Showing papers in "The Journal of Physical Chemistry in 2021"

CsPbBr₃, MAPbBr₃, and FAPbBr₃ Bromide Perovskite Single Crystals: Interband Critical Points under Dry N₂ and Optical Degradation under Humid Air

Abstract: We investigated the complex dielectric function of bromide perovskite single crystals (MAPbBr₃, FAPbBr₃ and CsPbBr₃) by spectroscopic ellipsometry from 1 to 5 eV in the 183–423 K temperature range and under a dry nitrogen environment. The temperature dependence of all critical points provided a benchmark for the interband electronic transition energies in the three bromide perovskites for a wide spectral range. We found that the number of transitions in CsPbBr₃ depends on the crystallographic phase, with the orthorhombic lattice showing an extra transition with respect to the tetragonal and the cubic phases. Through density functional theory calculations, we identified the changes in the electronic structure that accompany the tetragonal-to-orthorhombic transition, leading to splitting of the complex dielectric function along the a and b crystallographic directions. We thereon used a similar experimental approach to study the robustness of bromide perovskites under humid air. We found that FAPbBr₃ and CsPbBr₃ remain unchanged up to 383 K, whereas MAPbBr₃ is irreversibly modified at 363 K. The degradation of MAPbBr₃ impacts its optical transitions and introduces a pregap absorption tail. Together with this, a dramatic damage of the surface is observed with the formation of reverse truncated pyramids accompanied by local worsening of the carrier extraction barrier in the remaining flat sample surface.

Correlation between Photocorrosion of ZnO and Lattice Relaxation Induced by Its Surface Vacancies

Abstract: ZnO samples with different surface vacancy defects in nature were successfully synthesized via a hydrothermal method, and the correlation between the surface defects and photocorrosion was investigated by applying a photoelectric response testing system. Here, we report on findings showing that the defects of the oxygen vacancies (VOs) induce a local electric field attributable to an inward lattice relaxation, resulting in weakness and cleavage of the Zn–O bond (i.e., photocorrosion) around the VOs. In contrast, the zinc vacancies (VZₙs) inhibit photocorrosion by forming an outward lattice relaxation. These results suggest that the photocorrosion of ZnO is related to the lattice relaxation induced by the defects in ZnO. Furthermore, we have proved that the H₂, O₂, and H₂O molecules adsorbed operate as an electron acceptor to receive the excess electrons localized around VOs. These processes were found to construct an electron-transfer channel to induce a reduction or elimination of the local electric field around VOs, leading to the inhibition of photocorrosion. Thus, the present results open new insights into the understanding of the photocatalytic activity as well as photocorrosion of the ZnO crystal surface and their correlation.

Conversion of Formic Acid on Single- and Nano-Crystalline Anatase TiO₂(101)

Abstract: Understanding thermochemical transformations of formic acid (FA) on metal oxide surfaces is important for many catalytical reactions. Here we study thermally induced reactions of FA on a single-crystalline and nanocrystalline anatase TiO₂(101). We employ a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) to follow the FA surface intermediates and reaction products above room temperature. We find that the primary reaction products desorbing at about 300, 480, and 515 K are molecular water, carbon monoxide, and formaldehyde, respectively. Bidentate (BD) formate and bridging hydroxyl (HOb) are identified as central intermediates in the FA transformations. Bridging oxygen vacancies (VO) are also likely participants despite their low stability at the surface. Furthermore, the parallel studies on single crystals and faceted TiO₂(101) nanoparticles reveal the spectroscopic commonalities of surface species and of the thermal conversion of molecular and deprotonated forms of FA.

