Metal chalcogenides, pnictides, and carbides, labeled collectively as metal X-ides, have become an exciting new class of water oxidation electrocatalysts, but there is a lack of agreement regarding ...the composition of the “true” catalyst. The most prominent theories are that the X-ides are either completely oxidized, left unoxidized, or transformed into core@shell particles upon testing. Here, we examine examples of each conjecture, summarizing the conflicting viewpoints on catalyst identity and offering guidelines for more rigorous identification in the future. Most studies indicate that at least partial oxidation of the catalyst surface is critical to high performance, likely caused by an increased catalyst surface area upon oxidation or improved charge transfer in the X-ide cores. Therefore, more thorough and uniform long-term testing and nanoscale chemical analysis are essential to determine how these factors relate to catalyst performance.
To achieve practically high electrocatalytic performance for the oxygen evolution reaction (OER), the active surface area should be maximized without severely compromising electron and mass transport ...throughout the catalyst electrode. Though the importance of electron and mass transport has been studied using low surface area catalysts under low current densities (∼tens of mA/cm2), the transport properties of large surface area catalysts under high operating current densities (∼500 mA/cm2) for practical OER catalysis have rarely been explored. Herein, three-dimensional (3D) hierarchically porous anodized nickel foams (ANFs) with large and variable surface areas were synthesized via electrochemical anodization of 3D nickel foam and applied as OER electrocatalysts in Fe-free and unpurified KOH electrolytes. Using Fe-free and in situ Fe-doped ANF that were prepared in Fe-free and unpurified electrolytes, respectively, we investigated the interdependent effects of active surface area and transport properties on OER activity under practically high current densities. While activity increased linearly with active surface area for Fe-free ANF, the activity of Fe-doped ANF showed a nonlinear increase with active surface area due to lower electrocatalytic activity enhancement. Detailed investigations on the possible factors (Fe incorporation, mass transport, and electron transport) identified that electron transport limitations played the major role in restricting the activity enhancement with increasing active surface area for Fe-doped ANF, although Fe-doped ANF has electron transport properties better than those of Fe-free ANF. This study exemplifies the growing significance of electron transport properties in large surface area catalysts, especially those with superb intrinsic catalytic activity and high operating current density.
This paper reports a highly active and stable nonprecious metal electrocatalyst based on bimetallic nanoscale nickel molybdenum nitride developed for the hydrogen evolution reaction (HER). A ...composite of 7 nm Ni
2
Mo
3
N nanoparticles grown on nickel foam (Ni
2
Mo
3
N/NF) was prepared through a simple and economical synthetic method involving one-step annealing of Ni foam, MoCl
5
, and urea without a Ni precursor. The Ni
2
Mo
3
N/NF exhibits high activity with low overpotential (
η
10
of 21.3 mV and
η
100
of 123.8 mV) and excellent stability for the HER, achieving one of the best performances among state-of-the-art transition metal nitride based catalysts in alkaline media. Supporting density functional theory (DFT) calculations indicate that N sites in Ni
2
Mo
3
N with a N-Mo coordination number of four have a hydrogen adsorption energy close to that of Pt and hence may be responsible for the enhanced HER performance.
Ni
2
Mo
3
N catalyst is synthesized directly on Ni foam by a simple and economical method, which shows excellent hydrogen evolution performance in alkaline electrolyte.
