Direct Capturing and Regulating Key Intermediates for High-Efficiency Oxygen Evolution Reactions.

Developing efficient oxygen evolution reaction (OER) electrocatalysts can greatly advance the commercialization of proton exchange membrane (PEM) water electrolysis. However, the unclear and disputed reaction mechanism and structure-activity relationship of OER pose significant obstacles. Herein, the active site and intermediate for OER on AuIr nanoalloys are simultaneously identified and correlated with the activity, through the integration of in situ shell-isolated nanoparticle-enhanced Raman spectroscopy and X-ray absorption spectroscopy. The AuIr nanoalloys display excellent OER performance with an overpotential of only 246 mV to achieve 10 mA cm-2 and long-term stability under strong acidic conditions. Direct spectroscopic evidence demonstrates that * OO adsorbed on IrOx sites is the key intermediate for OER, and it is generated through the O-O coupling of adsorbed oxygen species directly from water, providing clear support for the adsorbate evolution mechanism. Moreover, the Raman information of the * OO intermediate can serve as a universal "in situ descriptor" that can be obtained both experimentally and theoretically to accelerate the catalyst design. It unveils that weakening the interactions of * OO on the catalysts and facilitating its desorption would boost the OER performance. This work deepens the mechanistic understandings on OER and provides insightful guidance for the design of more efficient OER catalysts.

Direct S-H Evidence Revealing the Photo-electrocatalytic Hydrogen Evolution Reaction Mechanism on CdS Using Surface-Enhanced Raman Spectroscopy.

Photoelectrocatalytic water splitting using metal sulfides is a promising method for green hydrogen production. However, in situ probing of the hydrogen evolution reaction (HER) on sulfides with excellent performance remains a challenge. Here, we construct Au@CdS core-shell nanoparticles to study the HER on CdS, a typical HER catalyst, by surface-enhanced Raman spectroscopy (SERS) using a "borrowing" strategy. We directly capture the spectroscopic evidence of S-H intermediate under HER condition, further verified by isotopic experiments. Moreover, the population of S-H intermediates is improved by injecting charge carriers through light illumination and the S-H bond is weakened by introducing Pt to form a Au@Pt@CdS structure to change the interfacial electronic structure, both of them resulting in significant HER performance improvement. These findings can deepen the understanding of the HER mechanism and offer strategies for designing of cost-effective HER catalyst with high performance.

Ultrafast and field-based detection of methamphetamine in hair with Au nanocake-enhanced Raman spectroscopy.

The disaster and devastation from abuse of Methamphetamine (MAMP) have a serious impact on people's mental and physical health. Developing a rapid and accurate method to screen drug suspects and thus control MAMP abuse is essential to social security. Hair analysis for MAMP detection is considered to be one of the most potential methods for monitoring drug abuse due to its convenient sample collection, easy for storage and long traceability period. However, the current accurate detection of MAMP in hair primarily utilizes hyphenated mass spectrometry (MS) techniques, but it is not suitable for field-based detection due to the bulky instrument. Hence, developing alternative portable detection techniques for rapid on-site detection of MAMP in hair is an urgent problem to be solved. Here, the high-performance Au nanocakes (Au NCs) were constructed as surface-enhanced Raman spectroscopy (SERS) substrates to detect MAMP in hair, realizing 5 min ultrafast and ultrasensitive detection utilizing a portable Raman spectrometer. Experiments and finite-difference time-domain (FDTD) simulations show that Au NCs have stronger enhancement than Au nanospheres (Au NPs), and 0.5 ppb (3.35 × 10-9 M) MAMP standard is stably detected by Au NCs as an enhanced substrate. A strategy of liquid-liquid microextraction was exploited to eliminate the interference of complex matrices in hair. This method exhibited excellent reproducibility and temporal stability across different drug addicts (relative standard deviation was 5.14% within 160 s). Our approach shows great promise in public safety, providing a rapid and accurate method to detect in hair by SERS.