Electromagnetic Field Gradient-Enhanced Raman Scattering in TERS Configurations

Abstract: Realizing in situ simultaneous measurement of full vibrational modes in tip-enhanced Raman spectroscopy (TERS) is of immense importance for improving the ability of quantitative analysis and measurement accuracy of the molecular spectrum. In this study, the electric field gradient effect in the TERS system was theoretically investigated by using the finite element method. The respective contributions of electric field and electric field gradient to dipole Raman and gradient Raman modes in the TERS spectrum were quantitatively distinguished. By comparing the relative intensity ratio between electric field gradient and electric field, the TERS configuration could be optimized to achieve a maximum of gradient Raman modes. Theoretical results indicate that molecular symmetries strongly influence the molecular hyperpolarizability, which significantly influences the plasmonic electric field gradient effects. Our theoretical results could reveal the contributions of electric field and electric field gradient to molecular vibration modes, useful for the design of the TERS platform with more spectrum information.

Phase Change-Driven Radical Population and Electrical Conductivity Increases of Organic Radical Crystals by Mild Heating

Abstract: Carrier doping in conductive radical crystal systems is one of the most important and challenging issues in employing radicals as charge carriers, i.e., electronic radicals for high-performance organic electronics. Here, we report an experimental demonstration of the increase in the radical population and the corresponding increase in electrical conductivity by water desorption-involved crystal structure change upon mild heating. The electrical conductivity of the dihydrophenazinium radical cation (H₂PNZ·⁺) crystal is enhanced by 4 orders of magnitude with an increase in radical population by 1.5 times according to the electron paramagnetic resonance signal after heating at 333 K. Single-crystal X-ray analyses show that the increase in radical population originates from the phase transformation from dimer-like radical units to monomer-like radical units upon mild heating in a narrow temperature range. The increase in intermolecular distance results in producing more independent radicals and reducing the band gap. Therefore, the electrical conductivity is increased by the synergistic effects of the radical population increase and band gap reduction.

Mapping of Guest Localization in Mesoporous Silica Particles by Solid-State NMR and Ab Initio Modeling: New Insights into Benzoic Acid and p-Fluorobenzoic Acid Embedded in MCM-41 via Ball Milling

Abstract: We present a novel strategy for mapping the localization of guest molecules (GMs) in mesoporous materials by combining mechanochemistry with solid-state nuclear magnetic resonance (ssNMR) spectroscopy. To this end, we consider model guest–host systems of benzoic acid (BA) and para-fluorobenzoic acid (4-FBA) embedded in mesoporous MCM-41 material and examine the recently proposed loading (MeLo) procedure for efficient encapsulation of molecular species. Application of high-resolution NMR experiments (¹H and ¹⁹F NMR) has allowed detection of a multimodal distribution of the spectral signals ascribed to embedded GMs. This peculiarity reflects the presence of distinct molecular ensembles subjected to intrinsically different local environments and exhibiting different dynamical behavior. Furthermore, a considerable fraction of an amorphous phase has been found as a byproduct of the ball-milling. The stability of the phase mixture was further checked by subjecting the samples to chemical and physical stimuli, and a detailed interpretation of the NMR data was corroborated by theoretical calculations. To this end, we have undertaken a challenge to predict the NMR spectra of the GMs@MCM-41 using advanced ab initio molecular dynamics (AIMD) simulations, providing an accurate and exhaustive analysis of the NMR spectra. On the basis of the ab initio modeling validated against the experimental results, we find that the multimodal signal distribution reflects the level of the pore filling, and can be ascribed to the presence of both interface and fluid molecular species trapped inside the pores. This has been confirmed for both BA@MCM-41 and 4-FBA@MCM-41 systems. The presence of the third, amorphous fraction can be linked to interstitial space between randomly ordered crystallites, pointing at the importance of the external surface in further stabilization of encapsulated materials. Altogether, a consistent experimental and theoretical methodology has been presented, paving the way for a more accurate analysis of complex nanoconfined systems.

