Lower Gibbs Free Energy Research Articles

Theoretical Analysis of Metals Supported on Tungsten Oxide Nanowires (W18O49) for Water Dissociation Reaction

AbstractPt is the cause of the high total cost of fuel cells and electrolyzers, leading to difficult commercialization. Here, various types of metal atoms, i.e., Pt, Pd, Ni, Ir, Ag, and Rh, suitable for catalysts are used and supported by W18O49 nanowires for oxygen evolution reaction (OER) by the density functional theory (DFT) method. Four adsorbate molecules involved in OER were tested on adsorption energy: OH, O, OOH, and OO. Although the adsorption energy of these adsorbate molecules indicates that W18O49 has low adsorption energy, the Gibbs free energy diagram demonstrates that W18O49 has high OER reaction energy. Pt, Pd, Ni, and Rh have the lowest Gibbs energy to initiate the reaction and reasonable Gibbs free energy for other OER reactions. Bimetallic or trimetallic active sites can be developed along with selecting other metals with Pt, Pd, Rh, and Ni to reduce the Gibbs free energy difference for the decomposition of OH to O and OOH to H2O. Ag metal can also be applied as a second or third metal because Ag exhibits a relatively low Gibbs free energy difference in the O to OOH step. A selectivity study of each step on bimetallic and trimetallic active sites needs to be performed.

Effect of wire diameter on structure and electrical properties of (Al + Al2O3)-sheathed MgB2 with Nb barrier

Effect of wire diameter and annealing on the properties of internal magnesium diffusion (IMD)-processed MgB2 composite wires is studied by establishing a correlation with the characteristics of the interfacial reaction zones developed at Mg − B and barrier (Nb) – sheath (Al + Al2O3) interfaces. The MgB2 superconductor phase grows at the Mg − B interface along with the B-rich MgB4 phase. Formation of MgB2 prior to MgB4 is anticipated owing to a relatively lower Gibbs free energy at annealing temperatures studied. An intermetallic NbAl3 phase has developed at the barrier – sheath interface whose thickness decreases with an increase in the wire diameter. Development of NbAl3 further provides mechanical reinforcement and aids at yielding a dense superconductor phase. Resistivity – voltage characteristics of wire indicates the formation of MgB2 through a huge drop in resistivity values across the core of the wire. An annealing temperature of 645 °C near the melting point of pure Mg (650 °C) and annealing time of 120 min results in highest critical current of the MgB2 wire. Annealing time beyond 120 min has shown an impediment to the flow current due to crack propagation possibly because of accumulation of stresses within the developed superconductor phase due to grain growth. A significantly greater resistance for the wire with the smallest diameter is attributed to the formation of a thicker NbAl3 phase. However, highest engineering current density has also been attained by the wire with the smallest diameter annealed at 645 °C for 60 min.

1T-WS2/NCN photocatalyst with unique trion behavior and cyano-defects: Photocatalytic hydrogen evolution and mechanistic insights

The interfacial catalytic reaction rate determines the hydrogen production performance of the photocatalyst. In this work, high specific surface area and cyano-defects g-C3N4 with 1T-WS2 as a co-catalyst was prepared to construct 1T-WS2/NCN photocatalytic system. The two-electron interfacial catalytic reaction pattern formed due to the unique trions behaviour of 1T-WS2 greatly enhanced the interfacial catalytic reaction rate. Under 180 min of light, the hydrogen production of the photocatalytic system could reach 5769.3 μmol/g, which was 91.6 times that of the pristine g-C3N4. It was found that the excellent electrical conductivity of 1T-WS2 with metallic nature and the low Gibbs free energy of its active edge facilitated the hydrogen production and release. Meanwhile, the unique trions behaviour of the two-electron catalytic reaction formed in 1T-WS2 is a significant factor in increasing the rate of photocatalytic hydrogen production. And the cyano-defects in g-C3N4 acts as an electron trap, which drives charge space separation and provides sufficient electrons for trions formation, facilitating two-electron catalytic hydrogen production. This study provides a new idea for the development of electron-hole separation to improve the photocatalytic hydrogen production activity, which has important theoretical significance and application value.

