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The effect of pressure on the spin-transition properties of the 2D coordination polymer {Fe(phpy)2[Ni(CN)4]} is reported. The study has been carried out by means of variable-temperature (10-310 K) magnetic susceptibility measurements at applied pressures of 105 Pa to 1.0 GPa and spectroscopic studies in the visible region at room temperature (105 Pa-3.0 GPa). As the pressure is increased, the characteristic temperature of the spin transition is displaced to higher temperatures and the thermal hysteresis loop disappears. A cooperative first-order spin transition characterized by a piezo-hysteresis loop about 0.3 GPa wide was observed at 293 K. [ABSTRACT FROM AUTHOR]

Barocaloric cooling, that is, lowering the temperature of a material under pressure action, is an attractive solid‐state effect that can potentially compete with volatile gas‐based cooling. To observe the barocaloric effect (BCE), it is necessary for materials to have high‐entropy, low‐hysteretic phase transitions with a large volume change between phases. Here details on a new FeII complex [Fe(L)(NCS)2], L = N1,N3‐bis((1‐propyl‐1H‐1,2,3‐triazol‐4‐yl)methylene)‐2,2‐dimethylpropane‐1,3‐diamine) possessing spin crossover (SCO) behavior near room temperature with large entropy and volume change are reported, which provides high sensitivity to external pressure. The observed BCE effect, characterized using variable pressure calorimetry, powder X‐ray diffraction, UV–vis, IR, and Raman spectroscopy, shows a colossal isothermal entropy change of >100 J kg−1 K−1 and a reversible adiabatic temperature change of ≈16 K at a pressure of 1 kbar, demonstrating a high refrigerant efficiency compared to other solid‐state materials. These results stimulate further investigations of SCO materials as barocaloric refrigerants, which depend on the proper design of their constituent organic ligands. [ABSTRACT FROM AUTHOR]

As one of the most likely alternatives to traditional vapor compression refrigeration technology, solid refrigeration technology based on the barocaloric effect (BCE) has attracted extensive attention in recent years. Spin-crossover (SCO) compounds are considered suitable for working at low driving pressures due to high-pressure sensitivity and small hysteresis width. However, the entropy change (ΔSSCO) of the SCO compound is smaller than that of other excellent barocaloric materials (plastic crystals and two-dimensional perovskites). Here, we report the BCE of the SCO compound {Fe(pz)2(BH3CN)2} (pz = pyrazine) with a smaller molar mass and a third source of entropy change besides electron and vibrational entropy changes. Compound {Fe(pz)2(BH3CN)2} exhibits high pressure sensitivity ( d T 1 / 2 d P = 20.2 K kbar−1) as well as entropy change (Δ S SCO = 202 J kg−1 K−1). The maximum values of reversible isothermal entropy change (ΔSit,rev,max) and adiabatic temperature change (ΔTad,rev,max) at 1 kbar are only 103 J kg−1 K−1 and ∼0 K, respectively, due to the hysteresis behavior. However, at sufficiently high driving pressures, ΔSit,rev,max exceeds 200 J kg−1 K−1, and ΔTad,rev,max can reach ∼47 K, which exceeds all SCO compounds reported in BCE studies and is comparable to some plastic-crystalline and two-dimensional perovskite barocaloric materials. The excellent BCE of the SCO compound {Fe(pz)2(BH3CN)2} is mainly due to its small molar mass, which makes the unit mass compound exhibit higher ΔSSCO, while the introduction of the third source of entropy change—the reorientation entropy change (ΔSreo), only plays a small role. This is expected to promote the practical application of SCO compounds as barocaloric refrigerants. [ABSTRACT FROM AUTHOR]

The idea of exploiting the inexhaustible solar energy-driven photocatalytic hydrogen evolution reaction (HER) to generate hydrogen energy, a sustainable clean energy source, to solve problems such as fossil fuel-induced greenhouse gas emissions is believed to be a feasible strategy. However, photocatalytic HER is inefficient due to insufficient light trapping capability and low charge separation efficiency of the single semiconductor material employed. Therefore, it remains a great challenge to achieve fast and efficient photocatalytic HER. In this work, a novel ternary Chl@Cu2O/Ti3C2Tx photocatalyst with an organic–inorganic heterojunction structure has been designed by employing a stepwise deposition process for efficient solar energy conversion to hydrogen. The results showed that the photocatalytic activity of Chl@Cu2O/Ti3C2Tx organic–inorganic heterojunction photocatalysts for HER was significantly improved compared with that of single Chl@Ti3C2Tx and Cu2O/Ti3C2Tx. The improvement of the photocatalytic performance of Chl@Cu2O/Ti3C2Tx compared to Chl@Ti3C2Tx and Cu2O/Ti3C2Tx is attributed to the construction of the Chl–Cu2O organic–inorganic heterojunction structure in Chl@Cu2O/Ti3C2Tx, which extends the absorption range of the spectra and dramatically enhances the separation efficiency of the photogenerated carriers in Chl@Cu2O/Ti3C2Tx composites. [ABSTRACT FROM AUTHOR]