Determining the Structure–Property Relationships of Quasi-Two-Dimensional Semiconductor Nanoplatelets

Abstract: We report a theoretical study of CdSe nanoplatelets aimed at identifying the main factors determining their photophysical properties. Using atomic configurations optimized with density functional theory calculations, we computed quasiparticle and exciton binding energies of nanoplatelets with two to seven monolayers. We employed many body perturbation theory at the GW level and solved the Bethe-Salpeter equation to obtain absorption spectra and excitonic properties. Our results, which agree well with recent experiments, were then used to design a model that allows us to disentangle the effects of quantum confinement, strain induced by passivating ligands, and dielectric environment on the electronic properties of nanoplatelets. We found that, for the model to accurately reproduce our first principle results, it is critical to account for surface stress and consider a finite potential barrier and energy-dependent effective masses when describing quantum confinement. Our findings call into question previous assumptions on the validity of an infinite barrier to describe carrier confinement in nanoplatelets, suggesting that it may be possible to optimize interfacial charge transfer and extraction by appropriately choosing passivating ligands. The model developed here is generalizable to core–shell platelets and enables the description of system sizes not yet directly treatable by first-principles calculations.

Ferromagnetism in Mn-Doped ZnO: A Joint Theoretical and Experimental Study

Abstract: We present a joint theoretical and experimental investigation on the origin of ferromagnetism in Mn-doped ZnO. Theoretical calculations revealed that the zinc vacancy (VZₙ) induces ferromagnetic ordering (FMO), whereas the oxygen vacancy (VO) quenches FMO in the Mn-doped ZnO system. This is further corroborated by the experimental results. Magnetic measurements revealed that Mn-doped ZnO shows room-temperature ferromagnetism (RTFM). Saturated magnetic moment per Mn²⁺ ion increases with oxygen partial pressure, indicating that the VZₙ enhances FMO in Mn-doped ZnO. Electron paramagnetic resonance and photoluminescence measurements revealed the presence of VZₙ in Mn-doped ZnO films. X-ray photoelectron spectroscopy measurements showed mixed oxidation states of Mn in Mn-doped ZnO films. Finally, we show that RTFM at very low doping concentrations is due to the overlapping of bound magnetic polarons. However, due to antiferromagnetic coupling at higher doping concentrations, the FMO weakens.

Novel Nickel-Based Single-Atom Alloy Catalyst for CO₂ Conversion Reactions: Computational Screening and Reaction Mechanism Analysis

Abstract: Catalytic conversion of CO₂ to methane and syngas are two promising routes for CO₂ utilization. Even though noble metals have shown high activity and stability for these reactions, cheaper nickel (Ni)-based catalysts are preferred. However, these are less active and are prone to deactivation due to carbon deposition. Boron-promoted Ni (NiB) was developed by the microstructural modification of Ni, and it showed improved stability, but with reduced activity. This is attributed to the high CO₂ activation barrier. Single-atom alloys (SAA) are adequately capable of performing selective catalysis to improve the catalyst performance. In this work, using first-principles-based computational screening, a novel and thermodynamically stable NiB SAA catalyst that reduces CO₂ activation barrier and improves the activity without affecting stability and selectivity of NiB is predicted. We considered 14 dopant elements that alloy with Ni (Ru, Pt, Pd, Rh, Co, Fe, Os, Ir, Re, W, Mo, Cu, Mn, and Zn) and evaluated the relative thermodynamic stability of the SAA configuration, compared to that of dimer or trimer structures. The relative stability of SAA was higher than that of the aggregates for only five metals (Pt, Pd, Rh, Cu, and Mn). We further calculated the CO₂ activation barrier on these five SAAs and found that Mn-NiB SAA was the only candidate on which there is a significant reduction of the CO₂ activation barrier (reduced by 56 kJ mol–¹). Subsequently, we studied CO₂ methanation (46 elementary reactions) and dry reforming of methane (DRM) (38 elementary reactions) reactions on Mn-NiB SAA. High CO₂ adsorption energy and low CO₂ and CO* activation barriers make Mn-NiB SAA a suitable catalyst for CO₂ methanation. Correspondingly, the low CO₂ and CH₄ activation barriers make Mn-NiB SAA a perfect candidate for DRM reaction. Prominently, the high endergonicity for CH₄ stepwise dehydrogenation combined with a low barrier for Boudouard reaction reduces the on-surface coke formation. Thus, we believe that Mn-NiB SAA can be a potential catalyst for CO₂ methanation and DRM reactions.

