratios that is useful for methodology testing and validation. Both datasets contain data from pristine samples and from environmental pollution. Spectra across both datasets have been manually assigned as one of 14 different plastic classifications. The consistency and quality of these datasets make them high-value resources for researchers active in diverse topics, including training ML models for microplastics research, for developing spectroscopic processing algorithms, or for those seeking datasets to test their methodologies against.

Laser-induced graphenization is a technique for patterning carbon materials directly onto polymer substrates under ambient conditions, but suitable polymer substrates are limited. Poly(furfuryl alcohol) (PFA) is a carbon precursor often derived from biomass waste, making it an environmentally friendly material. However, the structure of PFA is highly sensitive to synthetic conditions. This work prepares PFA with systematically varied chemical functionalities to examine how polymer structural factors affect laser-induced carbon formation. Three different series of PFA were prepared with varied chemical features, as confirmed by infrared spectroscopy, then lased by a CO 2 infrared laser. Raman spectroscopy on the resultant carbons revealed that the degree of PFA polymerization altered the degrees of structural order in the laser-induced carbon. Both glassy carbon and graphitic carbon were found in the three PFA series, with the latter produced when the PFA had the highest degree of polymerization. These results identify the structural features necessary to avoid amorphous glassy carbons, which will help guide precursor development for a controlled laser-induced graphene synthesis.

IrO 2 is a commonly employed anode catalyst for CO 2 electrolysis in membrane electrode assembly (MEA) systems. However, under high current densities, its structural reconstruction leads to activity loss and stability degradation, limiting the industrial viability of CO 2 electrolysis. Herein, we demonstrated a confinement reconstruction strategy to precisely regulate the structural evolution during electrolysis. Ethylene glycol serves as a structural modulator, protecting the catalyst surface, suppressing soluble species formation, and promoting ordered structural evolution. Single-atom Ru acts as a stability enhancer, forming robust Ir–O–Ru bridging structures that facilitate an ordered transformation from a 4-fold [RuO 4 ]/[IrO 4 ] to a 6-fold symmetry [RuO 6 ]/[IrO 6 ] octahedral framework, thereby enhancing structural rigidity and long-term stability. As a result, in MEA-based CO 2 electrolysis, the catalyst achieves a stable operation at 200 mA cm –2 for 480 h, maintaining a CO selectivity above 80%. Theoretical calculations further elucidate that the enhanced stability originates from the suppression of oxygen vacancy formation, making the lattice-oxygen-mediated mechanism (LOM) potentially less favorable. This work provides insights into the structural evolution of the OER catalysts under high-current-density conditions, paving the way for large-scale CO 2 electrolysis commercialization.

Aluminum–silicon (Al–Si)-coated steel is a mainstay material for manufacturing ultrahigh strength automotive parts through hot stamping. The coating protects the blanks from oxidation and decarburization as the steel is austenitized in a furnace. It also reacts with the steel to form solid Al–Fe–Si intermetallic phases that provide long-term corrosion protection. However, the coating extends the required heating times due to its high reflectance, and, in its molten state, it can also impregnate the furnace rollers, leading to their failure. This work demonstrates a strategy to avoid these issues by depositing an Fe-rich layer on the Al–Si coating. The introduction of a second Fe source both increases the blank’s ability to absorb thermal irradiation and introduces a secondary mechanism that accelerates the reactions that convert the metallic coating into the intermetallic layer. Cross-sectional Raman microscopic mapping reveals that the intermetallic phases grow from the steel/coating interface into the Al–Si coating only after the binary phase θ (Al 13 Fe 4 ) converts to η (Al 5 Fe 2 ) at 620 °C. With the Fe-rich overlayer, however, speciation maps show that the coating can be converted completely to solid-state intermetallic phases at temperatures only slightly above the Al–Si melting point. This strategy provides a promising new method to mitigate furnace roller contamination in industrial hot stamping manufacturing lines.

We report on the surface functionalization of molybdenum disulfide flakes (MoS 2 ) with 4-nitrobenzenediazonium (4-NBD) tetrafluoroborate salt to alter their electronic band gap and subsequent excitonic properties. The 2D materials composed of a few layers were grown by chemical vapor deposition over SiO 2 /Si. Raman and photoluminescence analyses of the MoS 2 functionalized surface were conducted using confocal measurements to evaluate the spontaneous formation of polyaryl clusters and azo bonds at the surface of MoS 2 and their stability under laser exposure. Gap mode tip-enhanced Raman and photoluminescence measurements on MoS 2 flakes transferred onto the Au surface were conducted in the vicinity of the polyaryl clusters, enabling the mapping of the excitons and revealing the formation of charged excitons. These results are indicative of a charge transfer process from the MoS 2 flake to the diazonium salts, which in turn act as a p-type dopant for MoS 2. Contact potential difference measurements were conducted to quantify the electronic work function of the modified MoS 2 surface.

Identification of catalytically relevant sites in solid-state materials and our ability to manipulate such sites is critical to designing improved electrocatalysts. In this work, we prepare a series of monolayer MoS 2 using chemical vapor deposition and install varied concentrations of defects through swift heavy ion irradiation. Electron micrographs indicate that the ion irradiation procedure generates pores within MoS 2 flakes, and Raman microscopic maps show that the defects exert a strongly localized influence. The localization is strong enough that spectra acquired across individual particles can be classified in a binary fashion: regions are either affected by the irradiation-induced pore or appear as pristine MoS 2. Besides providing insight into the nature of the defects within the monolayers, this feature enables spatial resolution of regions with significant densities of such pores. This capability is used to quantify the defect density across the sample series and show that the pores located within the MoS 2 flakes are particularly active sites for electrocatalytic hydrogen evolution.

