Synthesis of triphenylpyridines via an oxidative cyclization reaction using Sr-doped LaCoO3 perovskite as a recyclable heterogeneous catalyst

An La_0.6Sr_0.4CoO₃ strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the cyclization reaction between ketoximes and phenylacetic acids. The transformation proceeded via the oxidative functionalization of the sp³ C–H bond in phenylacetic acid. The La_0.6Sr_0.4CoO₃ catalyst demonstrated a superior performance to that of the pristine LaCoCO₃ as well as a series of homogeneous and heterogeneous catalysts. Furthermore, the La_0.6Sr_0.4CoO₃ catalyst was facilely recovered and reused without considerable decline in its catalytic efficiency. To the best of our knowledge, the combination of ketoximes with easily available phenylacetic acids is novel.

An La_0.6Sr_0.4CoO₃ strontium-doped lanthanum cobaltite perovskite was prepared via a gelation and calcination approach and used as a heterogeneous catalyst for the synthesis of triphenylpyridines via the cyclization reaction between ketoximes and phenylacetic acids.     

Precursor accumulation on nanocarbons for the synthesis of LaCoO3 nanoparticles as electrocatalysts for oxygen evolution reaction

Oxygen evolution reaction (OER) is a key step in energy storage devices. Lanthanum cobaltite (LaCoO₃) perovskite is an active catalyst for OER in alkaline solutions, and it is expected to be a low-cost alternative to the state-of-the-art catalysts (IrO₂ and RuO₂) because transition metals are abundant and inexpensive. For efficient catalysis with LaCoO₃, nanosized LaCoO₃ with a high surface area is desirable for increasing the number of catalytically active sites. In this study, we developed a novel synthetic route for LaCoO₃ nanoparticles by accumulating the precursor molecules over nanocarbons. This precursor accumulation (PA) method for LaCoO₃ nanoparticle synthesis is simple and involves the following steps: (1) a commercially available carbon powder is soaked in a solution of the nitrate salts of lanthanum and cobalt and (2) the sample is dried and calcined in air. The LaCoO₃ nanoparticles prepared by the PA method have a high specific surface area (12 m² g⁻¹), comparable to that of conventional LaCoO₃ nanoparticles. The morphology of the LaCoO₃ nanoparticles is affected by the nanocarbon type, and LaCoO₃ nanoparticles with diameters of less than 100 nm were obtained when carbon black (Ketjen black) was used as the support. Further, the sulfur impurities in nanocarbons significantly influence the formation of the perovskite structure. The prepared LaCoO₃ nanoparticles show excellent OER activity owing to their high surface area and perovskite structure. The Tafel slope of these LaCoO₃ nanoparticles is as low as that of the previously reported active LaCoO₃ catalyst. The results strongly suggest that the PA method provides nanosized LaCoO₃ without requiring the precise control of chemical reactions, harsh conditions, and/or special apparatus, indicating that it is promising for producing active OER catalysts at a large scale.

A simple synthetic process for LaCoO₃ nanoparticles based on the accumulation of precursors on nanocarbon supports was presented. The LaCoO₃ nanoparticles showed excellent OER activity owing to their high surface area and perovskite structure.     

Effects of LaCoO3 perovskite nanoparticle on Daphnia magna: accumulation,distribution and biomarker responses

Perovskite nanomaterials (PNMs) have been shown to be promising materials for the effective replacement of conventional energy source materials. With the increasing use of PNMs, they will inevitably enter aquatic environments, giving rise to concerns regarding the environmental impact of PNMs. To fill up the gap in information about the environmental effect of PNMs, Daphnia magna was exposed to a typical PNM LaCoO₃ for 48 h, to assess temporal patterns in PNM bioaccumulation and distribution. Synchrotron radiation based micro X-ray fluorescence spectroscopy (μ-XRF) was used to investigate the time dependent spatial distribution of LaCoO₃. Reactive oxygen species (ROS), superoxide dismutase (SOD) and Na⁺/K⁺-adenosine triphosphatase (ATPase) were measured as key biomarkers. The results showed that oxidative stress was observed at both LaCoO₃ concentrations and Na⁺/K⁺-ATPase was inhibited by high levels of LaCoO₃. The mode of action of LaCoO₃ was mainly dependent on the metal forms. At low LaCoO₃ levels, food ingestion was the main entry pathway into organisms and LaCoO₃ nanoparticle aggregates accumulated in the gut area. At high LaCoO₃ levels, both waterborne and dietary uptake was observed and the gut and thoracic limbs were the main target sites for LaCoO₃ nanoparticle aggregates and dissolved ions, respectively. LaCoO₃ was not found to translocate in daphnids during the 48 h exposure period at either concentration, suggesting that internalization did not occur. These findings help further our understanding of the fate of PNMs in aquatic organisms, as well as the associated biological responses to PNM exposure.