Doped MoS2 Polymorph for an Improved Hydrogen Evolution Reaction.

Green hydrogen produced from solar energy could be one of the solutions to the growing energy shortage as non-renewable energy sources are phased out. However, the current catalyst materials used for photocatalytic water splitting (PWS) cannot compete with other renewable technologies when it comes to efficiency and production cost. Transition-metal dichalcogenides, such as molybdenum disulfides (MoS2), have previously proven to have electronic and optical properties that could tackle these challenges. In this work, optical properties, the d-band center, and Gibbs free energy are calculated for seven MoS2 polymorphs using first-principles calculations and density functional theory (DFT) to show that they could be suitable as photocatalysts for PWS. Out of the seven, the two polymorphs 3Ha and 2R1 were shown to have d-band center values closest to the optimal value, while the Gibbs free energy for all seven polymorphs was within 5% of each other. In a previous study, we found that 3Hb had the highest electron mobility among all seven polymorphs and an optimal bandgap for photocatalytic reactions. The 3Hb polymorphs were therefore selected for further study. An in-depth analysis of the enhancement of the electronic properties and the Gibbs free energy through substitutional doping with Al, Co, N, and Ni was carried out. For the very first time, substitutional doping of MoS2 was attempted. We found that replacing one Mo atom with Al, Co, I, N, and Ni lowered the Gibbs free energy by a factor of 10, which would increase the hydrogen evolution reaction of the catalyst. Our study further shows that 3Hb with one S atom replaced with Al, Co, I, N, or Ni is dynamically and mechanically stable, while for 3Hb, replacing one Mo atom with Al and Ni makes the structure stable. Based on the low Gibbs free energy, stability, and electronic bandgap 3Hb, MoS2 doped with Al for one Mo atom emerges as a promising candidate for photocatalytic water splitting.

Exploring Regio- and Stereoselectivity in [3+2] Cycloaddition: Molecular Electron Density Theory Approach for Novel Spirooxindole-Based Benzimidazole with Pyridine Spacer

A new ethylene derivative was synthesized as a precursor for the [3+2] cycloaddition (32CA) reaction to access a novel spirooxindole embodied with benzimidazole with a pyridine spacer. The chalcone derivatives 3a–j is obtained with condensation of the acetyl derivative with aryl aldehydes. The one-pot multi-component reaction of the ethylene derivative, 5-Cl-isatin, and octahydroindole-2-carboxylic acid enables the construction of a highly functionalized quaternary center spirooxindole scaffold in a high chemical yield. A study using the Molecular Electron Density Theory (MEDT) explains the complete regio- and stereoselectivity of the reaction, resulting in the exclusive formation of the ortho/endo-cycloadduct under kinetic control. The low activation Gibbs free energy is the result of the supernucleophilic character of the in situ-generated azomethine ylide and the strong electrophilic character of the ethylene derivatives.

Vacancy-induced spontaneous H2 evolution by overall water splitting on MoTe2/Ti2CO2: A two-dimensional direct Z scheme heterostructure

Photocatalysts with direct Z-scheme heterostructures hold great potential for hydrogen production by solar-driven overall water splitting. However, building highly efficient direct Z-scheme heterostructures is still a challenge. This work develops a novel MoTe2/Ti2CO2 heterostructure as a potential direct Z-scheme photocatalyst for overall water splitting. First-principles computations utilizing density functional theory (DFT) have been used in this study to carefully study several aspects of the heterostructure, including vdW binding energy, formation of the internal electric field, electronic structure, optical properties, and thermodynamic feasibility of water splitting. According to these calculations, the superior interlayer interaction in MoTe2/Ti2CO2 results in a substantial built-in electric field suitable for carrier separation. These findings indicate that the suggested vdW heterostructure is a direct Z-scheme water-splitting photocatalyst with suitable band alignments and a broad light absorption range. This enhanced interaction resulted in a significant increase of photocurrent compared to both the monolayers. Additionally, it is envisaged that the heterojunction will serve as photocatalyst for the oxygen evolution process (OER) and the hydrogen evolution process (HER). The results demonstrate that MoTe2/Ti2CO2 heterostructure with Te vacancy has a low Gibbs free energy of the HER, favoring spontaneous hydrogen evolution in both acidic and neutral conditions. The suggested heterostructure exhibits a commendable solar-to-hydrogen efficiency of 15.25%, making it a viable option for commercial implementation. This research offers a theoretical foundation for designing novel, two-dimensional overall water-splitting photocatalysts in an effort to produce clean energy.