In situ Raman spectroscopy reveals the structure evolution and lattice oxygen reaction pathway induced by the crystalline-amorphous heterojunction for water oxidation.

One of the most successful approaches for balancing the high stability and activity of water oxidation in alkaline solutions is to use amorphous and crystalline heterostructures. However, due to the lack of direct evidence at the molecular level, the nano/micro processes of amorphous and crystalline heterostructure electrocatalysts, including self-reconstruction and reaction pathways, remain unknown. Herein, the Leidenfrost effect assisted electrospray approach combined with phase separation was used for the first time to create amorphous NiO x /crystalline α-Fe2O3 (a-NiO x /α-Fe2O3) nanowire arrays. The results of in situ Raman spectroscopy demonstrate that with the increase of the potential at the a-NiO x /α-Fe2O3 interface, a significant accumulation of OH can be observed. Combining with XAS spectra and DFT calculations, we believe that more OH adsorption on the Ni centers can facilitate Ni2+ deprotonation to achieve the high-valence oxidation of Ni4+ according to HSAB theory (Fe3+ serves as a strong Lewis acid). This result promotes the electrocatalysts to follow the lattice oxygen activation mechanism. This work, for the first time, offers direct spectroscopic evidence for deepening the fundamental understanding of the Lewis acid effect of Fe3+, and reveals the synergistic effect on water oxidation via the unique amorphous and crystalline heterostructures.

Exploring the Effect of Pd on the Oxygen Reduction Performance of Pt by In Situ Raman Spectroscopy.

Directly monitoring the oxygen reduction reaction (ORR) process in situ is very important to deeply understand the reaction mechanism and is a critical guideline for the design of high-efficiency catalysts, but there is still lack of definite in situ evidence to clarify the effect between adsorbed intermediates and the strain/electronic effect for enhanced ORR performance. Herein, in situ surface-enhanced Raman spectroscopy (SERS) was employed to detect the intermediates during the ORR process on the Au@Pd@Pt core/shell heterogeneous nanoparticles (NPs). Direct spectroscopic evidence of the *OOH intermediate was obtained, and an obvious red shift of the *OOH frequency was identified with the controllable shell thickness of Pd. Detailed experimental characterizations and density functional theory (DFT) calculations demonstrated that such improved ORR activity after inducing Pd into Au@Pt NPs can be attributed to the optimized adsorbate-substrate interaction due to the strain and electronic effect, leading to a higher Pt-O binding energy and a lower O-O binding energy, which was conducive to O-O dissociation and promoted the subsequent reaction. Notably, this work illustrates a relationship between the performance and strain/electronic effect via the intermediate detected by SERS and paves the way for the construction of ORR electrocatalysts with high performance.

Unmasking the Critical Role of the Ordering Degree of Bimetallic Nanocatalysts on Oxygen Reduction Reaction by In Situ Raman Spectroscopy.

Precise control and accurate understanding of the ordering degree of bimetallic nanocatalysts (BNs) are challenging yet crucial to acquire advanced materials for the oxygen reduction reaction (ORR). AuCu BNs with various ordering degrees were synthesized to evaluate the influence of ordering degree on the ORR at a molecular level using in situ Raman spectroscopy. The activity of AuCu BNs was improved by over 2 times after a disorder-to-order transition, making the performance of highly ordered AuCu BNs exceed that of benchmark Pt/C. Direct Raman spectroscopic evidence of key intermediate (*OH) demonstrates that the active site is the combination site of Au and Cu. Moreover, two distinct *OH species are observed on the ordered and disordered structure, and the ordered site is more beneficial for ORR due to its lower affinity to *OH. This work deepens the understanding on the important role of ordering degree on BNs and enables the design of improved catalysts.

In Situ Raman Probing of Hot-Electron Transfer at Gold-Graphene Interfaces with Atomic Layer Accuracy.