The instability of PNMs in water is of environmental concern. This study shows that in daphnids over 48 h, the mode of action of a representative PNM LaCoO₃ is dependent on Co species, which results in the differences in uptake, accumulation, distribution and toxicity.     

The spin–orbit–phonon coupling and crystalline elasticity of LaCoO3 perovskite

Based on an integrated study of magnetic susceptibility, specific heat, and thermal expansion of single-crystal LaCoO₃ free from cobalt and oxygen vacancies, two narrow spin gaps are identified before and after the phonon softening of gap size ΔE ∼ 0.5 meV in a CoO₆-octahedral crystal electric field (CEF) and the thermally activated spin gap Q ∼ 25 meV, respectively. Significant excitation of Co³⁺ spins from a low-spin (LS) to a high-spin (HS) state is confirmed by the thermal activation behavior of spin susceptibility χ^S of energy gap Q ∼ 25 meV, which follows a two-level Boltzmann distribution to saturate at a level of 50% LS/50% HS statistically above ∼200 K, without the inclusion of a postulated intermediate spin (IS) state. A threefold increase in the thermal expansion; coefficient (α) across the same temperature range as that of thermally activated HS population growth is identified, which implies the non-trivial spin–orbit–phonon coupling caused the bond length of Co^3+(LS↔HS)–O fluctuation and the local lattice distortion. The unusually narrow gap of ΔE ∼ 0.5 meV for the CoO₆ octahedral CEF between e_g–t_2g indicates a more isotropic negative charge distribution within the octahedral CEF environment, which is verified by the Electron Energy Loss Spectroscopy (EELS) study to show nontrivial La–O covalency.

Considering the before and after phonon softening, the gap in a CoO₆-octahedral crystal electric fields (CEF) and the thermally activated spin gap, were observed of ∼0.5 meV and Q ∼ 25 meV in defect-free LaCoO₃ single crystal, respectively.     

Enhancing the water splitting performance via decorating Co3O4 nanoarrays with ruthenium doping and phosphorization

Hydrogen is the most promising renewable energy source to replace traditional fossil fuels for its ultrahigh energy density, abundance and environmental friendliness. Generating hydrogen by water splitting with highly efficient electrocatalysts is a feasible route to meet current and future energy demand. Herein, the effects of Ru doping and phosphorization treatment on Co₃O₄ nanoarrays for water splitting are systemically investigated. The results show that a small amount of phosphorus can accelerate hydrogen evolution reaction (HER) and the trace of Ru dopant can significantly enhance the catalytic activities for HER and oxygen evolution reaction (OER). Ru-doped cobalt phosphorous oxide/nickel foam (CoRuPO/NF) nanoarrays exhibit highly efficient catalytic performance with an overpotential of 26 mV at 10 mA cm⁻² for HER and 342 mV at 50 mA cm⁻² for OER in 1 M KOH solution, indicating superior water splitting performance. Furthermore, the CoRuPO/NF also exhibits eminent and durable activities for alkaline seawater electrolysis. This work significantly advances the development of seawater splitting for hydrogen generation.

CoRuPO/NF shows low overpotentials in HER and OER.     