Phase, chemical constitution and effective depth of oxide overgrowth and its influence on SmPrCo5 magnetic properties

Ternary Sm-Pr-Co magnets with CaCu5-type hexagonal structure exhibit superior magnetic properties and investigating their oxidation behavior is crucial for assessing their stability under the service conditions. In this regard, here thermal oxidation of sintered SmPrCo5 magnet is carried out by varying the oxidation temperature from 373 to 973 K for 5 hr under atmospheric pressure. Owing to their lower negative Gibbs free energy, rare earth oxides of Sm2O3 and Pr2O3 are found to initially form, making cobalt free from SmPrCo5 lattice. Cobalt is then observed to migrate to the oxide surface, because of the difference in chemical potential between the surface and inner layers, leading to Co3O4 formation. Moreover, here a comprehensive discussion on the occurrences of different oxide phases and their depth distributions in the oxide layer are being made. Finally, a correlation between these oxide phases and the alterations in their magnetic properties is discussed.

Cu aggregation behavior on interfacial reaction of Sn-3.0Ag-0.5Cu/ENIG solder joints

Cu aggregation of Sn-3.0Ag-0.5Cu /electroless nickel-immersion gold (ENIG) solder joints generally affect the formation of intermetallic compounds (IMCs) at the interfacial region. Herein, the interfacial microstructure of Sn-3.0Ag-0.5Cu/ENIG solder joint is compared with the joints obtained with Cu-free solder or substrate to reveal the Cu contributor and its metallurgical behavior. Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) were used to observe the IMCs and elemental distribution, respectively. The Gibbs free energy of the Sn-Ni-Cu system was calculated by JMatPro software. The results showed that Cu had significant aggregation in the interface. The solder is the major contributor to Cu aggregation in the interface, while the ENIG substrate also makes a minor contribution. The Cu6Sn5 is then formed at the interface because of its lowest Gibbs free energy in the system. These results are of significance for subsequently controlling the interfacial microstructure and performance of joints.

Laser welding of SiCp/2024Al composites with novel TiB2/2024Al composite filler

The effect of TiB2/2024Al filler on the microstructure and mechanical properties of SiCp/2024Al joint was investigated in this paper. The results indicated that needle-like Al4C3 phase with a content of 18.9% was the dominant precipitate in the weld zone (WZ) of direct welded (DWed) joint, which weakened the tensile strength (109.3 MPa) of DWed joint. The introduction of TiB2/2024Al filler changed the microstructure of WZ. In the molten pool, TiC precipitate was precipitated preferentially due to the higher chemical affinity of Ti and C atoms and lower Gibbs free energy of TiC formation. The dispersed TiC blocks presented strong fine grain strengthening and precipitation strengthening effects. Compared with DWed joint, the strength of SiCp/2024Al joint with TiB2/2024Al filler increased to 268 MPa, approximately 72.9% of that of base metal (BM).

Entropy-Mediated High-Entropy MXenes Nanotherapeutics: NIR-II-Enhanced Intrinsic Oxidase Mimic Activity to Combat Methicillin-Resistant Staphylococcus Aureus Infection.

Bacterial infections, such as bacterial keratitis (BK) and subcutaneous abscess, pose significant challenges to global healthcare. Innovative and new antibacterial agents and antibacterial strategies are in demand to control infections in this era of high drug resistance. Nanotechnology gradually emerged as an economically feasible and effective anti-infection treatment. High-entropy MXenes (HE MXenes) are used to confer desirable properties with exposed active sites to high-entropy atomic layers, whose potential application in the field of biomedicine remains to be explored. Herein, we fabricate the monolayer HE MXenes by implementing transition metals with high entropy and low Gibbs free energy to fill the gap in the biocatalytic performance of non-high-entropy MXenes. HE MXenes are endowed with extremely strong oxidase mimic activity (Km = 0.227mM) and photothermal conversion efficiency (65.8%) in the second near-infrared (NIR-II) biowindow as entropy increases. Subsequently, HE MXenes realize NIR-II-enhanced intrinsic oxidase mimic activity for killing methicillin-resistant Staphylococcus aureus and rapidly removing the biofilm. Furthermore, HE MXenes can effectively treat BK and subcutaneous abscess infection induced by MRSA as nanotherapeutic agents with minuscule side effects. Overall, monolayer HE MXenes demonstrate promising clinical application potential in the treatment of drug-resistant bacterial infections and promote the healing of infected tissues. This article is protected by copyright. All rights reserved.