[Display omitted] Ti 3 C 2 T x MXene, a new family of 2D materials, has been successfully used as an ideal co-catalyst for enhancing photocatalytic activity due to its excellent physicochemical properties. Bacteriochlorophyll (BChl), one of the most abundant photosynthetically active pigments, has been studied in the field of artificial photosynthesis due to its constant and efficient absorption of solar energy and conversion of the excitation energy into chemical energy. In this work, five BChl- a derivatives with a range of different side chain groups in the C17 substituent were prepared and deposited on the Ti 3 C 2 T x MXene surface by a solvent evaporation process to form BChl-n@Ti 3 C 2 T x (n = 1∼5) composite photocatalysts, which were used for the visible light-driven photocatalytic hydrogen evolution reaction (HER) in an aqueous solution. Among the five BChls investigated, BChl-5 possessing a quaternary ammonium terminal showed the best performance and BChl-5@Ti 3 C 2 T x composite exhibited an excellent hydrogen production activity of as high as 51 μmol/h/g cat. This is primarily attributed to the excellent photophysical and photochemical properties provided by the self-aggregation of BChl-5 in the BChl-5@Ti 3 C 2 T x composite and the outstanding charge transport capability of the 2D Ti 3 C 2 T x MXene. At the same time, the fast carrier separation and transfer capability between BChl-5 and Ti 3 C 2 T x provides reliable charge transport for photocatalytic HER. [ABSTRACT FROM AUTHOR]

Hydrogen energy is an abundant, clean, sustainable and environmentally friendly renewable energy source. Therefore, the production of hydrogen by photocatalytically splitting water on semiconductors has been considered in recent years as a promising and sustainable strategy for converting solar energy into chemical energy to replace conventional energy sources and to solve the growing problem of environmental pollution and the global energy crisis. However, highly efficient solar‐driven photocatalytic hydrogen production remains a huge challenge due to the poor visible light response of available photocatalytic materials and the low efficiency of separation and transfer of photogenerated electron‐hole pairs. In the present work, organic heterojunction structures based on bacteriochlorophyll (BChl) and chlorophyll (Chl) molecules were introduced and used for solar‐driven photocatalytic hydrogen production from water under visible light. Also, noble metal‐free photocatalyst was successfully constructed on Ti3C2Tx nanosheets by simple successive deposition of Chl and BChl, which was used for the photocatalytic splitting water to hydrogen evolution reaction (HER). The results show that the optimal BChl@Chl@Ti3C2Tx composite has a high HER performance with 114 μmol/h/gcat, which is much higher than the BChl@Ti3C2Tx and Chl@Ti3C2Tx composites. [ABSTRACT FROM AUTHOR]