Heteroatom-doped nanoporous carbon from recyclable Pueraria lobata and its dual activities for oxygen reduction and hydrogen evolution reactions

Many efficient and non-precious metal catalysts for oxygen reduction or hydrogen evolution reactions have been developed, but bifunctional catalysts for both oxygen reduction reaction and hydrogen evolution reactions are seldom reported despite their advantages. Herein, we designed the bulk preparation of heteroatom-doped nanoporous carbon catalysts using widely available and recyclable Pueraria lobata powder as the carbon source. The typical product was N, P and Fe Tri-doped nano-porous carbon (N,P,Fe-NPC) with high surface area (BET surface area of 776.68 m² g⁻¹ and electrochemical surface area of 55.0 mF cm⁻²). The typical N,P,Fe-NPC sample simultaneously exhibited high activities for oxygen reduction and hydrogen evolution reactions. Because of the high surface area and the tri-doping of N, P and Fe elements, the prepared material may have applications in other fields such as gas uptake, sensors, sewage treatment, and supercapacitors. The suggested approach is low-cost, simple and readily scalable.

The bulk preparation of an N, P and Fe Tri-doped nano-porous carbon sample using recyclable Pueraria powder, which exhibits dual activities.     

Enhancing the efficiency of the hydrogen evolution reaction utilising Fe3P bulk modified screen-printed electrodes via the application of a magnetic field

We report the fabrication and optimisation of Fe₃P bulk modified screen-printed electrochemical platforms (SPEs) for the hydrogen evolution reaction (HER) within acidic media. We optimise the achievable current density towards the HER of the Fe₃P SPEs by utilising ball-milled Fe₃P variants and increasing the mass percentage of Fe₃P incorporated into the SPEs. Additionally, the synergy of the application of a variable weak (constant) external magnetic field (330 mT to 40 mT) beneficially augments the current density output by 56%. This paper not only highlights the benefits of physical catalyst optimisation but also demonstrates a methodology to further enhance the cathodic efficiency of the HER with the facile application of a weak (constant) magnetic field.

We report the fabrication and optimisation of Fe₃P bulk modified screen-printed electrochemical platforms (SPEs), enhancing their performance towards the hydrogen evolution reaction (HER), within acidic media, by the application of a magentic field.     

N-doped mixed Co,Ni-oxides with petal structure as effective catalysts for hydrogen and oxygen evolution by water splitting

Developing electrocatalytic nanomaterials for green H₂ energy is inseparable from the exploration of novel materials and internal mechanisms for catalytic enhancement. In this work, nano-petal N-doped bi-metal (Ni, Co) and bi-valence (+2, +3) (Ni_1−xCo_x)²⁺Co₂³⁺O₄ compounds have been in situ grown on the surface of Ni foam. The N³⁻ atoms originate from the amino group in urea and doped in the compound during annealing. The as-synthesized N-doped (Ni_1−xCo_x)²⁺Co₂³⁺O₄ nano-petals demonstrate commendable hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) bi-functional catalytic efficiency and stability. Electrochemical measurements confirm that the nitrogen doping significantly improves the catalytic kinetics and the surface area. Density functional theory calculations reveal that the improved HER and OER kinetics is not only due to the synergistic effect of bi-metal and bi-valence, as well as the introduction of defects such as oxygen vacancies, but also it more depends on the shortened bond length between the nitrogen N³⁻ atoms and the metal atoms, and the increased electron density of the metal atoms attached to the N³⁻ atoms. In other words, the change of lattice parameters caused by nitrogen doping is more conducive to the catalytic enhancement than the synergistic effect brought by bi-metal. This study provides an experimental and theoretical reference for the design of bi-functional electrocatalytic nanomaterials.

Developing electrocatalytic nanomaterials for green H₂ energy is inseparable from the exploration of novel materials and internal mechanisms for catalytic enhancement.     