Research has shown microplastic particles to be pervasive pollutants in the natural environment, but labor-intensive sample preparation, data acquisition, and analysis protocols continue to be necessary to navigate their diverse chemistry. Machine learning (ML) classification models have shown promise for identifying microplastics from their Raman spectra, but all attempts to date have focused on the lower energy “fingerprint” region of the spectrum. We explore strategies to improve ML classification models based on the k -nearest-neighbor algorithm by including other regions of the Raman spectra. The information content inherent in C–H bonds, which occur in the higher frequency region of 2500–3600 cm –1, is found to be particularly powerful in improving classification model performance. Variations in the relative intensity of peaks arising from C–H vibrations improve identification capabilities for plastics that the fingerprint region alone struggles with, such as resolving acrylonitrile butadiene styrene from polystyrene and identifying poly(vinyl chloride), polyurethane, and polyoxymethylene. Testing of strategies to both acquire and analyze data across the two regions is explored for their efficacy and their compatibility with real-world sampling restrictions. We find that localized normalization of spectra, independently acquired in the two regions, provides the most direct and effective route to improving the ML classification performance.

Colloids are vital in many fields, including optics, energy conversion, and photothermal therapy. TiC colloids and their variants are synthesized in acetonitrile using a “green” femtosecond laser fragmentation method under varying laser-power conditions. Three distinct types of TiC colloids are produced: i) pure TiC spherical nanoparticles at low laser power (<0.5 W) through thermodynamically driven spheroidization; ii) TiC nanoparticles with a multilayer graphene shell at moderate laser power (0.5-1 W) via carbon nucleation from acetonitrile decomposition; and iii) nitridated TiC nanoparticles with a multilayer graphene shell at high laser power (1.5-2 W), achieved through nitrogen incorporation facilitated by the breakdown of strong C≡N bonds in acetonitrile. The structural integrity of TiC in all three types of nanoparticles is maintained. The nanoparticle formation mechanisms are investigated, along with the effects of the interfaces and nitridation of TiC on its light-to-heat conversion. Photothermal measurements show a significant improvement in light-to-heat conversion for TiC nanoparticles with shells compared to pure TiC. This enhancement is attributed to light trapping within the colloidal solution, caused by the refractive index mismatch between TiC and graphene. Comparative scattering, absorption, and extinction efficiencies calculations based on Mie theory confirm this effect. Further photothermal enhancement is observed in the nitridated nanoparticles attributed to superior absorption in the NIR region. This study presents a sustainable approach for synthesizing photothermally efficient TiC-based colloids with nanoscale interface engineering and doping, enabling applications in energy and biomedical technologies.

The cathodic electrochemical exfoliation of transition metal dichalcogenide (TMD) crystals using bulky tetraheptylammonium ions (THA + ) is now commonly used to produce high-quality, high-aspect-ratio flakes for diverse applications. However, most studies apply excessively high potential differences (−6 V to −8 V) between a crystal and counter electrode. This study reveals that applying extremely high potentials results in THA + intercalation, while simultaneously causing intercalant decomposition into heptane gas. This gas formation creates additional internal pressure that enhances the macroscopic expansion of the material beyond what would be expected from intercalation alone. To address this, we use a spring-loaded, three-electrode cell to control the working electrode potential. Using this setup, we compare the exfoliation yield and quality of micron-sized MoS 2 powders and large single crystals, both intercalated potentiostatically at –2.5 and −6 V vs the ferrocenium/ferrocene (Fc + /Fc) redox couple. At –2.5 V vs Fc + /Fc, complete THA + intercalation is achieved with minimal electrolyte decomposition and the associated macroscopic expansion. The intercalates exfoliate with high concentrations (∼0.5–1 mg/mL) of flakes that exhibit narrow thickness distributions (95% below 3 nm), and high aspect ratios ( L / t > 150). While all samples exfoliated in our 3-electrode cell exhibit strong photoluminescence and Raman spectra congruent with single-layer MoS 2, materials exfoliated at –2.5 V demonstrate superior quality, with fewer 1T phase impurities and adsorbates related to THA + decomposition as determined by X-ray photoelectron spectroscopy. These findings highlight a pathway to improve the quality of electrochemically exfoliated MoS 2, enhancing its already strong potential for future optoelectronic applications.

Abstract Uniform deposition of metal oxides on 2D materials, while preserving their structural integrity, is a crucial step to realize the integration of 2D materials in practical devices. In this study, we demonstrate the rapid nucleation of ZnO using atmospheric-pressure spatial chemical vapor deposition (AP-SCVD) on 2D transition metal dichalcogenides MoS 2 and WS 2. The high precursor partial pressure and uniform precursor delivery afforded by the AP-SCVD process, as compared to conventional atomic layer deposition (ALD), led to rapid ZnO nucleation on both CVD-grown MoS 2 and WS 2 in as little as 5 AP-SCVD oscillations and complete film closure was achieved on CVD-grown WS 2 flakes in less than 60 AP-SCVD oscillations. The ZnO nuclei formed larger interconnected clusters on MoS 2, whereas more-isolated islands were formed on the WS 2. Raman and photoluminescence (PL) spectroscopy revealed that AP-SCVD is a benign process that does not damage the underlying 2D materials and rather helps to passivate defects via oxygen/water adsorption from the air, when performed in an appropriate temperature window. Deposition of the ZnO was found to impact the optical and structural properties of CVD-grown MoS 2 and WS 2 differently. For the MoS 2 –ZnO heterostructure, electron doping and strain dominate, resulting in a reduction in the PL of MoS 2, whereas for the WS 2 –ZnO, strain and dielectric screening have a larger impact, resulting in an enhanced PL.