Here, we develop a strategy to improve the visible-light-driven photocatalytic hydrogen evolution activity of g-C3N4 by compositing it with low-cost Ni(OH)2 nanoplatelets and inexpensive and ...earth-abundant halloysite nanotubes. The Ni(OH)2@g-C3N4/halloysite nanocomposite photocatalysts with different amounts of Ni(OH)2 (0.5–10 wt%) were prepared, and a synergistic effect of Ni(OH)2 platelets and halloysite nanotubes on physicochemical properties and photocatalytic hydrogen evolution activity of g-C3N4 was investigated. As expected, the Ni(OH)2@g-C3N4/halloysite nanocomposite photocatalyst prepared with 1 wt% Ni(OH)2 exhibited the highest photocatalytic hydrogen evolution rate (18.42 μmol h−1) which is much higher than that of g-C3N4 (0.43 μmol h−1) and Ni(OH)2@g-C3N4 (9.12 μmol h−1). Such enhancement in photocatalytic activity of Ni(OH)2@g-C3N4/halloysite nanocomposite photocatalyst is attributed to efficient transfer of photogenerated electrons from the g-C3N4 to Ni(OH)2 cocatalyst interface and trapping of photogenerated holes on the negatively charged surfaces of halloysite nanotubes. In addition, adsorption affinity of the water and methanol molecules was modeled using different surfaces of Ni(OH)2, halloysite-7Å, and g-C3N4 and it is found that combining the g-C3N4 with halloysite-7Å and Ni(OH)2 can significantly improve the adsorption of water and methanol molecules on the surface of the developed nanocomposite. This study offers a simple approach for developing an efficient and inexpensive nanocomposite for effective and applied photocatalytic water splitting methodology for hydrogen production and other possible optoelectronic and photocatalytic applications.
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•Inexpensive and efficient Ni(OH)2@g-C3N4/halloysite nanocomposite was developed.•Nanocomposite with 1 wt% Ni(OH)2 exhibited the highest hydrogen evolution rate.•Ni(OH)2 nanoplatelets favor the efficient transfer of photogenerated electrons.•Negatively charged surface of halloysite nanotubes trapped photogenerated holes.•Deposition of Ni(OH)2 on g-C3N4/halloysite improves adsorption of H2O and CH3OH.
Transition metal oxynitride perovskites AB(O,N)3 are an emerging class of inorganic materials with superior light harvesting ability (narrow band-gap energy) and moderate photostability. They can be ...utilized as visible-light-active photocatalysts for efficient solar water splitting. The photocatalytic activity of transition metal oxynitride perovskites has been significantly improved by changing the synthesis approach, reducing the defect density that acts as a recombination center for photogenerated charge carriers, increasing the conductivity, and engineering their band structures, namely the conduction and valence band positions, through an A/B-site doping/substitution. In this minireview, we briefly overview our recent advances on enhancing the photocatalytic activity of selected transition metal oxynitride perovskites (i.e., LaTiO2N, BaTaO2N, and BaNbO2N) and related works. Additionally, the challenges and future perspectives of these materials are discussed.
During photoelectrochemical (PEC) water splitting, the reactions occur on the surface of the photoelectrode. Therefore, the properties of the interfaces between the various components of the ...electrode (semiconductor/semiconductor, semiconductor/catalyst, or photoelectrode/electrolyte) affect the PEC performance of the composite material. Notably, surface trap states may hinder charge transfer and transport properties, and also cause Fermi pinning, affecting the quasi-Fermi level and onset potential under illumination, which may in turn influence the PEC performance of the corresponding tandem cells. In this study, plate-like WO3 array films prepared by an aqueous chemical growth method were employed to highlight the effect of interfacial properties on the performance of a WO3-based photoanode. The Mott–Schottky and linear sweep voltammetry experiments prove the existence of surface trap states and Fermi pinning for pristine WO3, which are alleviated after an “etching” treatment and disappeared after surface passivation by a Ga2O3 layer. Both etching and passivation increase the oxygen evolution activity and the Faradaic efficiency for the oxygen evolution reaction (OER). After loading a permeable catalyst (FeOOH), the photocurrent is further increased, and there is a synergistic effect between loading of the electrocatalyst with etching or passivation. The onset potentials of the samples follow the trends: etch-WO3/FeOOH < WO3/FeOOH ≤ WO3/Ga2O3/FeOOH < etch-WO3 < WO3 < WO3/Ga2O3, indicating that the interfacial properties have a significant effect on the PEC performance. Meanwhile, the modified WO3-based electrode was combined with a dye-sensitized solar cell to fabricate tandem cell, which showed 2.42-fold photocurrent density compared with the pristine WO3-based tandem cell.