Real-Time Imaging of Laser-Induced Nanowelding of Silver Nanoparticles in Solution

Abstract: Nanowelding of metallic nanoparticles induced by laser illumination is of particular interest because it provides convenient and controlled means for shape-conversion of nanoparticles and fabrication of nanodevices. However, the kinetics of the laser-induced nanoparticle nanowelding remains largely unexplored. Herein, we exploited fluorescence microscopy to directly image the real-time nanowelding kinetics of silver nanoparticles (AgNPs) when illuminated with a continuous wave laser at 405 nm. We observed that the laser illumination induced the AgNPs to form higher-order branched structures or assemblies. More importantly, we quantified the sizes of the laser-induced assemblies and found that the dependence of the average size (A) of the assemblies on the illumination time t followed A ∝ 1 – e–ᵗ/τ. An analytical model based on simple polymerization was developed to predict and understand the measured kinetics. We experimentally verified the model by varying the laser power and the concentration of AgNPs. Furthermore, we improved the model by taking into account the merging of assemblies and predicted that the laser-induced assembling kinetics was diffusion-limited, which was then verified experimentally with AgNPs in 50% glycerol. Lastly, in contrast to the single-phased ohmic nanocontact produced by the laser-induced nanowelding, we found that the formed higher-order structures were separated into different photoluminescent domains and different regions of the same laser-induced assembly showed asynchronous, uncorrelated blinking behaviors. This work is expected to facilitate the development of better nanowelding strategies of metallic nanoparticles for broader applications.

Is Solid Copper Oxalate a Spin Chain or a Mixture of Entangled Spin Pairs

Abstract: Macroscopic assemblies of interacting spins give rise to a broad spectrum of behaviors determined by the spatial arrangement of the magnetic sites and the electronic interactions between them. Compounds of copper(II), in which each copper carries spin 1/2, exhibit a vast variety of physical properties. For antiferromagnetically coupled spin sites, there are two limiting scenarios: spin chains in which the spins can exhibit a long-range order or a mixture of dimers in which the spins within each pair are entangled but do not interact with the spins from other dimers. In principle, the two types can be distinguished on the basis of experimental observations and modeling using empirically parameterized effective Hamiltonians, but in practice, ambiguity may persist for decades, as is the case for copper oxalate. Here, we use high-level ab initio calculations to establish the validity of the nearest-site Heisenberg model and to predict the interaction strength between the magnetic sites. The computed magnetic susceptibility provides an unambiguous interpretation of magnetic experiments performed over half a century, clearly supporting the infinite spin-chain behavior of solid copper oxalate.

Probing Acid–Base Properties of Anatase TiO₂ Nanoparticles with Dominant {001} and {101} Facets Using Methanol Chemisorption and Surface Reactions

Abstract: In the present study, we investigate the surface acid–base properties of anatase TiO₂ nanomaterials with dominant {101} and {001} facets via methanol titrations. Two anatase nanoparticles, TiO₂(101) and TiO₂(001), with well-defined morphology are prepared. TiO₂(101) is predominantly enclosed by the {101} facets (>90%), and TiO₂(001) contains ∼46% {001} facets and ∼54% {101} facets. Upon adsorption of methanol at 423 K, diffuse reflectance infrared Fourier transform spectroscopy measurements show that both molecular and dissociative adsorption occur on TiO₂(101), while dissociative adsorption dominates on TiO₂(001). During methanol temperature-programmed desorption, TiO₂(001) mainly generates the acid–base product dimethyl ether and thermal cracking products CO and H₂, as anticipated. In contrast, substantial amounts of formaldehyde and methane also desorb from TiO₂(101), suggesting strong participation of surface defects (e.g., oxygen vacancies).