Abstract Although intricate structural assemblies contribute to enhancing the activity of electrocatalytic CO 2 reduction (ECR) to C 2+ products, blindly coupling multiple design strategies may not yield the expected results, and even inhibit the activity of intrinsic catalytic sites. Therefore, elucidating the promoting or inhibitory effects of each design strategy on the CO 2 ‐to‐C 2+ conversion to clarify the real active sites and dynamic oxidation processes is of paramount importance. Here, commonly used grain boundaries (GBs), oxidation states, and alloying strategies are focused on, constructing four different types of catalysts structures: original Cu GBs, oxygen‐enriched grain boundary oxidation (GBO), Ag‐enriched GBO, and Cu/Ag GBs. Multiple operando characterizations reveal that GBs and GBO strengthen the resistance of the oxidative Cu species to the electrochemical reduction. The in situ generated strongly oxidative hydroxyl radicals alter the local reaction environment on the catalyst surface, inducing and stabilizing oxidative Cu δ+ species. Catalytic activity comparisons indicate that the oxidation state of Cu plays a decisive role in the CO 2 ‐to‐C 2+ conversion, and the nanoalloy effect tends to favor the CH 4 production in intricate GBs assemblies. Theoretical calculations suggest that weak CO adsorption on GBO structures facilitates hydrogenation, promoting C–C coupling toward C 2+ products.

. These structural descriptors are composition-dependent within individual sample series but inconsistent across fabrication protocols, revealing fabrication-dependence in catalyst microstructure. Correlations between structural parameters from different techniques show that Fe-O resists bond length changes, forcing distortion of Co environments. We find the difference in O-M-O bond angles between Co and Fe sites to correlate with electrocatalytic behavior across both sample series, which we attribute to asymmetric distortion of potential energy surfaces for the Co(iii) to Co(iv) oxidation. A Tafel slope consistent with a rate-limiting step without electron transfer emerges as the O-Co-O angle decreases, implying a distortion-induced transition in the rate-limiting step. The fabrication dependence of electronic and bonding structure in the catalysts should be considered in theoretical and high-throughput analyses of electrocatalyst materials.

Compositional tuning of layered perovskite oxides provides a means of systematically studying how local distortions affect fundamental aspects of electrochemical reaction pathways. Structural analysis of a family of samples La 1.2 Sr 0.8 Ni 1– y Co y O 4 shows that Ni-rich compositions have an expanded crystalline c axis, which is anisotropically compressed by systematic Co incorporation. Raman spectra reveal the strong growth of a symmetry forbidden mode, which suggests that Co acts through localized distortions. Crystallographic and spectroscopic parameters describing this structural distortion correlate to the measured Tafel slopes for the oxygen reduction reaction for all Ni-containing samples, which is attributed to the distortion of potential energy surfaces by the Jahn–Teller expansion of d 7 Ni(III) cations. Incorporation of Co not only minimizes the distortion but alters the apparent selectivity of the oxygen reduction reaction away from H 2 O 2 and toward H 2 O. Rotating ring-disk electrochemical measurements, however, indicate that the apparent change in selectivity is due to activation of a first-order chemical disproportionation of H 2 O 2 that is activated by Co in the lattice. These outcomes will support efforts to design electrocatalysts and reactors for the electrochemical synthesis of H 2 O 2.

Abstract The electrochemical production of hydrogen peroxide through the oxygen reduction reaction (ORR) presents the interesting situation where the fully reduced form, H 2 O, is significantly favored by thermodynamics. Perovskite oxide based catalysts with slow kinetics, or large overpotentials, have been shown to enable production of H 2 O 2 with good selectivity, but this comes at the expense of energy efficiency and production rates. We analyze the structure and electrocatalytic ORR capabilities of a perovskite oxide series based on LaCr 1‐x Co x O 3. We find that the crystal lattice distorts in two ways as Co content increases – anisotropic compression of the lattice is accompanied by a pinching of angles within the structure, which gives way to compression of B−O bonds at higher Co contents. Product selectivity is observed to change as a function of both catalyst composition and rotation rates of rotating ring‐disk electrode. The rotation‐dependent behavior is attributed to accelerated displacement of surface‐bound peroxide intermediates by a pH gradient at the catalyst surface, and composition‐dependent changes in selectivity are found to correlate to the angular distortion of the crystal lattice. These results highlight the importance of local environments in electrocatalyst reactions and demonstrate that structure and local pH can be used to manipulate product selectivity.

MoSe 2 nanomaterials are promising photothermal agents for noninvasive cancer treatment. Their surfaces usually need to be functionalized with biocompatible polymers to improve their biocompatibility and colloidal stability and to reduce their cytotoxicity. Herein PEGylated MoSe 2 nanomaterials are produced by femtosecond laser ablation of MoSe 2 powder in aqueous poly(ethylene glycol) (PEG) solutions. Quantum dots are produced by laser ablation for 30 min with a power of 1.5 W, while larger spherical nanoparticles are produced by laser ablation for 10 min with various powers. PEG molecules attach to the nanomaterials through both physical absorption and Mo–O chemical bonds. A higher concentration of PEG in the solution results in more PEG being attached, and increasing the laser ablation power leads to more PEG molecules being attached through chemical bonds. Notably, the attachment of PEG to the nanomaterials through Mo–O bonds can efficiently suppress the oxidation of the MoSe 2 nanomaterials to MoO 3 nanoparticles. Both the MoSe 2 quantum dots and spherical nanoparticles demonstrate high photothermal conversion efficiencies (PTCEs), and the PTCEs of the quantum dots are overall higher than those of the nanoparticles, making them a promising candidate agent for photothermal cancer therapy.