Plasmonic metals under photoexcitation can generate energetic hot electrons to directly induce chemical reactions. However, the capability and fundamental insights of the transportation of these hot electrons at plasmonic metal-2D material interfaces remain unclear. Herein, hot-electron transfer at Au-graphene interfaces has been in situ studied using surface-enhanced Raman spectroscopy (SERS) with atomic layer accuracy. Combining in situ SERS studies with density functional theory calculations, it is proved that hot electrons can be injected from plasmonic Au nanoparticles to graphene and directly penetrate graphene to trigger photocatalytic reactions. With increasing graphene layers, the transportation of hot electrons decays rapidly and would be completely blocked after five layers of graphene. Moreover, the transfer of hot electrons can be modulated by applying an external electric field, and the hot-electron transfer efficiency under electrochemical conditions is improved by over three times in the presence of a monolayer of graphene.

In Situ Raman Observation of Oxygen Activation and Reaction at Platinum-Ceria Interfaces during CO Oxidation.

Understanding the fundamental insights of oxygen activation and reaction at metal-oxide interfaces is of significant importance yet remains a major challenge due to the difficulty in in situ characterization of active oxygen species. Herein, the activation and reaction of molecular oxygen during CO oxidation at platinum-ceria interfaces has been in situ explored using surface-enhanced Raman spectroscopy (SERS) via a borrowing strategy, and different active oxygen species and their evolution during CO oxidation at platinum-ceria interfaces have been directly observed. In situ Raman spectroscopic evidence with isotopic exchange experiments demonstrate that oxygen is efficiently dissociated to chemisorbed O on Pt and lattice Ce-O species simultaneously at interfacial Ce3+ defect sites under CO oxidation, leading to a much higher activity at platinum-ceria interfaces compared to that at Pt alone. Further in situ time-resolved SERS studies and density functional theory simulations reveal a more efficient molecular pathway through the reaction between adsorbed CO and chemisorbed Pt-O species transferred from the interfaces. This work deepens the fundamental understandings on oxygen activation and CO oxidation at metal-oxide interfaces and offers a sensitive technique for the in situ characterization of oxygen species under working conditions.

Maximizing metal utilization by coupling cross-linked PtRu multi-atom on an atomically dispersed ZnFeNC support.

Minimizing noble-metal (NM) usage by exposing all metal atoms on surfaces for catalysis has been a longstanding goal in the development of highly efficient NM catalysts. Here, we realized the full utilization of Pt and Ru atoms by anchoring the cross-linked PtRu multi-atom on the surface of atomically dispersed Zn, Fe and N tri-doped carbon nanomaterials through Pt-N and Ru-N bonds. This supported bimetallic PtRu multi-atom catalyst has extremely high atom efficiency and shows excellent stability and high activity towards the methanol oxidation reaction (MOR). Density functional theory (DFT) calculations reveal that the strongly coupled Pt-N and Ru-N bonds are critical for stabilizing multi-atom PtRu.

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1-xPbxZn2Ga2O7.

Lone pair cations like Pb2+ are extensively utilized to modify and tune physical properties, such as nonlinear optical property and ferroelectricity, of some specific structures owing to their preference to adopt a local distorted coordination environment. Here we report that the incorporation of Pb2+ into the polar "114"-type structure of CaBaZn2Ga2O7 leads to an unexpected cell volume expansion of CaBa1-xPbxZn2Ga2O7 (0 ≤ x ≤ 1), which is a unique structural phenomenon in solid state chemistry. Structure refinements against neutron diffraction and total scattering data and theoretical calculations demonstrate that the unusual evolution of the unit cell for CaBa1-xPbxZn2Ga2O7 is due to the combination of the high stereochemical activity of Pb2+ with the extremely strained [Zn2Ga2O7]4- framework along the c-axis. The unprecedented cell volume expansion of the CaBa1-xPbxZn2Ga2O7 solid solution in fact is a macroscopic performance of the release of uniaxial strain along c-axis when Ba2+ is replaced with smaller Pb2+.

