In recent decades, the most common photocatalyst used in environmental applications has been TiO₂, which exhibits a large band gap and can only be photoactivated under UV light. Therefore, the scientific interest has recently focused on the development of visible-light-activated photocatalysts such as g-C₃N₄. However, the photocatalytic performance of g-C₃N₄ is significantly hindered by the rapid recombination of photogenerated charges. An interesting approach to overcome this drawback is to fabricate Schottky junctions by combining g-C₃N₄ with materials demonstrating high metallicity.

In the present study, g-C₃N₄ was synthesized by forming a urea slurry that was dried and then calcinated at 550 ^oC for 4 h (CNU). The resulting CNU was ground and calcinated again at 550 ^oC for 2 h (3×) to achieve thermal exfoliation of its stacked 2D layers (CNUex3). In addition, a quantity of CNUex3 was then subjected to protonation using an HCl solution (pCNUex3). Ti₃C₂T_X MXene was prepared via a top-down methodology using HF to achieve wet-chemical etching of the Al layers from the Ti₃AlC₂ Max Phase. Subsequently, low amounts of Ti₃C₂T_X were coupled with either CNUex3 or pCNUex3 via a simple sonication methodology to form x%-CNMX or x%-pCNMX 2D/2D Schottky junctions (x = 1, 3 or 5), respectively. The structural, morphological and optical characteristics of all the synthesized materials were investigated using various characterization techniques (PXRD, SEM, ATR-FTIR, Raman spectroscopy, DRS, etc.). Furthermore, their photocatalytic performance was evaluated through laboratory-scale experiments using the antihypertensive drug Valsartan as a model pollutant, since it is commonly detected in various aquatic matrices. The results revealed that the combination of either CNUex3 or pCNUex3 with low amounts of Ti₃C₂T_X improves their photocatalytic performance due to the successful suppression of exciton recombination as a result of the formation of a Schottky barrier at the interface of the two materials.

Graphitic carbon nitride (g-C₃N₄) is an intriguing two-dimensional (2D) material characterized by remarkable features, such as visible light absorption, superior thermal stability, and a large abundance of its components in the Earth's crust. In contrast to conventional photocatalysts like titanium dioxide (TiO₂), g-C₃N₄ may function well in visible light, which constitutes a substantial segment of the solar spectrum. This makes it a more sustainable alternative for environmental cleanup. Nonetheless, the efficacy of pure g-C₃N₄ in photocatalysis is impeded by challenges such as the rapid recombination of electron–hole pairs and a very limited surface area. To tackle these issues, g-C₃N₄ is modified with metal sulfides such as zinc sulfide (ZnS) and bismuth sulfide (Bi₂S₃) by a simple, sustainable process employing starch. These adjustments boost charge carrier separation and improve light absorption, leading to a significant increase in photocatalytic efficiency.
These composite materials have shown remarkable efficacy in effectively degrading coumarin and para-nitrophenol upon exposure to visible light. The amalgamation of g-C₃N₄ and metal sulfides markedly enhances degradation rates, making them very useful for environmental remediation applications. This advancement offers an effective and eco-friendly method for degrading organic contaminants present in wastewater and industrial discharges.

This work was supported by the National Research, Development, and Innovation Office of Hungary in the frame of the bilateral Hungarian-Vietnamese S&T Cooperation Program (project code 2019-2.1.12-TÉT_VN-2020-00009) and by the Ministry for Innovation and Technology of Hungary from the National Research, Development and Innovation Fund, financed under the 2021 Thematic Excellence Program funding scheme (grant number TKP2021-NKTA-21).

The effect of modifying TiO₂ with Pd, as well as of Cu and Ag co-catalysts on TiO₂, on the photocatalytic efficiency exhibited by these catalysts in the photocatalytic water splitting and nitrate reduction reaction was studied. Mono- (Pd, Cu, and Ag) and bimetallic (Pd-Cu and Pd-Ag) nanoparticles were prepared by a modified protocol of the alkaline polyol method [1]. The size, shape and specific morphologies of nanoparticles can be controlled from the preparation method [2, 3]. The physical and chemical properties of the Pd-M (M=Cu, Ag) catalysts supported on TiO₂ were investigated using different methods such as TEM, XRD, XPS, H₂-TPR, UV-Vis, CO chemisorption and fractal analysis. The photocatalytic activity of the prepared catalysts was tested by evaluating the generation of hydrogen from water under UV light (150 W high-pressure lamp with inner irradiation). The catalytic and photocatalytic removal of nitrates from aqueous solutions was examined using TiO₂-based materials containing bimetallic nanoparticles.