Hole Dopants Disentangling Peierls–Mott Relevance States of VO₂ by First-Principles Calculation

Abstract: The formation mechanism of the metastable M₂-phase VO₂, which is believed to be a true Mott insulator, has attracted great attention for understanding the intriguing physics of the metal–insulator transition of VO₂ and the promising application in ultrafast electronic switching devices. Herein, we conducted the hole-doping calculation regardless of the type of element and revealed theoretically that the hole carriers disentangle the complex Mott–Peierls relevance states of M₁-phase VO₂. The hole induces the zigzag dimerized V–V chains to separate into two different states: one remains paired but straight and the other remains zigzag but unpaired. The dedimerization weakens the intradimer hopping, which makes the superexchange interaction come into effect, consequently resulting in the formation of the spin antiferromagnetic ordering along the zigzag unpaired V–V chains, indicating that the Mott correlation plays a dominant role in the formation of M₂-VO₂. This work gives an insight into the mechanism of stabilizing the “true” Mott insulator M₂-VO₂, which would offer an opportunity for the realization of Mott transition field-effect transistors.

CsPbBr₃ Nanocrystal Light-Emitting Diodes with Efficiency up to 13.4% Achieved by Careful Surface Engineering and Device Engineering

Abstract: Perovskite nanocrystals (NCs) have gained great interest for next-generation light-emitting diodes (LEDs) prized for their spectrally tunable, narrow luminescence and their defect-tolerant electronic structure. Here, we provide a careful surface engineering to balance the passivation and charge injection by controlling the ligand density of didodecyl dimethyl ammonium bromide on NCs, which not only offers an improved photoluminescence quantum yield of >70% for NCs but also guarantees the effective charge injection and transportation of CsPbBr₃ NC films for fabricating LEDs. Additionally, the optimization of the hole transport layer was implemented to offer more balanced charge transportation, which further improved the performance of CsPbBr₃ NC-based LEDs with a peak external quantum efficiency of 13.4%.

Rational Design of Cobaltate MCo₂O₄₋δ Hierarchical Nanomicrostructures with Bunch of Oxygen Vacancies toward Highly Efficient Photocatalytic Fixing of Carbon Dioxide

Abstract: Cobaltate MCo₂O₄₋δ (M = Zn, Ni, Cu) spinel composites have been known as promising catalysts for the manufacture of fuels and fine chemicals by CO₂ photocatalytic fixing reaction (CO₂PFR), whereas CO₂PFR product selectivity based on different cobaltate remains poorly understood. Herein, various cobaltate spinel composites with different cations distribution hierarchical nanomicrospheres (HNMs) were rationally designed and synthesized. We found that among the cobaltate MCo₂O₄₋δ (M = Zn, Ni, Cu) spinel catalysts, CuCo₂O₄₋δ DSHoMs show the most substantially promoted CO production rate (26.54 μmol h–¹), while the CH₄ yields of NiCo₂O₄₋δ SMs were ≈1.7 times higher than that of CuCo₂O₄₋δ DSHoMs. As collectively evidenced by PEC, in situ DRIFTS, TPD, and theoretical results, NiCo₂O₄₋δ SMs feature promotional charge transferability and super CH₄ selectivity, while CuCo₂O₄₋δ DSHoMs feature higher light adsorption, high transient photocurrent density, and preferential selectivity of CH₄ evolution. This work unearths atomic-level insights into the correlation between the distribution of metal cations in cobalt-based spinel structures and selectivity regulation for CO₂PFR and deeper understanding of the intrinsic relationship of metal species property between various cobaltate spinel oxides which lays a firm foundation for controlling and tailoring of the selectivity of the desired products during CO₂PFR.