Strong f-f Excitation and Bright Red Emission in Cd4 Gd1-x Eux O(BO3 )3 (0≤x≤1): Near-UV LED Pumped Red Phosphor with Low Thermal Quenching.

Searching efficient red phosphors under near-UV or blue light excitation is practically important to improve the current white light-emitting diodes (WLEDs). Eu2+ - and Mn4+ -based red phosphors have been extensively studied. Here we proposed that Eu3+ is also a promising activator when it resides on a noncentrosymmetric coordination site. We proved that Cd4 GdO(BO3 )3 is a good host, which has a significantly distorted coordination for Eu3+ . A careful crystallographic study was performed on the solid solutions of Cd4 Gd1-x Eux O(BO3 )3 (0≤x≤1) by Rietveld refinements. The as-doped Eu3+ cations locate at the Gd3+ site and are well separated by CdO8 , CdO6 and BO3 groups; thus, only a slight concentration quenching was observed at ≈80 atom % Eu3+ . Most importantly, the parity-forbidden law of 4f-4f transitions for Eu3+ are severely depressed, thus the absorptions at ≈393 and ≈465 nm are remarkable. Cd4 Gd0.2 Eu0.8 O(BO3 )3 can be pumped by a 395 nm LED chip to give a bright red emission, and when mixed with other commercial blue and green phosphors, it can emit the proper white light (0.3657, 0.3613) with a suitable Ra ≈87 and correlated colour temperature ≈4326 K. In-situ photoluminescence study indicated the low thermal quenching of these borate phosphors, especially under 465 nm excitation. Our case proves the practicability to develop near-UV excited red phosphors in rare-earth-containing borates.

Ca2 PbGa8 O15 : Rational Design, Synthesis, and Structure Determination of a Purely Tetrahedra-Based Intergrowth Oxide.

Intergrowth oxides, like Aurivillius, Ruddlesden-Popper phase, comprise functional layers and exhibit interesting physical properties. The hitherto known intergrowth structures mainly were composed of closed-packing of oxygen ions, and it is very challenging to develop new types of intergrowth structures. We proposed the possible match between the tridymite and grossite, both of which are purely tetrahedra-based structures. We synthesized Ca2 PbGa8 O15 ((Ca0.5 Pb0.5 Ga2 O4 )2 (CaGa4 O7 )) and its structure was solved by ab-initio method. Pb2+ is vitally important to stabilize this first example of tetrahedra-based intergrowth oxide. The appropriate size difference between Pb2+ and Ca2+ causes the layered type cationic ordering, and reduced the thermodynamic potential, in addition, the high hybridization between Pb 6s6p and O 2p orbitals further consolidate the covalency of the tetrahedra-base framework.

Strong Lewis Base Ga4B2O9: Ga-O Connectivity Enhanced Basicity and Its Applications in the Strecker Reaction and Catalytic Conversion of n-Propanol.

Heterogeneous solid base catalysis is valuable and promising in chemical industry, however it is insufficiently developed compared to solid acid catalysis due to the lack of satisfied solid base catalysts. To gain the strong basicity, the previous strategy was to basify oxides with alkaline metals to create surficial vacancies or defects, which suffers from the instability under catalytic conditions. Monocomponent basic oxides like MgO are literally stable but deficient in electron-withdrawing ability. Here we prove that a special connectivity of atoms could enhance the Lewis basicity of oxygen in monocomponent solids exemplified by Ga4B2O9. The structure-induced basicity is from the µ3-O linked exclusively to five-coordinated Ga3+. Ga4B2O9 behaved as a durable catalyst with a high yield of 81% in the base-catalyzed synthesis of α-aminonitriles by Strecker reaction. In addition, several monocomponent solid bases were evaluated in the Strecker reaction, and Ga4B2O9 has the largest amount of strong base centers (23.1 µmol/g) and the highest catalytic efficiency. Ga4B2O9 is also applicable in high-temperature solid-gas catalysis, for example, Ga4B2O9 catalyzed efficiently the dehydrogenation of n-propanol, resulting in a high selectivity to propanal (79%). In contrast, the comparison gallium borate, Ga-PKU-1, which is a Brönsted acid, preferred to catalyze the dehydration process to obtain propylene with a selectivity of 94%.