TEM and CO chemisorption measurements confirmed that the particle size is in the nanometric range. TEM images were examined in detail using the "box-counting method" to determine the fractal dimension [4]. The studied catalysts showed fractal behavior. H₂-TPR and XPS analyses provided information about the metal–support interaction and oxidation states of metallic components. Bimetallic Pd-Cu nanoparticles dispersed on TiO₂ with various molar ratios showed better activity for photocatalytic water splitting and nitrate reduction reaction compared to the Pd-Ag/TiO₂ sample. Depending on the surface atomic composition of the co-catalyst, an estimation of the optimal Pd-M (M=Cu, Ag) molar ratio was made to improve the photocatalytic performance in the H₂ generation reaction. A better understanding of the role of bimetallic nanoparticles is of the utmost importance for the design of efficient photocatalysts.

The commonly used β-blocker nadolol is regularly detected in wastewater and purified wastewater, representing a serious environmental problem. Photocatalysis has emerged as an innovative strategy to address such pollutants, with TiO₂ being a widely studied photocatalyst due to its potential in water treatment. However, its practical application is hindered by the high recombination rate of photo-generated electron–hole pairs. Therefore, the modification of TiO₂ to increase photocatalytic performance is one of the most important objectives in photocatalyst studies. For the modification of catalysts, various polymers can be used, such as polyvinylidene fluoride, hydroquinone superlattice, polypropylene, and poly(methyl methacrylate) (PMMA). Among these, PMMA stands out as a low-cost, non-toxic, water-insoluble polymer. In this study, powder PMMA was modified with silver, combined with TiO₂ nanopowder, and applied for the degradation of nadolol under UV-LED radiation. The degradation kinetics were monitored using high-performance liquid chromatography, and pH changes were observed using a pH meter. After 120 min of UV-LED irradiation, the materials demonstrated a significantly higher removal efficiency of nadolol compared to direct photolysis, specifically a 94 % removal efficiency of nadolol, significantly outperforming direct photolysis. The photocatalytic activity results demonstrated the practical applicability of the novel materials. The degradation followed pseudo-first-order kinetics, as evidenced by the calculated rate constant.

This study investigates the influence of platinum (Pt) loading on the photothermal NO₂ reduction activity of TiO₂/Pt catalysts, focussing on the relationship between Schottky barrier height (SBH) and catalytic performance. Noble metals such as Pt generate “hot electrons” under visible light due to their localised surface plasmon resonance (LSPR) properties. These electrons are injected into the TiO₂ conduction band, and the efficiency depends on the SBH at the TiO₂/Pt interface. Catalysts with different Pt loadings (0.5-2 wt%) were synthesised by wet impregnation and then extensively characterised using techniques such as XRD, TEM, SEM-EDS, XPS and UV-vis DR spectroscopy to evaluate their optical, electronic and structural properties. The results showed that higher Pt loading reduced the SBH from 0.42 eV for TiO₂/0.5%Pt to 0.16 eV for TiO₂/2%Pt. This reduction facilitates faster electron injection and minimises recombination, which increases the photothermal NO₂ reduction performance. The average size of Pt nanoparticles increased from 1.1 nm to 1.5 nm with increasing Pt content, but the morphology and crystallinity of TiO₂ remained stable. Catalytic tests showed that TiO₂ was inactive below 100 °C, while TiO₂/Pt catalysts enabled NO₂ reduction at temperatures as low as 30 °C under visible light. However, a higher SBH value reduced the activity due to increased electron–hole recombination at the TiO₂/Pt interface. The results confirm the multifunctionality of TiO₂/Pt catalysts and demonstrate their suitability for hybrid photothermal reaction systems for NO_x reduction at low temperatures. These results underline the crucial role of optimising Pt loading to balance SBH and improve catalytic performance in photothermal applications.