Achieving large-scale electrochemical CO2 reduction to multicarbon products with high selectivity using membrane electrode assembly (MEA) electrolyzers in neutral electrolyte is promising for carbon neutrality. However, the unsatisfactory multicarbon products selectivity and unclear reaction mechanisms in an MEA have hindered its further development. Here, we report a strategy that manipulates the interfacial microenvironment of Cu nanoparticles in an MEA to suppress hydrogen evolution reaction and enhance C2H4 conversion. In situ multimodal characterizations consistently reveal well-stabilized Cuδ+-OH species as active sites during MEA testing. The OH radicals generated in situ from water create a locally oxidative microenvironment on the copper surface, stabilizing the Cuδ+ species and leading to an irreversible and asynchronous change in morphology and valence, yielding high-curvature nanowhiskers. Consequently, we deliver a selective C2H4 production with a Faradaic efficiency of 55.6% ± 2.8 at 316 mA cm−2 in neutral media. The use of membrane electrode assembly electrolyzers in a neutral electrolyte offer potential for large-scale CO2 reduction to multicarbon products, but progress is hindered by low selectivity and unclear reaction mechanisms. Here, the authors regulate the interfacial microenvironment of Cu to stabilize Cuδ + -OH active species for selective C2H4 production.

Raman spectroscopic measurements on a series of perovskites as a function of the temperature and gaseous atmosphere reveal chemical reactions between CO 2 and La 2 CuO 4 that alter the thermal phase transitions and inhibit the synthesis of pristine La 2 CuO 4. Variable temperature Raman spectra acquired under O 2 environments show that La 2 CuO 4 synthesized under ambient conditions undergoes not only the two previously documented phase transitions but also a loss of Raman vibrations at ca. 200 °C that we attribute to desorption of chemisorbed CO 2. Samples heated to 700 °C and then cooled to room temperature under O 2 show a Raman spectrum with many fewer peaks; subsequent exposure to CO 2 at room temperature reveals a spontaneous surface reaction through an increased number of peaks in the spectrum. A subsequent heat ramp under CO 2 shows desorption of the additional CO 2 -related peaks, a ca. 50 °C change in temperature for the two thermal phase transitions, and signs of La 2 O 2 (CO 3 ) formation at temperatures above 538 °C. The original La 2 CuO 4 spectrum is obtained only after heating in the presence of CO 2, which suggests that synthesis under ambient conditions leads to incorporation of CO 2 into the structure. Partial substitution of Ni for Cu to obtain La 2 Cu 0.9 Ni 0.1 O 4 facilitates manipulation of oxygen vacancies in the lattice by heating in oxygen deficient or rich atmospheres but also prevents thermal phase transitions and inhibits direct reactions with CO 2. We attribute both features to strengthened bonds between B-site cations and oxygen, which significantly alters chemical reactivity. We anticipate that in situ variable temperature Raman spectroscopy will be an effective tool for exploring chemical reactivity and guide synthetic protocol development for solid state materials.

High Resolution Image Download MS PowerPoint Slide Hot-stamped ultrahigh strength steel components are pivotal to automotive light-weighting. Steel blanks, often coated with an aluminum-silicon (Al-Si) layer to protect them from oxidation and decarburization, are austenitized within a furnace and then simultaneously quenched and formed into shape. The Al-Si coating melts within the furnace and reacts with iron from the steel to yield an intermetallic phase that provides some long-term corrosion protection. During the intermediate liquid phase, some of the coating may transfer to the furnace components, leading to maintenance costs and operational downtime. A detailed understanding of the coating transformation mechanism is needed to avoid such production issues while ensuring that final intermetallic coatings conform to specifications. We introduce cross-sectional Raman microscopic mapping as a method to rapidly elucidate the coating transformation mechanism. Raman spectroscopic fingerprints for relevant intermetallic compounds were determined using synthesized Al-Fe-Si ternary and Al-Fe binary compounds. These fingerprints were used to map the spatial distribution of intermetallic compounds through cross sections of Al-Si-coated 22MnB5 specimens that were heated at temperatures between 570 and 900 °C. These chemical maps show that the intermetallic fraction of the coating does not grow significantly until formation of η (Al 5 Fe 2 ) at the steel interface, suggesting that η facilitates extraction of iron from the steel and subsequent diffusion through the coating. Under the heating conditions used here, a series of reactions ultimately lead to a silicon-rich τ 2 (Al 3 FeSi) phase on top of the binary η phase. The technique presented here simplifies structural analysis of intermetallic compounds, which will facilitate prototyping of strategies to optimize hot stamping.