High-Throughput Screening of Nitrogen Reduction Reaction on Single Atom@1T'-MoS2

Electrochemical synthesis of ammonia through nitrogen reduction reaction (NRR) is cost-effective and energy-effective compared to the conventional Haber-Bosch process. However, the slow activation kinetics of N2 hindered the development of active and selective catalysts. In this work, we showed that transition metal (TM) single atoms anchored on monolayer molybdenum disulfide (MoS2) with 1T phase could promote its phase transition to 1T' due to broken surface symmetry induced by TM atoms. By combining the high-throughput screening with DFT calculations in transition metals, two TM elements (V, Ti) anchored on 1T'-MoS2 were proved to be promising candidates in N2 fixation. The V and Ti single-atom catalyst exhibits low Gibbs free energy barrier, and Ti single-atom catalyst shows the highest selectivity towards NH3. Moreover, the stability of single-atom TM@1T'-MoS2 was measured with the ab initio molecular dynamics (AIMD) simulations. The density of states (DOS) and surface charge analysis were conducted to reveal the origin of improved activities. Our results pave the way to design more innovative single-atom catalysts for high-performance electrocatalytic NRR.

Controlling basal plane sulfur vacancy in water splitting MoSx/NiF electrocatalysts through electric-field-assisted pulsed laser ablation

Eco-friendly, efficient, and durable electrocatalysts from earth-abundant materials are crucial for water splitting through hydrogen and oxygen generation. However, available methods to fabricate electrocatalysts are either hazardous and time-consuming or require expensive equipment, hindering the large-scale, eco-friendly production of artificial fuels. Here, we present a rapid, single-step method for producing MoSx/NiF electrocatalysts with controlled sulfur-vacancies via electric-field-assisted pulsed laser ablation (EF-PLA) in liquid and in-situ deposition on nickel foam, enabling efficient water splitting. Electric-field parameters efficiently control S-vacancy active sites in electrocatalysts. Higher electric fields yield a MoSx/NiF electrocatalyst with a larger density of S-vacancy sites, suited for HER due to lower Gibbs free energy for H∗ adsorption, while lower electric fields produce an electrocatalyst with lower S-vacancy sites, better suited for OER, as shown by both experimental and theoretical results. The present work opens a horizon in designing high-efficiency catalysts, for a wide range of chemical reactions.

Thermo-kinetic analysis of pyrolysis of thermally pre-treated sewage sludge from the food industry

Due to the specific characteristics of sewage sludge from the food industry, including its high fat content, its treatment is quite complex. Therefore, in this study, the effect of the pre-treatment processes torrefaction (T) and hydrothermal carbonization (HTC) on the pyrolysis of industrial sewage sludge (SS) from the vegetable oil industry was investigated by thermogravimetric analysis. Kinetic and thermodynamic analysis was performed using the Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO), and Friedman (FRI) iso-conversional kinetic models. In addition, the influence of water replacement by whey in the hydrothermal carbonization process was investigated on the subsequent pyrolysis kinetics.The activation energy (Eα) values for pyrolysis of industrial sewage sludge ranged from 49 to 372 kJ/mol. Pre-treatment (torrefaction, hydrothermal carbonization) of sewage sludge increases the activation energy significantly: the Eα values for torrefied (T-SS) and hydrothermally treated (HTC-SS) samples ranged from 177 to 689 kJ/mol and from 161 to 486 kJ/mol, respectively. The variations in activation energy and the generally lower activation energies for the HTC-SSW sample (158–445 kJ/mol) indicate that the use of whey in the HTC process affects the hydrochar properties and subsequent pyrolysis kinetics significantly. According to the results, the pre-treatment of the samples is reflected in better thermochemical properties and stability of the treated samples, as well as in the thermodynamic parameters of pyrolysis, since the pre-treated samples (especially the torrefied sample, T-SS) exhibited higher entropies and enthalpies and lower Gibbs free energies.