Green emission of indium oxide via hydrogen treatment

In this work, we prepared hydrogen treated indium oxide (H₂-In₂O₃) and investigated the effect of hydrogen treatment on the optical and photoluminescence properties of In₂O₃. Hydrogen treatment has no influence on the crystal structure, but alters the intrinsic electronic structure and optical properties via introducing hydrogen induced defects such as shallow donor states (near the conduction band) and singly ionized oxygen vacancies in H₂-In₂O₃. Both air-In₂O₃ (air calcinated) and H₂-In₂O₃ show intense blue emission under UV excitation (280 nm). However, hydrogen treated In₂O₃ exhibited an additional green emission, which is absent in air-In₂O₃. This green emission arises from the passivation of singly ionized oxygen vacancies by hydrogen treatment. Hydrogen treatment could be a promising strategy to tune the electronic and optical properties of In₂O₃.

H₂-treated In₂O₃ gives rise to photoemission ranging from blue to green-yellow, while air-calcined In₂O₃ shows only blue emission. EPR and optical spectroscopies reveal singly ionized oxygen vacancies induced by H₂ treatment responsible for the green-yellow emission.     

Enhancing the low temperature NH3-SCR activity of FeTiOx catalysts via Cu doping: a combination of experimental and theoretical study

A series of Fe_αCu_1−αTiO_x catalysts with variable Cu doping amounts was directly synthesized by the sol–gel method and their catalytic performances were tested for the selective catalytic reduction of NO with ammonia. The highest activity was achieved on Fe_0.9Cu_0.1Ti catalyst. NO conversion was above 80% and N₂ selectivity exceeded 90% on this catalyst in the temperature range of 200–375 °C. High NO and NH₃ oxidation activities facilitated the high NH₃-SCR activities of the catalysts in the low temperature range, while too strong NH₃ oxidation ability resulted in the decline of NH₃-SCR activity. DFT calculations based on the Fe and Cu co-doping TiO₂ model showed that the barrier of NH₃ activation is dramatically reduced as compared to pure Fe doping. This is due to the lowered p-band of lattice O. However, such activated O will also strongly decrease the barrier for the dissociation of NH₂ to NH species, which will lead to the formation of N₂O. Both Brønsted and Lewis acid sites over Fe_0.9Cu_0.1Ti catalyst are involved in the NH₃-SCR reaction. The adsorption of NO_x is strong in the low temperature range, and large amounts of nitrates were decomposed on the catalyst surface in the high temperature range.

A series of Fe_αCu_1−αTiO_x catalysts with variable Cu doping amounts was directly synthesized by the sol–gel method and their catalytic performances were tested for the selective catalytic reduction of NO with ammonia.     

Facile doping of nickel into Co3O4 nanostructures to make them efficient for catalyzing the oxygen evolution reaction

Designing a facile and low-cost methodology to fabricate earth-abundant catalysts is very much needed for a wide range of applications. Herein, a simple and straightforward approach was developed to tune the electronic properties of cobalt oxide nanostructures by doping them with nickel and then using them to catalyze the oxygen evolution reaction (OER) in an aqueous solution of 1.0 M KOH. The addition of a nickel impurity improved the conductivity of the cobalt oxide, and further increased its activity towards the OER. Analytical techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and powder X-ray diffraction (XRD) were used to investigate, respectively, the morphology, composition and crystalline structure of the materials used. The nickel-doped cobalt oxide material showed randomly oriented nanowires and a high density of nanoparticles, exhibited the cubic phase, and contained cobalt, nickel and oxygen as its main elements. The nickel-doped cobalt oxide also yielded a Tafel slope of 82 mV dec⁻¹ and required an overpotential of 300 mV to reach a current density of 10 mA cm⁻². As an OER catalyst, it was shown to be durable for 40 h. Electrochemical impedance spectroscopy (EIS) analysis showed a low charge-transfer resistance of 177.5 ohms for the nickel-doped cobalt oxide, which provided a further example of its excellent OER performance. These results taken together indicated that nickel doping of cobalt oxide can be accomplished via a facile approach and that the product of this doping can be used for energy and environmental applications.

Designing a facile and low-cost methodology to fabricate earth-abundant catalysts is very much needed for a wide range of applications.     