Electrocatalytic nitrate reduction reaction (NO 3 – RR) technology provides a promising solution to recover the nitrate nutrition from wastewater through catalyzing nitrate reduction into value-added NH 3. However, the selectivity and efficiency of electrocatalysts are frustrated due to the imbalance of H adsorption (for NO 3 hydrogenation) and unavoidable adjacent H self-coupling on active sites, resulting in competitive hydrogen evolution reaction (HER). Here, we report a PdCu single-atom alloy (SAA) catalyst that allows isolated Pd sites to produce H for the hydrogenation process of NO 3 on neighboring Cu sites, which can restrain the H self-coupling through extending the distance between two H and thus effectively suppress competitive HER. Consequently, the PdCu SAA catalyst exhibits an ultrahigh NH 3 Faraday efficiency (FE) of 97.1% with a yield of 15.4 μmol cm –2 h –1 from the electrocatalytic NO 3 – RR in the neutral electrolyte, outperforming most of the reported catalysts. Single-crystal experiments and theoretical calculations further prove that the introduction of atomic Pd on the Cu (100) surface could serve as the main active site and greatly decrease the energy barrier of the rate-determining step (RDS) on Cu from Δ G = 0.39 eV (NOO → NOOH) to Δ G = 0.10 eV of NOH → NHOH on PdCu SAA.

Wettability of microplastics may change due to chemical or physical transformations at their surface. In this work, we studied the adsorption of spherical nucleic acids (SNAs) with a gold nanoparticle core and linear DNA of the same sequence to probe the wettability of microplastics. Soaking microplastics in water at room temperature for 3 months resulted in the enhancement of SNA adsorption capacity and affinity, whereas linear DNA adsorption was the same on the fresh and soaked microplastics. Drying of the soaked microplastics followed by rehydration decreased the adsorption of the SNA, suggesting that the effect of soaking was reversible and related to physical changes instead of chemical changes of the microplastics. Raman spectroscopy data also revealed no chemical transformations of the soaked microplastics. Heating of microplastics over a short period induced a similar effect to long-term soaking. We propose that soaking or heating removes air entrapped in the nanosized pores at the water-plastic interface, increasing the contact surface area of the SNA to afford stronger adsorption. However, such wetted porosity would not change the adsorption of linear DNA because of its much smaller size.

Iron ions are observed in a previously unobserved coordination environment in iron–nickel oxyhydroxide. Complementary characterization techniques suggest the environment sits atop the two-dimensional Ni(OH) 2 sheets.

A fundamental understanding of the active sites in heterogeneous catalysts is extremely important in the development of effective catalysts. In general, it is difficult to identify the catalytically active sites due to their structural complexity, including the size and specific atomic configurations. In this paper, we prepare different Pt species (single atoms, fully exposed clusters, and nanoparticles) on a nanodiamond/graphene (ND@G) hybrid support to understand their evolution in structure for low-temperature CO oxidation. Remarkably, the atomically dispersed and fully exposed Pt clusters with an ensemble of a few Pt atoms showed the maximum atom utilization of low-coordinated metal sites. As determined by a catalytic performance evaluation, detailed characterizations, and theoretical calculations, the 0.5 wt % Ptn/ND@G catalyst showed a catalytic performance for CO oxidation at low temperature superior to those of single-atom and nanoparticle catalysts, which was attributed to the weakened CO adsorption and facilitated O2 dissociative adsorption on these atomically dispersed and fully exposed Pt cluster catalysts.

Abstract The atomic proximity of two disparate structural motifs in interleaved structures such as that adopted by La 2 NiO 4, which alternates rock salt and perovskite motifs, offers potential synergistic effects and increased tunability for electrocatalyst behavior. Structural and voltammetric analysis of the intersecting composition series LaSrNi 1−y Fe y O 4+δ and La 2−x Sr x Ni 0.7 Fe 0.3 O 4+δ show that tuning different cation sites uniquely affects structure and behavior. Correlations between X‐ray diffraction and Raman spectroscopy results reveal that structural changes in the a‐b plane are uniformly matched by the c ‐axis following La−Sr exchange, while Ni−Fe exchange distorts the structure by expanding the crystal c‐axis and generating oxygen defects with a distinct signature in Raman spectra. Correlations between rotating ring‐disk electrode voltammetry results and structural parameters show that only Fe substitution has a direct effect on the electrocatalytic oxygen reduction reaction. We link systematic changes observed for onset of electrocatalytic oxygen reduction reaction, Tafel slope, and selectivity to distortions in the B‐site environments. This analysis demonstrates that correlational analysis linking electrochemical behavior parameters to multiple structural characterization techniques is an effective means to probe the effect of individual compositional substitution on electrocatalytic behavior.

Raman spectroscopy is commonly used in microplastics identification, but equipment variations yield inconsistent data structures that disrupt the development of communal analytical tools. We report a strategy to overcome the issue using a database of high-resolution, full-window Raman spectra. This approach enables customizable analytical tools to be easily created─a feature we demonstrate by creating machine-learning classification models using open-source random-forest, K-nearest neighbors, and multi-layer perceptron algorithms. These models yield >95% classification accuracy when trained on spectroscopic data with spectroscopic data downgraded to 1, 2, 4, or 8 cm–1 spacings in Raman shift. The accuracy can be maintained even in non-ideal conditions, such as with spectroscopic sampling rates of 1 kHz and when microplastic particles are outside the focal plane of the laser. This approach enables the creation of classification models that are robust and adaptable to varied spectrometer setups and experimental needs.