Optimizing the performance of photocatalytic H2 generation for ZnNb2O6 synthesized by a two-step hydrothermal method.

Semiconductor-based photocatalytic H2 generation is a promising technique and the development of efficient photocatalysts has attracted great attention. Columbite-ZnNb2O6 is a wide-bandgap semiconductor capable of photocatalytic water splitting. Here we employed a two-step hydrothermal method to first dissolve Nb2O5 with a highly basic aqueous solution and further react it with Zn2+ to form nanosized ZnNb2O6. The reaction time plays an important role on its morphology and photocatalytic performance in water reduction. The sample synthesized through 7 days of reaction was the optimal one with an appropriate crystallinity and a large specific surface area, however the severe surficial defects prohibited its photocatalytic activity in pure water. The H2 generation at a rate of 23.6(5) µmol h-1 g-1 emerged when 20 vol% methanol was used as the hole-sacrificial agent. Most remarkably, once metal or metal oxide cocatalysts, including Pt, Au, NiO, RuO2, Ag2O, and Pd/PdO, were loaded appropriately, the photocatalytic H2 generation rate ultimately achieved 3200(100) or 680(20) µmol h-1 g-1 with or without using methanol, respectively. Apparent quantum yields (AQYs) at 295 nm were investigated by changing the experimental parameters, and the optimal AQYs are 4.54% and 9.25% in water and methanol solution, respectively. Further post-modifications like bandgap engineering may be performed on this highly efficient nano-ZnNb2O6.

ZnCr2S4: Highly effective photocatalyst converting nitrate into N2 without over-reduction under both UV and pure visible light.

We propose several superiorities of applying some particular metal sulfides to the photocatalytic nitrate reduction in aqueous solution, including the high density of photogenerated excitons, high N2 selectivity (without over-reduction to ammonia). Indeed, ZnCr2S4 behaved as a highly efficient photocatalyst, and with the assistance of 1 wt% cocatalysts (RuOx, Ag, Au, Pd, or Pt), the efficiency was greatly improved. The simultaneous loading of Pt and Pd led to a synergistic effect. It offered the highest nitrate conversion rate of ~45 mg N/h together with the N2 selectivity of ~89%. Such a high activity remained steady after 5 cycles. The optimal apparent quantum yield at 380 nm was 15.46%. More importantly, with the assistance of the surface plasma resonance effect of Au, the visible light activity achieved 1.352 mg N/h under full arc Xe-lamp, and 0.452 mg N/h under pure visible light (λ > 400 nm). Comparing to the previous achievements in photocatalytic nitrate removal, our work on ZnCr2S4 eliminates the over-reduction problem, and possesses an extremely high and steady activity under UV-light, as well as a decent conversion rate under pure visible light.

Y(1-x)Sc(x)BaZn3GaO7 (0 ≤ x ≤ 1): Structure Evolution by Sc-Doping and the First Example of Photocatalytic Water Reduction in "114" Oxides.