Effect of shielding gas on surface formation during laser welding of Al-Si coated press-hardened steels

The Al-Si coating was negative to the laser welding of press-hardened steel (PHS). This study carried out the comparative analysis of the shielding gas (Pure Ar and CO2 shielding gas) on the surface formation of welded seam. The results showed that the intermittent umbrella-shaped Fe-Al intermetallic compounds (IMCs) on both sides of the weld were presented for the Ar shielding gas, but it was disappeared for CO2 shielding gas. The adjacent Al-Si coating on the humps of the molten pool was melted by the heat conduction of the liquid metal, and flowed into the molten pool at the gap of two humps. The reaction between Fe and Al occurred to form Fe-Al IMCs as umbrella-shaped. The CO2 shielding gas provided a reaction opportunity with liquid Al to form the floated film on the surface of molten pool due to low Gibbs free energy, which presented particles oxides on the weld surface after solidification.

Duplex Interpenetrating-Phase FeNiZn and FeNi3 Heterostructure with Low-Gibbs Free Energy Interface Coupling for Highly Efficient Overall Water Splitting

HighlightsFree-standing bimodal porous interpenetrating-phase FeNiZn and FeNi3 intermetallic heterostructure is in situ built on NiFe foam by dealloying way.FeNiZn/FeNi3@NiFe exhibits superior electrocatalytic activities toward both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with very low overpotentials and prolonged catalytic durability in alkaline medium.The strong synergy between FeNiZn alloy and FeNi3 intermetallic generates modulated electron structure and thus well optimizes the intermediates chemisorption toward OER and HER.

Tailoring high-index-facet and oxygen defect of black In2O3−x/In2O3 as highly photothermal catalyst for boosting photocatalytic hydrogen evolution and contaminant degradation

Defect engineering and the high index surface (HIF) can be regarded as two effective methods to tuning the microstructure of catalysts and photocatalytic activities. So far, the integration of the synergistic effects of oxygen vacancies and HIF advantages into semiconductors to enhance photocatalytic performance has received little attention. Herein, Black In2O3−x/In2O3 with oxygen vacancy and exposed (321) surface active crystal plane has been synthesized as excellent photocatalysts via a facile method. The catalysts of Black In2O3−x/In2O3 can reach the highest photocatalytic hydrogen evolution rate (1046 μmol h−1 g−1) and degradation efficiency of MB (0.017 min−1) with 120 min, respectively. As a special phase junction, Black In2O3−x/In2O3 could effectively boost the separation and transfer of photogenerated charges because of unique defect homojunction microstructure resulting in exposing abundant photocatalyst redox active sites. The experiment characterization and density functional theory (DFT) calculations can further employ to unveil mechanism of enhanced hydrogen evolution reaction (HER) activity. It is obvious that enhanced HER activity was attributed to synergy of oxygen defect and exposed (321) surface active crystal planes due to electron distribution and lower Gibbs free energy of H adsorption. This work might open up new avenues to integrate exposed facets and oxygen vacancy for enhancing the photocatalytic hydrogen evolution with renewable energy sources and environmental remediation.

Low-temperature synthesis of colloidal few-layer WTe2 nanostructures for electrochemical hydrogen evolution