The rapid growth in microplastic pollution research is influencing funding priorities, environmental policy, and public perceptions of risks to water quality and environmental and human health. Ensuring that environmental microplastics research data are findable, accessible, interoperable, and reusable (FAIR) is essential to inform policy and mitigation strategies. We present a bibliographic analysis of data sharing practices in the environmental microplastics research community, highlighting the state of openness of microplastics data. A stratified (by year) random subset of 785 of 6,608 microplastics articles indexed in Web of Science indicates that, since 2006, less than a third (28.5%) contained a data sharing statement. These statements further show that most often, the data were provided in the articles’ supplementary material (38.8%) and only 13.8% via a data repository. Of the 279 microplastics datasets found in online data repositories, 20.4% presented only metadata with access to the data requiring additional approval. Although increasing, the rate of microplastic data sharing still lags behind that of publication of peer-reviewed articles on environmental microplastics. About a quarter of the repository data originated from North America (12.8%) and Europe (13.4%). Marine and estuarine environments are the most frequently sampled systems (26.2%); sediments (18.8%) and water (15.3%) are the predominant media. Of the available datasets accessible, 15.4% and 18.2% do not have adequate metadata to determine the sampling location and media type, respectively. We discuss five recommendations to strengthen data sharing practices in the environmental microplastic research community.

Structural defects present in hematite photoanodes are analyzed by correlating parameters derived from Raman spectra, band structure measurements and photoelectrochemical (PEC) oxygen evolution reaction performance. A series of photoanodes with varying quality of hematite are prepared by sintering akaganeite films with a systematically varied protocol. Data acquired through Raman microscopy, linear sweep voltammetry, and electrochemical impedance spectroscopy is parametrized and systematically compared to detect relationships amongst the data. Correlations between the structural information contained within the Raman spectra and PEC parameters enable the identification of three types of structural defects within the material. Each defect type is found to exert unique influence on PEC behavior and each became dominant under specific fabrication conditions. A straightforward approach to detecting defects in photoelectrodes and identifying their influence on photoelectrode properties and behavior is provided.

Heterogeneous electrocatalytic reactions are believed to occur at a minority of coordination sites through a series of elementary reactions that are balanced by minor equilibria. These features mask changes in reaction sites, making it challenging to directly identify and analyze reaction sites or intermediates while studying reaction mechanisms. Systematic perturbations of a reaction system often yield systematic changes in material properties and behavior. Correlations between measurable changes in parameters describing the structure and behavior, therefore, serve as powerful tools for distinguishing active reaction sites. This review explores structure–property correlations that have advanced understanding of behavior and reaction mechanisms in heterogeneous electrocatalysis. It covers correlations that have advanced understanding of the contributions of the local reaction environment to reactivity, of structure and bonding within solid-state materials, of geometric or mechanical strain in bonding environments, and of the impact of structural defects. Such correlations can assist researchers in developing next generation catalysts by establishing catalyst design principles and gaining control over reaction mechanisms.

Abstract The consistent fabrication of high performance α -Fe 2 O 3 photoanodes for the oxygen evolution reaction remains a challenge. We work towards resolving this issue by developing in situ variable temperature Raman spectroscopy as a means to better understand the formation of α -Fe 2 O 3, using the conversion of γ -FeOOH to α -Fe 2 O 3 under varied gaseous environments as a model case. The sensitivity of Raman spectroscopy to structural changes provides mechanistic insights that are not readily available in more conventional approaches, such as thermal gravimetric analysis and differential scanning calorimetry. The Raman spectra are combined with conventional thermal analyses to interpret the photoelectrocatalytic performance of a series of α -Fe 2 O 3 photoanodes prepared by systematic variation of a three-stage annealing protocol. The combined results suggest that protohematite, a form of α -Fe 2 O 3 where trapped hydroxyl ligands are balanced by Fe(III) vacancies, forms between 200 °C and 400 °C in a reaction environment-dependent fashion. This protohematite is shown to be remarkably persistent once formed, degrading photoelectrocatalytic performance. This research advances understanding of the γ -FeOOH to α -Fe 2 O 3 structural transformation, illustrates a powerful method to study solid state phase transitions, and provides guidance for the synthesis of high quality α -Fe 2 O 3 from a convenient precursor.

We analyze the stability of anodically electrodeposited cobalt hydroxide films during operation as electrocatalysts for water oxidation and show that the stability considerations of these films are a complex issue concerning multiple distinct phenomena. We combine the electrochemical analysis with the quantitative analysis of cobalt content using X-ray fluorescence spectroscopy, transient behavior analysis using in operando XANES electrochemical analysis, and structural analysis using cryogenic X-ray absorption spectroscopy to show that three distinct behavioral regimes exist. Each behavioral regime suffers from a degree of corrosion, but both self-healing and self-repair mechanisms are capable of partially stabilizing the cobalt content within the catalyst film. However, even if the absolute cobalt content of the film is fully stabilized, the catalytic behavior of the film degrades over time. This behavior is attributed to observed structural evolution of the material, where mid- and long-range structural order increases during operation. The increased structural order appears independent of whether self-healing or self-repair mechanisms are activated and results in fundamental changes in the catalytic behavior, observable as decreased turnover frequency and increased Tafel slopes. These results reveal that the cobalt content or electrocatalytic performance of the catalyst films can be stabilized by controlling the applied voltage or solution composition, but simultaneous stabilization of both is not currently possible.