"114" oxides have shown intriguing physical properties while their performance in photocatalysis has not yet been reported probably due to the instability in aqueous solution. YBaZn3GaO7 is an exception, which is stable and indeed shows observable photocatalytic H2 evolution (∼2 µmol/h/g) in methanol aqueous solution under UV light. This activity was enhanced to 23.6 µmol/h/g by a full replacement of Y(3+) by Sc(3+). Optical absorption spectra and theoretical calculations show no significant difference upon Sc(3+)-doping. Instead, a systematic analysis of the structure evolution by Rietveld refinements for Y(1-x)Sc(x)BaZn3GaO7 (0 ≤ x ≤ 1) suggests that the increase of the catalytic activity is likely due to the decrease of the structural defects and thus the lower level of recombination rate of e(-) and h(+). In detail, Sc(3+) substitution leads to a shrinkage of YO6 octahedra, and successively the adjustment of the Zn(2+)/Ga(3+) occupancy behaviors in tetrahedra sites. The photocatalytic H2 evolution rate was further optimized to 118.2 µmol/h/g in methanol solution and 42.9 µmol/h/g in pure water for 1 wt % Pt-loaded ScBaZn3GaO7. Here, the relatively less investigated nonmagnetic "114" oxides were, for the first time, proved to be good candidates for photocatalytic water reduction.

Co-molten solvothermal method for synthesizing chalcopyrite CuFe(1-x)Cr(x)S2 (x ≤ 0.4): high photocatalytic activity for the reduction of nitrate ions.

In literature, it is very difficult to obtain sulfides with Cr(3+) in tetrahedral coordination. Here, a thiourea-oxalic acid co-molten solvothermal method was applied to synthesize chalcopyrite CuFe(1-x)Cr(x)S2 (x ≤ 0.4) solid solutions. We propose that oxalic acid plays an important role in the crystallization of CuFe(1-x)Cr(x)S2 and can considerably restrain the formation of other undesirable impurities. The successful incorporation of Cr(3+) was confirmed by powder XRD, SEM and EDX mapping (2D elemental distribution). The UV-Vis reflectance spectra of CuFe(1-x)Cr(x)S2 suggest that the bandgap energies decrease from 0.80 to 0.61 eV along with an increase in the Cr(3+) concentration. All the CuFe(1-x)Cr(x)S2 (0 ≤ x ≤ 0.4) samples show considerably higher photocatalytic activities than P25 toward the reduction of nitrate ions in aqueous solution. We speculate that the thiourea-oxalic acid co-molten method may not only be effective to synthesize Fe(3+)-Cr(3+) sulfides, but can also be helpful to incorporate Cr(3+) to other sulfide systems with MS4 tetrahedra.

Coordination environment evolution of Eu3+ during the dehydration and re-crystallization processes of Sm(1-x)Eu(x)[B9O13(OH)4]·H2O by photoluminescent characteristics.

There are limited photoluminescence (PL) studies for rare earth borates with crystalline water molecules, which are usually supposed to have low PL efficiency because the vibrations of H2O or -OH may lead to emission quenching. We investigated the PL properties of Sm(1-x)Eu(x)[B9O13(OH)4]·H2O (x = 0-1.00) and their dehydrated products α-Sm(1-x)Eu(x)B5O9. There is no quenching effect in those studied polyborates because the large borate ionic groups isolate the Eu(3+) activators very well. Sm(3+) and Eu(3+) are basically separated luminescent activators. Comparatively, Sm(3+) shows a very small emission intensity, which can be almost ignored, therefore our interest is focused on the Eu(3+) luminescence. By TG-DSC and powder XRD experiments, we defined three weight-loss steps for Eu[B9O13(OH)4]·H2O and a re-crystallization process to α-EuB5O9, during which luminescent spectra of Eu(3+) are recorded. It shows an interesting variety and therefore is a good medium to understand the coordination environment evolution of Eu(3+), even for the intermediate amorphous phase. In fact, the coordination symmetry of Eu(3+) in the amorphous state is the lowest. The high efficiency of the f-f transitions and large R/O value (3.8) imply this amorphous phase is potentially a good red-emitting UV-LED phosphor. Anhydrous α-EuB5O9 shows the highest luminescent efficiency excited by Eu(3+) CT transition. In addition, α-Sm(1-x)Eu(x)B5O9 was synthesized by a sol-gel method directly for the first time, and α-EuB5O9 shows superior PL properties due to its better crystallinity. A lot of hydrated polyborates with crystalline water molecules remain unexplored and our study shows their potential as good phosphors.