Photocatalytic hydrogen production is among the top ten emerging technologies in chemistry. In particular, the photoreforming (PR) of organic substrates is an attractive way to obtain green H₂ together with the valorisation of waste or biomass. This reaction combines water reduction with the oxidation of a sacrificial agent using a semiconductor. In this context, it is possible to use, as sacrificial agents, new emerging water pollutants as plastic materials like polystyrene (PS) and polyethylene (PE) to obtain H₂ with a contextual water purification. Furthermore, an important point is the choice of photocatalysts. Carbon-based materials are prospective candidates for their remarkable electrical, thermal, and mechanical properties. For this reason, the focus of this work was to study the performance of composites based on metal carbides (MCs) (SiC, MoC, TiC) with different amounts of graphitic carbon nitride (g-C₃N₄). For the preparation of these catalysts, thermal polymerization was used, whereas for increasing the production of H₂, Pt (1 wt%) was added with wetness impregnation. The photocatalytic tests were performed with 50 mg of the photocatalyst, homogeneously suspended in an aqueous solution containing previously pre-treated PS or PE, with the photoreactor irradiated for 5 h using a solar lamp. A good production of H₂ was verified from all the investigated sacrificial agents. PE PR using the TiC1%g-C₃N₄_Pt sample allowed more H₂ (750 umol H₂/(gcat*h)) to be produced compared to PS PR (170umol H₂/(gcat*h)). This can be due to the easier C–C bond cleavage and C–C bond coupling of PE with respect to PS during the pre-treatment and the photoreforming reaction. The results obtained in this work pave the way for future environmental perspectives in which the pollutants can be considered new raw materials to obtain H₂, contextually preserving the water by the emerging contaminants.

Medium-temperature (up to 200°C) polymer membrane water (steam) electrolyzers are attractive due to improved thermodynamics and kinetics, and reduced specific electric energy consumption for electrolytic gas generation. Moreover, applications of alternative polymer membranes (such as acid-doped polybenzimidazoles) less sensitive to the presence of the impurities in water (in contrast to Nafion® and its analogs) significantly simplify the water treatment system.

The aim of this study is to develop electrocatalytic layers with high and stable proton and electron conductivity and mass transfer efficiency under conditions of strong electric/thermal/chemical fields for medium-temperature polymer membrane water electrolysis. The most severe conditions are associated with anodes, in which active oxygen and oxygen radicals are generated at increased temperatures and low pH values. Both mathematical modeling and experimental techniques have been applied to create a three-dimensional stable conducting network of pores, electrolytes, and supported catalytically active nanoparticles. The results show that IrO₂ supported with Ta₂O₅ or TaC could be potentially used as catalysts for anode of medium-temperature electrolyzers with polybenzimidazole-based membranes doped with H₃PO₄. In particular, even a few % of Ta₂O₅ improves IrO₂ activity and stability towards the oxygen evolution reaction, as well as reducing noble metal loading. It was shown that the optimum catalyst structure and composition depend on the applied synthesis method (chemical or physical). Electrocatalytic layers with a component concentration gradient are recommended. Numerical calculations have shown that in the anode electrocatalytic layer, the main reaction zone is shifted towards the gas diffusion electrode. Hence, during the formation of the catalyst layer, it is recommended to increase the concentration of the catalyst across the catalyst layer thickness (from the membrane towards the gas diffusion electrode). For the cathode, the use of carbon-supported Pt nanoparticles with a reverse concentration gradient is recommended.

The research was supported by RSF (project No. 25-29-00545).

Abstract

Introduction:

Manganese oxide octahedral molecular sieves (OMSs) are promising catalysts for oxygen reduction reactions (ORRs) due to their cost-effectiveness and durability. However, their practical application is hindered by inherent limitations, including low electrical conductivity and insufficient intrinsic catalytic activity.