Uncertainty regarding the nature of structural defects in hematite and their specific impacts on material properties and photoelectrocatalytic water oxidation inhibits their development as photoanodes. We perform structure-property analysis on a series of hematite films fabricated by annealing lepidocrocite films with varied temperatures, annealing times and atmospheres and find a gradient in the magnitude of a crystal lattice distortion by tracking the relative intensity of a formally Raman inactive vibrational mode. Structure-property analysis reveals that this feature in the Raman spectrum correlates to photocurrent density, semiconductor band positions, and the onset of photoelectrocatalysis. We propose that the onset of photoelectrocatalysis is linked to the location of defects that act as intraband recombination sites; an increase in the degree of structural distortion shifts these states towards the conduction band, thereby facilitating recombination. Analysis of the nature of the key Raman vibrations, X-ray diffraction patterns, and the synthetic conditions leads us to assign the distortion to iron vacancies that are induced by the trapping of protons within the crystal lattice. The ability to rapidly diagnose a specific structural defect will aid in the optimization of fabrication protocols for hematite photoanodes.

The chemistry underlying the activation of nickel hydroxide toward electrocatalytic oxygen evolution by incorporation of iron remains a subject of debate. We extract insights into the role of geometric strain on the electrochemical behavior in this class of materials by blending aluminum, gallium, or iron into a disordered nickel hydroxide lattice. The electrochemical behavior and electronic structure of the three binary composition series are found to be similarly influenced by each additive cation. Density functional theory models indicate that the additive cations asymmetrically impede the voltage-induced expansion and contraction of the nickel hydroxide host lattice, a feature that is supported by near-infrared spectroscopy. Reaction coordinate diagrams suggest that this distortion decreases the activation energy for electron transfer by decreasing the extent to which the lattice can expand and contract but that iron is unique in its ability to favor oxidation by distorting the shape of the potential energy surface of the oxidized state to lower the electron transfer coefficient. These results reveal that interference with voltage-induced structural changes by incorporation of suitably sized ions alters the electrochemical behavior but that overall electrocatalytic performance cannot be linearly controlled by such geometric distortions.

coordination. Our results show that in water oxidation catalysis at near-neutral pH, anion type and exchange dynamics correlate with the catalyst structure and redox properties.

Understanding the mechanism for electrochemical water oxidation is important for the development of more efficient catalysts for artificial photosynthesis. A basic step is the proton-coupled electron transfer, which enables accumulation of oxidizing equivalents without buildup of a charge. We find that substituting deuterium for hydrogen resulted in an 87% decrease in the catalytic activity for water oxidation on Co-based amorphous-oxide catalysts at neutral pH, while 16O-to-18O substitution lead to a 10% decrease. In situ visible and quasi-in situ X-ray absorption spectroscopy reveal that the hydrogen-to-deuterium isotopic substitution induces an equilibrium isotope effect that shifts the oxidation potentials positively by approximately 60 mV for the proton coupled CoII/III and CoIII/IV electron transfer processes. Time-resolved spectroelectrochemical measurements indicate the absence of a kinetic isotope effect, implying that the precatalytic proton-coupled electron transfer happens through a stepwise mechanism in which electron transfer is rate-determining. An observed correlation between Co oxidation states and catalytic current for both isotopic conditions indicates that the applied potential has no direct effect on the catalytic rate, which instead depends exponentially on the average Co oxidation state. These combined results provide evidence that neither proton nor electron transfer is involved in the catalytic rate-determining step. We propose a mechanism with an active species composed by two adjacent CoIV atoms and a rate-determining step that involves oxygen–oxygen bond formation and compare it with models proposed in the literature.

Multiply charged anions (MCAs) display unique photophysics and solvent-stabilizing effects. Well-known aqueous species such as SO42– and PO43– experience spontaneous electron detachment or charge-separation fragmentation in the gas phase owing to the strong Coulomb repulsion arising from the excess of negative charge. Thus, anions often present low photodetachment thresholds and the ability to quickly eject electrons into the solvent via charge-transfer-to-solvent (CTTS) states. Here, we report spectroscopic evidence for the existence of a repulsive Coulomb barrier (RCB) that blocks the ejection of “CTTS-like” electrons of the aqueous B12F122– dianion. Our spectroscopic experimental and theoretical studies indicate that despite the exerted Coulomb repulsion by the nascent radical monoanion B12F12–•aq, the photoexcited electron remains about the B12F12–• core. The RCB is an established feature of the potential energy landscape of MCAs in vacuo, which seems to extend to the liquid phase highlighting recent observations about the dielectric behavior of confined water.

Iron is observed to induce geometric distortions in oxidized nickel oxyhydroxide that activate the material for the oxygen evolution reaction.

The identification of recombination centers in perovskite solar cells is highly challenging.

The emergence of disordered metal oxides as electrocatalysts for the oxygen evolution reaction and reports of amorphization of crystalline materials during electrocatalysis reveal a need for robust structural models for this class of materials. Here we apply a combination of low-temperature X-ray absorption spectroscopy and time-resolved in situ X-ray absorption spectroelectrochemistry to analyze the structure and electrochemical properties of a series of disordered iron-cobalt oxides. We identify a composition-dependent distribution of di-μ-oxo bridged cobalt-cobalt, di-μ-oxo bridged cobalt-iron and corner-sharing cobalt structural motifs in the composition series. Comparison of the structural model with (spectro)electrochemical data reveals relationships across the composition series that enable unprecedented assignment of voltammetric redox processes to specific structural motifs. We confirm that oxygen evolution occurs at two distinct reaction sites, di-μ-oxo bridged cobalt-cobalt and di-μ-oxo bridged iron-cobalt sites, and identify direct and indirect modes-of-action for iron ions in the mixed-metal compositions.