First-principles study of vanadium carbides as electrocatalysts for hydrogen and oxygen evolution reactions

Vanadium carbides have attracted much attention as highly active catalysts in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), while a satisfactory understanding of the underlying mechanisms still remains a challenge. Herein we apply first-principles calculations to systematically analyze the crystal structures, electronic properties, free energies during the HER and OER processes, surface energies and crystal formation energies of the three types of vanadium carbides, i.e., V₄C₃, V₈C₇ and VC. We show that all these vanadium carbides are metallic, which enables efficient electron transport from the bulk to the surface of the catalysts. All these vanadium carbides exhibit excellent HER performance but show poor OER catalytic activity. In particular, the V₈C₇ (110) surface shows the best catalytic performance for its relatively small |ΔG(H*)| value (−0.114 eV) for HER. Emergence of natural carbon vacancies gives rise to large surface energy, proper hydrogen adsorption energy, low crystal formation energy and weak bond strength in V₈V₇, which guarantees its leading position among the three vanadium carbides. In addition, a remarkable resemblance between VC/V₈C₇ and Pt in their electronic structures on (110) and (111) surfaces are found, which indicates a Pt-like HER mechanism in these vanadium carbides. Our results thus bring new insights to the theoretical understanding of the excellent HER performance of vanadium carbides.

The origin of excellent performance of vanadium carbides (VC and V₈V₇) in hydrogen and oxygen evolution reactions (HER and OER) is revealed by first-principles calculations. It is found that the underlying mechanisms in HER/OER processes are Pt-like.     

MOF derived carbon based nanocomposite materials as efficient electrocatalysts for oxygen reduction and oxygen and hydrogen evolution reactions

The escalating global energy demands and the formidable risks posed by fossil fuels coupled with their rapid depletion have inspired researchers to embark on a quest for sustainable clean energy. Electrochemistry based technologies, e.g., fuel cells, Zn–air batteries or water splitting, are some of the frontrunners of this green energy revolution. The primary concern of such sustainable energy technologies is the efficient conversion and storage of clean energy. Most of these technologies are based on half-cell reactions like oxygen reduction, oxygen and hydrogen evolution reactions, which in turn depend on noble metal based catalysts for their efficient functioning. In order to make such green energy technologies economically viable, the need of the hour is to develop new noble metal free catalysts. Porous carbon, with some assistance from heteroatoms like N or S or earth abundant transition metal or metal oxide nanoparticles, has shown excellent potential in the catalysis of such electrochemical reactions. Metal–organic frameworks (MOFs) containing metal nodes and organic linkers in an ordered morphology with inherent porosity are ideal self-sacrificial templates for such carbon materials. There has been a recent spurt in reports on such MOF-derived carbon based materials as electrocatalysts. In this review, we have presented some of this research work and also discussed the practical reasons behind choosing MOFs for this purpose. Different approaches for synthesizing such carbonaceous materials with unique morphologies and doping, targeted towards superior electrochemical activity, have been documented in this review.

Hetero-atom doped porous carbon materials derived from MOFs are efficient noble metal-free electrocatalysts.     

Bandgap engineering of a lead-free defect perovskite Cs3Bi2I9 through trivalent doping of Ru3+

Inorganic defect halide compounds such as Cs₃Bi₂I₉ have been regarded as promising alternatives to overcome the instability and toxicity issues of conventional perovskite solar cells. However, their wide indirect bandgaps and deep defect states severely limit their photoelectronic conversion efficiency when implemented in devices. Trivalent cation substitution has been proposed by previous calculations allowing the engineering of their band structures, but experimental evidences are still lacking. Herein we use the trivalent cation Ru³⁺ to partially replace Bi³⁺ in Cs₃Bi₂I₉, and reveal their structural and optoelectronic properties, as well as the environmental stability. The Ru-doped Cs₃Bi₂I₉ shows a decreasing bandgap with the increasing doping levels and an overall up-shift of band structure, owing to the dopant-induced defect states and thus enhanced phonon–electron coupling. As a result, upon Ru³⁺ doping, the narrowed bandgap and the upward shift of the band structures might facilitate and broaden their applications in optoelectronic devices.

Inorganic defect halide compounds such as Cs₃Bi₂I₉ have been regarded as promising alternatives to overcome the instability and toxicity issues of conventional perovskite solar cells.     