In this work, a novel multiferroic‐like nanocomposite is designed and obtained using the high‐pressure torsion (HPT) method. The crystal structure, phase composition, morphology, ferromagnetic (FM), and ferroelectric (FE) properties of the initial powders and ferroelectric/ferromagnetic nanocomposites are studied comprehensively. The initial powders and their composite show the perovskite and spinel crystalline phases for the FE and FM fractions, respectively. After HPT, the particle sizes of the initial powders are decreased significantly. It is shown that the novel nanocomposite consists of exchange‐interacting FE and FM phases and demonstrates improved magnetic and electrical properties in low fields at room temperature. A giant increase in residual polarization with an increase in external high‐pressure is found in new composite. The obtained results make it possible to consider the novel nanocomposite as a new functional material for its use both in electronic devices for monitoring ultra‐high‐pressure and in integrated circuits of high‐speed computing nanosystems with low switching energy. The HPT method is a promising method for obtaining new heterophase nanosystems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Organic heterojunction of Chl–g-C 3 N 4 was constructed for visible-light driven H 2 evolution. • Highly efficient electron-hole separation occurred through the organic heterojunction structure. • Chl@g-C 3 N 4 /MXene-based photocatalytic system were developed for noble metal-free HER. • This work provides directions for preparation of organic heterojunction structures suitable for HER. Sustainable conversion of solar energy to hydrogen energy by photocatalytic splitting of water through the use of semiconductor photocatalytic materials is considered to be a promising and sustainable alternative proposal to replace conventional fossil fuels. However, efficient solar-driven photocatalytic hydrogen production has remained a great challenge due to the prevailing problems of existing semiconductor photocatalytic materials. It is shown that constructing heterostructures is a very powerful way to improve the photocatalytic hydrogen production capacity of composites under visible light. Therefore, in this work, an organic heterojunction structure based on chlorophyll (Chl) molecules and graphitic carbon nitride (g-C 3 N 4) is reported and exploited for efficient solar energy-driven photocatalytic hydrogen evolution reaction (HER). Meanwhile, noble metal-free photocatalysts were successfully constructed with Ti 3 C 2 T x MXene nanosheets through a simple stepwise complexation process of Chl, g-C 3 N 4 , and Ti 3 C 2 T x for the achievement of HER for the photocatalytic decomposition of water. The results show that the optimal Chl@g-C 3 N 4 /Ti 3 C 2 T x organic heterojunction composite has a remarkably advanced HER performance of 131 μmol/h/g cat , as compared with the traditional Chl@Ti 3 C 2 T x and g-C 3 N 4 /Ti 3 C 2 T x composites. The results provide new ideas for exploring MXene based catalysts for highly efficient conversion of solar energy. [ABSTRACT FROM AUTHOR]

This article focuses on electrochemical tuning of the magnetic phase transition. The ability of certain molecules and molecule-based materials to change the nature of their electronic ground state under the presence of an external perturbation is well-known. This leads to functional materials in which a physical property (absorption of light, magnetic moment, electrical conductivity, can be tuned through the application of an external stimulus. abundant in the literature on spin crossover compounds. Less common is the observation of this behavior in magnetically ordered molecule-based materials.

The thermal spin-crossover behaviour, photoexcitation and subsequent relaxation, as well as the pressure-induced spin-crossover behaviour at 298 K are discussed for the non-porous two-dimensional Hofmann-like coordination polymer [Fe(3-Clpy)2Pd(CN)4] ( 1). The title compound undergoes a two-step, cooperative thermal-induced SCO with critical temperatures Tc1↓ = 159.6 K and Tc1↑ = 164.5 K for the first step and Tc2↓ = 141.4 K and Tc2↑ = 148.4 K for the second step. Irradiation of the low-spin state with green light (514 nm) at 10 K induced the photoexcitation of around 60 % of the iron(II) centres to the high-spin state (LIESST effect). The subsequent cooperative relaxation processes were recorded at several temperatures and analysed. The effect of cooperativity on the photoexcitation was investigated by the LITH (light-induced thermal hysteresis) procedure. At 300 K, the coordination polymer 1 underwent a cooperative spin-crossover transition at around 0.6 GPa. The thermodynamic analysis of the pressure-induced SCO transition afforded the enthalpy and cooperativity parameters Δ HHL( p) = 22.01 kJ mol-1 and Γ( p) = 7.47 kJ mol-1, consistent with previous results. [ABSTRACT FROM AUTHOR]

The effect of applied pressure on the magnetic properties of the Prussian blue analogue K0.4Fe4[Cr(CN)6]2.8 · 16H2O (1) has been analyzed by dc and ac magnetic susceptibility measurements. Under ambient conditions, 1 orders ferromagnetically at a critical temperature (Tc) of 18.5 K. Under application of pressure in the 0-1200 MPa range, the magnetization of the material decreases and its critical temperature shifts to lower temperatures, reaching Tc = 7.5 K at 1200 MPa. Pressure-dependent Raman and Mössbauer spectroscopy measurements show that this striking behavior is due to the isomerization of some CrIIICNFeII linkages to the CrIIINCFeII form. As a result, the ligand field around the iron(II) centers increases, and the diamagnetic low-spin state is populated. As the number of diamagnetic centers in the cubic lattice increases, the net magnetization and critical temperature of the material decrease considerably. The phenomenon is reversible: releasing the pressure restores the magnetic properties of the original material. However, we have found that under more severe pressure conditions, a metastable sample containing 22% CrIIINCFeII linkages can be obtained. X-ray absorption spectroscopy and magnetic circular dichroism of this metastable sample confirm the linkage isomerization process. [ABSTRACT FROM AUTHOR]