An electrochemical conditioning protocol was developed for studying amorphous phases of nickel hydroxide films (NiO x ) that renders congruent and reproducible redox behavior that enables more standardized mechanistic studies. Dynamic oxidative behavior was measured for the conditioned films, where the initial peak associated with the oxidation of NiO x contains 1.3–2.0 times the charge for that measured in subsequent oxidative scans and the corresponding reductive peak. It is shown that charge trapping does not fully account for this excess charge. The magnitude of excess anodic charge is affected by pH, OH – diffusion through the film, and voltammetric scan rates.

The dynamic behavior of the anodic peak for amorphous nickel oxy/hydroxide (a-NiOx) films in basic media was investigated. Chronocoulometry of films with known nickel concentrations reveals that a total of four electrons per nickel site comprise the signature anodic peak at 1.32 V during the first oxidative scan, and two electrons are passed through the associated cathodic peak on the reverse scan. The anodic and cathodic signals each contain two electrons on the successive scans. Catalytic oxygen evolution reaction (OER) was detected within the anodic peak, which is at a lower potential than is widely assumed. In order to rationalize these experimental results, we propose that the four-electron oxidation event is the conversion of the film from nickel(II) hydroxide (Ni(II)-OH) to a higher valent nickel peroxide species (e.g., Ni(IV)-OO or Ni(III)-OO·). The subsequent reduction of the nickel peroxide species is confined by a chemical step resulting in the accumulation of Ni(II)-OOH, which is then oxidized by two electrons to form Ni(IV)-OO during the subsequent oxidative scan on the time scale of a cyclic voltammetric experiment. Our proposed mechanism and the experimental determination that each nickel site is oxidized by four electrons helps link the myriad of seemingly disparate literature data related to OER catalysis by nickel electrodes. The faster catalysis that occurs at higher oxidative potentials is derived from a minority species and is not elaborated here.

A photochemically initiated decomposition reaction was used to synthesize a series of amorphous mixed-metal oxide films. The series, consisting of mixed-metal oxides included binary and ternary mixtures containing Fe, Co and/or Ni, enabled a systematic evaluation of amorphous metal oxides with complex compositions. X-ray diffraction results indicate that the as-deposited films are amorphous and can be converted to their crystalline form by heating. Infrared spectroscopy of the films indicates the photochemically driven process drives complete loss of the organic constituents from the film. X-ray photoelectron spectroscopy confirmed the loss of the organic ligand and enabled estimation of metal oxidation states. Electrochemical characterization revealed significant variation in catalytic performance toward the oxygen evolving reaction (OER) upon variation of metal oxide composition. Analysis of the Tafel behavior was used to distinguish amorphous mixed-metal oxide films to enable several film compositions to exhibit excellent electrocatalytic OER performance.

Light-driven decomposition of Ir(acac) 3 spin-cast on a conducting glass substrate produces a thin conformal film of amorphous iridium oxide, a -IrO x. The decomposition process, which was carried out under an ambient atmosphere at room temperature and tracked by Fourier transform infrared (FTIR) spectroscopy, appears to proceed by way of a ligand-to-metal charge transfer (LMCT) process. The amorphous nature of the films is based on the lack of any observable Bragg reflections by powder X-ray diffraction techniques; the elemental composition was corroborated by X-ray photoelectron spectroscopy (XPS) measurements. The films are found to be excellent electrocatalysts for mediating the oxygen evolution reaction (OER) in acidic media, as evidenced by the onset of catalysis at 130 mV and a Tafel slope of 34 mV dec –1. These parameters enable current densities of 1 and 10 mA cm –2 to be reached at 190 and 220 mV, respectively. Exposing the films to higher temperatures (500 °C) renders a film of crystalline iridium oxide, c -IrO x, which displays a Tafel slope of 60 mV dec –1, thus requiring an additional 50 mV to reach a current density of 1 mA cm –2. The film of a -IrO x reported here is among the best OER electrocatalysts reported to date.

Photochemical metal-organic deposition (PMOD) was used to prepare amorphous metal oxide films containing specific concentrations of iron, cobalt, and nickel to study how metal composition affects heterogeneous electrocatalytic water oxidation. Characterization of the films by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy confirmed excellent stoichiometric control of each of the 21 complex metal oxide films investigated. In studying the electrochemical oxidation of water catalyzed by the respective films, it was found that small concentrations of iron produced a significant improvement in Tafel slopes and that cobalt or nickel were critical in lowering the voltage at which catalysis commences. The best catalytic parameters of the series were obtained for the film of composition a-Fe20Ni80. An extrapolation of the electrochemical and XPS data indicates the optimal behavior of this binary film to be a manifestation of iron stabilizing nickel in a higher oxidation level. This work represents the first mechanistic study of amorphous phases of binary and ternary metal oxides for use as water oxidation catalysts, and provides the foundation for the broad exploration of other mixed-metal oxide combinations.

Large-scale electrolysis of water for hydrogen generation requires better catalysts to lower the kinetic barriers associated with the oxygen evolution reaction (OER). Although most OER catalysts are based on crystalline mixed-metal oxides, high activities can also be achieved with amorphous phases. Methods for producing amorphous materials, however, are not typically amenable to mixed-metal compositions. We demonstrate that a low-temperature process, photochemical metal-organic deposition, can produce amorphous (mixed) metal oxide films for OER catalysis. The films contain a homogeneous distribution of metals with compositions that can be accurately controlled. The catalytic properties of amorphous iron oxide prepared with this technique are superior to those of hematite, whereas the catalytic properties of a-Fe(100-y-z)Co(y)Ni(z)O(x) are comparable to those of noble metal oxide catalysts currently used in commercial electrolyzers.