Electronic Origin of Catalytic Activity of TiH₂ for Ammonia Synthesis

Abstract: Transition metals with a higher-lying Fermi level (a smaller work function) such as Ti can easily activate nitrogen and hydrogen reductively to form nitrides and hydrides on its surface, but ammonia synthesis does not occur. H atoms adsorbed on the surface of Ti have a negative charge, identified as H–. In order for an N–H bond to form on the Ti surface, the hydrogen atom must dump its extra charge into empty levels above the Fermi level of the surface. The high Fermi level of Ti impedes this process. Although the Fermi level of TiH₂ is as high as that of Ti, TiH₂ has been reported to act as an ammonia synthesis catalyst under Haber–Bosch conditions, characterized by its robust catalytic activity. To clarify the electronic aspects of the catalytic activity of TiH₂, a theoretical study on the surface of TiH₂ using density functional theory calculations is carried out. The negative charge of the H atom about to bond to the N atom in ammonia synthesis on the TiH₂ surface is as large as that of the H atom on the Ti surface. However, since surface hydrides of TiH₂ present nearby interact with the H atom in an antibonding manner and the electronic state of the H atom is destabilized, the energy required for dumping the extra charge upon the formation of the N–H bond is substantially reduced. Nitrides in the surface of TiH₂, the formation of which during the reaction has been suggested experimentally, make the amount of the charge of the H atom smaller by oxidizing the Ti atoms nearby. This further reduces the activation barrier associated with the N–H bond formation, leading to a good agreement between theory and experiment.

Anion Substitution Effects on the Structural, Electronic, and Optical Properties of Inorganic CsPb(I₁–ₓBrₓ)₃ and CsPb(Br₁–ₓClₓ)₃ Perovskites: Theoretical and Experimental Approaches

Abstract: Several inorganic perovskites of iodine, bromine, and chlorine halides have emerged as candidates for various optoelectronic devices. High-quality CsPb(I₁–ₓBrₓ)₃ and CsPb(Br₁–ₓClₓ)₃ (x = 0.00, 0.25, 0.50, 0.75, and 1.00) inorganic perovskite thin films were prepared in this study using a thermal evaporation system. Experiments and first-principles calculations were conducted to elucidate the structural, electronic, and optical properties of the prepared films at room temperature. The thin-film perovskite band gap was tuned from 1.85 to 3.13 eV by replacing I– with Br– and then Cl–. Dominant excitonic effects on the onset of optical absorption led us to explicitly account for enhancing absorption through the Sommerfield factor, enabling us to extract the electronic band gap and the exciton binding energy correctly. We correlated our experimental results with the theory of first principles and gained insight into the lattice parameters, electronic structure, excitonic binding energy (Eb), dielectric constant (e), and reduced effective mass (μ) of the carriers. With increasing concentration (x) of Br and Cl, the Eb increased from 39.44 meV for pure CsPbI₃ to 63.04 and 96.73 meV for pure CsPbBr₃ and CsPbCl₃, respectively, because of a decrease in the dielectric constant and the almost constant value of μ at ∼0.051 mₑ. The Urbach energy (EU) was calculated and found to fluctuate between 28 and 77 meV.

Photochromism and Persistent Luminescence in Ni-Doped ZnGa₂O₄ Transparent Glass-Ceramics: Toward Optical Memory Applications

Abstract: Persistent luminescence and photochromism are two fascinating optical properties that involve charge trapping via defects and their release due to an external stimulation. In both processes, it is possible to define “1” (or “on”) and “0” (or “off”) optical states. Consequently, materials with one of these phenomena find major interest in view of designing smart, anticounterfeiting, and optical information storage devices. Combining both processes within a single material can lead to a new generation of information storage phosphors, in which it may be possible to obtain three different optical states by playing on the external stimulations applied on the material. For that purpose, the elaboration of nickel-doped ZnGa₂O₄ spinel transparent nano-glass-ceramics is presented in this work. The short-wave infrared (SWIR) emission, a broad band located at ca. 1275 nm, arises from Ni²⁺ cations located in gallium octahedral sites. SWIR persistent luminescence, arising from the same doping ion transition, can also be monitored after UV charging. Interestingly, UV irradiation not only leads to persistent luminescence charging but also to a reversible photochromism effect. By means of a complete optical characterization study combined with electron paramagnetic resonance measurements, the origins of both the persistent luminescence emission and the photochromism are explored. Finally, a discussion on the advantages of such a material combining both persistent luminescence and photochromism properties, leading to three possible optical states of the material, is dressed in view of possible optical memory systems.