Method:

To address these challenges, we employed a novel surface reduction etching treatment using sodium borohydride (NaBH₄) to optimize the oxygen vacancy content of OMS materials. This method involves immersing OMS samples in varying concentrations of NaBH₄ solution followed by vacuum annealing, leading to the controlled introduction of oxygen vacancies.

Results:

The NaBH₄ treatment significantly increased the number of oxygen vacancies on the OMS surface. These vacancies act as crucial active sites, facilitating the adsorption and dissociation of oxygen molecules, thereby improving ORR activity. Furthermore, the treatment was found to regulate the Mn³⁺/Mn⁴⁺ ratio on the nanorod surface, further promoting catalytic efficiency. Notably, the OMS material treated with 6mmol/L NaBH₄ exhibited a remarkable half-wave potential of 0.74 V in an alkaline medium of 0.1M KOH electrolyte, which is comparable to the state-of-the-art platinum catalyst (0.837 V).

Conclusions:

The optimized OMS materials exhibited significantly improved ORR performance compared to pristine OMS. This enhancement is attributed to the increased availability of active sites and the improved interaction between oxygen molecules and the OMS surface. The NaBH4 surface etching treatment provides a simple and scalable approach to unlock the full potential of OMS materials for ORR catalysis, paving the way for advancements in energy storage and conversion technologies.

CuFeO₂ (CFO) has been recently identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. However, this p-type semiconductor suffers from poor photo-induced electron–hole separation and charge collection. In this study, CFO thin films were successfully sputtered via a magnetron sputtering technique, and their PEC experiments were carried out under front-side chopped illumination in 1M NaOH electrolyte against a Ag/AgCl reference electrode. In argon, the thin films produced low photocurrent, while in oxygen, it enhanced the photocurrent and rose to 0.55 mAcm^-2 at 0.4 V vs. RHE (V_RHE). The synthesis parameters dictated the water splitting efficiency; when the power of the Fe target increased from 160 W to 200 W, the photo-current density enhanced and reached the highest value of 0.55 at 0.4 V vs. RHE (V_RHE) due to its lower charge transfer resistance according to EIS data. Similarly, the Cu power was optimized in which the highest J-V was produced with a power of 40 W rather than 60 and 80 W. The other constituent of CFO that affects the PEC activity is oxygen; an oxygen flow rate of 2 sccm (with O₂:Ar = 1:9) was optimal; increasing this further reduced the J-V. AFM spectroscopy revealed that the roughness of the thin films increased as a function of Fe and Cu sputtering power, whereas SEM-EDAX inferred a homogeneous distribution of the elemental constituents.

Metal–air batteries offer high energy density but suffer from sluggish oxygen electrochemistry. This work focuses on developing efficient bifunctional catalysts, combining NiFe-LDH with manganese-based materials, to address this challenge and enable the practical application of these promising energy-storage devices.

Manganese-based catalysts were synthesized via a sol–gel autocombustion method. NiFe-LDH materials were prepared through a hydrothermal method. The synthesized materials were subsequently compounded with pristine and nitrogen-doped carbon nanofibers to create composite electrodes. This combination aimed to improve the overall electrochemical performance of the materials. Electrochemical measurements were conducted in a conventional three-electrode cell configuration. An alkaline electrolyte was used, with a Hg/HgO electrode serving as the reference electrode and a graphite rod as the counter electrode. The electrocatalytic activity of the materials towards the ORR and OER was assessed through a series of electrochemical techniques, including cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy.

Electrochemical evaluation using CV and LSV in a three-electrode cell revealed promising electrocatalytic activity of the synthesized NiFe-LDH and manganese-based materials towards both OER and ORR. The materials exhibited significant current densities and favorable onset potentials, consistent with their structural and compositional characteristics. These findings demonstrate their potential for high-performance metal–air battery applications.

This study demonstrates the feasibility of NiFe-LDH and manganese-based spinels as bifunctional electrocatalysts for metal–air batteries. A comprehensive characterization confirmed their desired properties, and an electrochemical evaluation revealed promising activity for both ORR and OER. While promising, further research is crucial to optimize their performance, including long-term stability studies and the exploration of strategies such as doping and support engineering. These findings contribute significantly to the advancement of sustainable energy technologies.