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•4f electron configurations in the Pr2W2O9, Nd2W2O9, and Eu2W2O9 crystals.•Nitridation effects for the SrWO4, La2W2O9, Pr2W2O9, Nd2W2O9, and Eu2W2O9 crystals.•Two-step synthesis of ...AW(O,N)3 (A=Sr, La, Pr, Nd or Eu) perovskites.•Photocatalytic water oxidation of AW(O,N)3 (A=Sr, La, Pr, Nd or Eu) perovskites.
To expand the family of transition metal oxynitride perovskites, the two-step synthesis of a series of tungsten-based metal oxynitride perovskites (EuW(O,N)3, NdW(O,N)3, PrW(O,N)3, LaW(O,N)3, and SrW(O,N)3) and their visible-light-driven photocatalytic water oxidation activity with the assistance of CoOx (2wt% Co) cocatalyst were studied in this work. The XRD results revealed that the cubic perovskite LnW(O,N)3 (Ln=Pr, Nd, and Eu) and SrW(O,N)3 phases and tetragonal perovskite LaW(O,N)3 phase were successfully synthesized by nitriding their corresponding oxide precursors at 900°C for 10–25h under an NH3 flow, with minor secondary phases in only PrW(O,N)3 and NdW(O,N)3. The highly porous structures of EuW(O,N)3, LaW(O,N)3, and SrW(O,N)3 were formed from the segregation of nanocrystals with average sizes of 140, 92, and 160nm, respectively. The surfaces of the NdW(O,N)3 and PrW(O,N)3 crystal structures were covered with plate-like crystals which can be identified as W5N4. No clear absorption edges were observed in the UV–Vis diffuse reflectance spectra of the tungsten-based metal oxynitrides owing to the extensive amount of reduced tungsten species (W5+ and W4+) or metallic tungsten and anion deficiency. Within 5h of the photocatalytic water oxidation half-reaction, the CoOx-loaded SrW(O,N)3 crystal structures exhibited the highest photostability and O2 evolution rate of 3.3μmolh−1 compared with CoOx-loaded LnW(O,N)3 (Ln=La, Pr, Nd, and Eu) crystal structures due possibly to the highest O/N ratio and more positively positioned top of valence band of SrW(O,N)3. The present work is expected to stimulate research into the development of more stable and efficient tungsten-based metal oxynitride perovskites in the future.
Oxynitride perovskites and related phases have received considerable attention due to their potential application for visible-light-responsive photocatalyst and nontoxic inorganic pigments. The ...changes in bonding and structure by a partial replacement of O2– by N3– give rise to interesting dielectric behavior. Here, we report on the fabrication of highly crystalline La2TiO5 crystals by chloride flux growth method and their subsequent nitridation to form the LaTiO2N crystals using NH3 gas. The flux-grown La2TiO5 crystals had a columnar structure grown in the ⟨001⟩ direction. Using the NaCl flux, larger columnar La2TiO5 crystals were grown compared to those grown using the KCl flux. With increasing solute concentration, the aspect ratio of columnar La2TiO5 crystals decreased significantly. The columnar La2TiO5 crystals with smooth surface were readily converted by nitridation at 950 °C for 45 h followed by acid treatment into the LaTiO2N crystals with a highly porous structure that formed from the strong segregation of nanocrystals, leading to the largest specific surface area (16.5–18.4 m2·g–1). For the La2TiO5 crystals grown using the chloride fluxes, the wavelength of the absorption edges was approximately 320 nm (E g = 3.87 eV), whereas the absorption edges exhibited by the LaTiO2N crystals obtained by nitridation were approximately 600 nm (E g = 2.06 eV). Particularly, the LaTiO2N crystals prepared in this study by nitriding the precursor La2TiO5 crystals did not show a noticeable absorption in the near-infrared region above 600 nm, which is generally attributable to some reduced Ti3+ species and nitrogen deficiency, even after a long nitridation process. The fabricated LaTiO2N crystals with low defect density will be advantageous for various applications that specially require higher specific surface area.
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