Enhanced gas sensing performance of perovskite YFe1−xMnxO3 by doping manganese ions

Perovskite YFe_1−xMn_xO₃ with a hierarchical structure were prepared by a simple hydrothermal method and used as gas sensing materials. The structure, morphology and composition of YFe_1−xMn_xO₃ were investigated using X-ray diffraction, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The gas sensing test showed that all YFe_1−xMn_xO₃ perovskites with different Mn doping concentrations displayed fast response and recovery characteristics to multiple analytes as well as good stability and recoverability. With the increase of Mn doping concentration, the response of YFe_1−xMn_xO₃ to four kinds of target atmospheres first increases, then decreases. The sensing performance of YFe_1−xMn_xO₃ is best when x = 0.05. Compared with pure YFeO₃, the responses of YFe_0.95Mn_0.05O₃ to 1000 ppm of CH₂O, C₂H₆O, H₂O₂ and 100% relative humidity were increased by 835%, 1462%, 812% and 801%, respectively. The theoretical detection limit of YFe_0.95Mn_0.05O₃ for H₂O₂ and CH₂O is 1.75 and 2.55 ppb, respectively. Furthermore, the possibility of buildings a sensor array based on YFe_1−xMn_xO₃ with different doping concentrations was evaluated by principal component analysis and radar chart analysis. It is feasible to realize the visual and discriminative detection of the target analyte by constructing sensor arrays through radar chart analysis and database construction.

The gas sensitive performance of perovskite YFe_1−xMn_xO₃ can be tailored effectively by simple manganese ion doping.     

Photolysis of hydrogen peroxide, an effective disinfection system via hydroxyl radical formation

The relationship between the amount of hydroxyl radicals generated by photolysis of H(2)O(2) and bactericidal activity was examined. H(2)O(2) (1 M) was irradiated with laser light at a wavelength of 405 nm to generate hydroxyl radicals. Electron spin resonance spin trapping analysis showed that the amount of hydroxyl radicals produced increased with the irradiation time. Four species of pathogenic oral bacteria, Staphylococcus aureus, Aggregatibacter actinomycetemcomitans, Streptococcus mutans, and Enterococcus faecalis, were used in the bactericidal assay. S. mutans in a model biofilm was also examined. Laser irradiation of suspensions in 1 M H(2)O(2) resulted in a >99.99% reduction of the viable counts of each of the test species within 3 min of treatment. Treatment of S. mutans in a biofilm resulted in a >99.999% reduction of viable counts within 3 min. Other results demonstrated that the bactericidal activity was dependent on the amount of hydroxyl radicals generated. Treatment of bacteria with 200 to 300 μM hydroxyl radicals would result in reductions of viable counts of >99.99%.     

Unraveling the microstructural and optoelectronic properties of solution-processed Pr-doped SrSnO3 perovskite oxide thin films

The inorganic stannous-based perovskite oxide SrSnO₃ has been utilized in various optoelectronic applications. Facilitating the synthesis process and engineering its properties, however, are still considered challenging due to several aspects. This paper reports on a thorough investigation of the influence of rare-earth (praseodymium) doping on the microstructural and optoelectronic properties of pure and Pr-doped SrSnO₃ perovskite oxide thin films synthesized by a two-step simple chemical solution deposition route. Structural analysis indicated the high quality of the obtained phase and the alteration generated from the insertion of impurities. Surface scanning illustrated the formation of homogenous and crack-free SrSnO₃ thin films with a nanorod morphology, with an augmentation in size as the dopant ratios increased. Optical properties analysis showed an enhancement in the samples optical absorption with wide-range bandgap tuning. First-principles calculations revealed the exchange interactions between the 3d–4f states and their impact on the electronic properties of the pristine material. Hall-effect measurements revealed an immense decrement in the resistivity of the films upon increment of doping ratios, passing from 7.3 × 10⁻² Ω cm for the undoped sample to 4.8 × 10⁻² Ω cm for 7% Pr content, while a reverse trend was observed on the carrier mobility, rising from 2.5 to 7.6 cm² V⁻¹ s⁻¹ for 7% Pr content. The results emphasized the efficiency of the simple synthesis route to produce high-quality samples. The current findings will contribute to paving the way towards expanding the utilization of simple and cost-effective chemical solution deposition methods for the fast and large area growth of stannous-based perovskite oxides for optoelectronic applications.

Unraveling the optical, electronic and electrical properties of high-quality nanorod morphology spray-coated Pr-doped SrSnO₃ perovskite oxide thin films.     

The effect of heteroatom doping on the active metal site of CoS2 for hydrogen evolution reaction

The exploration of cost-effective hydrogen evolution reaction (HER) electrocatalysts through water splitting is important for developing clean energy technology and devices. The application of CoS₂ in HER has been drawing more and more attention due to its low cost and relatively satisfactory HER catalytic performance. And CoS₂ was found to exhibit excellent HER catalytic performance after appropriate doping according to other experimental investigations. However, the theoretical simulation and the intrinsic catalytic mechanism of CoS₂ remains insufficiently investigated. Therefore, in this study, density functional theory is used to investigate the HER catalytic activity of CoS₂ doped with a heteroatom. The results show that Pt-, N- and O-doped CoS₂ demonstrates smaller Gibbs free energies close to that of Pt, compared with the original CoS₂ and CoS₂ doped with other atoms. Furthermore, HER catalytic performance of CoS₂ can be improved by tuning d-band centers of H adsorption sites. This study provides an effective method to achieve modified CoS₂ for high-performance HER and to investigate other transition metal sulfides as HER electrode.

The linear relationship between ΔG_H* and d-band centers of H adsorption sites.     

Enhancing the photocatalytic hydrogen production activity of BiVO4 [110] facets using oxygen vacancies

The activity of the hydrogen evolution reaction (HER) during photoelectrochemical (PEC) water-splitting is limited when using BiVO₄ with an exposed [110] facet because the conduction band minimum is below the H⁺/H₂O potential. Here, we enhance the photocatalytic hydrogen production activity through introducing an oxygen vacancy. Our first-principles calculations show that the oxygen vacancy can tune the band edge positions of the [110] facet, originating from an induced internal electric field related to geometry distortion and charge rearrangement. Furthermore, the induced electric field favors photogenerated electron–hole separation and the enhancement of atomic activity. More importantly, oxygen-vacancy-induced electronic states can increase the probability of photogenerated electron transitions, thus improving optical absorption. This study indicates that oxygen-defect engineering is an effective method for improving the photocatalytic activity when using PEC technology.

An oxygen-vacancy-induced internal electric field enhances the photocatalytic hydrogen production activity of a BiVO₄ [110] facet.     

Electrocatalytic reduction of oxygen at platinum nanoparticles dispersed on electrochemically reduced graphene oxide/PEDOT:PSS composites

Composites of commercially available graphene oxide (GO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with solvent additive ethylene glycol (EG) were investigated as an alternative support for Pt nanoparticles towards the electrocatalytic reduction of oxygen. The surface characteristics of the materials were examined using atomic force microscopy (AFM), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS). Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) at rotating disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) were used to characterise the electrocatalytic activities of the composites materials. The structural and electrochemical studies reveal that the addition of EG favours the homogeneous distribution of Pt particles with reduced particle size and improves the electrocatalytic properties. A 30% and 16% increase in electrochemically active surface area (ECSA), a 1.2 and 1.1 fold increase in specific area activity (SA), and a 1.5 and 1.2 fold increase in mass activity (MA) were observed for 30% and 40% Pt loading on PEDOT:PSS after the addition of EG. A composite of rGO and PEDOT:PSS(EG) was investigated for different (w/w) ratios of PEDOT:PSS and rGO. The 1 : 2 w/w ratio showed an enhanced catalytic activity with high limiting current, more positive onset potential, higher SA and MA with lower H₂O₂ yield compared to PEDOT:PSS(EG) and rGO and previously reported values for PEDOT:PSS.

Composites of commercially available graphene oxide and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) with solvent additive ethylene glycol were investigated as an alternative support for Pt nanoparticles towards the electrocatalytic reduction of oxygen.