High-quality transition metal tellurides, especially for WTe2, have been demonstrated to be necessarily synthesized under close environments and high temperatures, which are restricted by the low formation Gibbs free energy, thus limiting the electrochemical reaction mechanism and application studies. Here, we report a low-temperature colloidal synthesis of few-layer WTe2 nanostructures with lateral sizes around hundreds of nanometers, which could be tuned the aggregation state to obtain the nanoflowers or nanosheets by using different surfactant agents. The crystal phase and chemical composition of WTe2 nanostructures were analyzed by combining the characterization of X-ray diffraction and high-resolution transmission electron microscopy imaging and elements mapping. The as-synthesized WTe2 nanostructures and its hybrid catalysts were found to show an excellent HER performance with low overpotential and small Tafel slope. The carbon-based WTe2–GO and WTe2–CNT hybrid catalysts also have been synthesized by the similar strategy to study the electrochemical interface. The energy diagram and microreactor devices have been used to reveal the interface contribution to electrochemical performance, which shows the identical performance results with as-synthesized WTe2–carbon hybrid catalysts. These results summarize the interface design principle for semimetallic or metallic catalysts and also confirm the possible electrochemical applications of two-dimensional transition metal tellurides.

Microstructure and properties of composite carbide (Ti,V)C enhanced Ni-based coatings by laser cladding

The strengthening mechanism of composite carbide (Ti,V)C enhanced Ni-based coatings was discussed by combined calculations and tests to obtain the influence mechanism of the microstructure and properties. The first principle was used to calculate the microstructure, formation energy, mechanical properties, anisotropy, and electronic properties of composite ceramic phase (Ti,V)C. It explored the effect of changes in Ti and V content on the properties of composite carbides. The influence mechanism of different Ti/V ratios on the performance and organization of coatings was analyzed by experiments to clarify the evolution process of the molten pool at different contents. The properties and crystal parameters of the composite phase greatly changed with increased V. The recombination of covalent bonds was an important reason for the increased hardness of the composite carbide. TiC with low Gibbs free energy precipitated preferentially at the solidification stage of the molten pool. The subsequent VC tended to form (Ti,V)C composite carbides with TiC as heterogeneous nucleation sites. When powder ratio Ti:V = 3:1, the hardness and wear resistance of the coating were better than those reinforced by single carbide TiC through hardness and friction wear tests. The difference in grain morphologies in the coating was the main reason for the change in coating performance. Research results provide a theoretical reference and basis for preparing composite carbide-reinforced coatings by laser cladding.

A highly dispersed and surface-active Ag-BTC catalyst with state-of-the-art selectivity in CO2 electroreduction towards CO

Electrochemical CO2 reduction reaction (CO2RR) towards CO is considered as one of the most promising routes to alleviate carbon emissions and achieve a carbon-neutral cycle. In this work, a highly dispersed Ag-BTC catalyst (BTC refers to the 1,3,5-benzenetricarboxylate ligand) with abundant catalytically active sites has been synthesized via a facile one-step hydrothermal method. Efficient, highly selective, and stable electrochemical reduction of CO2 into CO has been achieved on the Ag-BTC electrocatalyst. The faradaic efficiency of CO production from the CO2RR is around 95%, which is within the range of highest values reported to date. A correlation between the structure of the Ag-BTC electrode and its enhanced catalytic performance has been established through the combination of characterization results and electrochemical measurements. Furthermore, a lower Gibbs free energy change (∆G) for the rate-determining step regarding the formation of intermediate COOHads also leads to the superior catalytic performance of CO2RR on Ag-BTC.

Fluorenone-based covalent organic frameworks with efficient exciton dissociation and well-defined active center for remarkable photocatalytic hydrogen evolution

Precisely modulating both the fine structures and exciton behaviors of covalent organic framework (COF)-based photocatalysts for efficient photocatalytic hydrogen evolution is still a great challenge. Herein, we report two COFs, fluorenone-based COF (FOO-COF) and fluorenyl-based COF (FO-COF), for photocatalytic H2 evolution. The results demonstrated that FOO-COF achieved a remarkable stability and hydrogen evolution rate of 119.1 mmol/h/g, surpassing those of many previously reported COF-based photocatalysts, thus resulting in an apparent quantum efficiency up to 20.5% at 435 nm. The joint observations from temperature-dependent photoluminescence, the delayed fluorescence and phosphorescence spectroscopy confirm that FOO-COF shows a stronger carrier separation and migration ability, much lower exciton binding energy, and faster dissociation speed of singlet and triplet excitons than FO-COF. More importantly, the femtosecond time-resolved transient absorption spectra (fs-TAS) revealed the great difference between the two COFs in electron trapping and exciton dissociation after Pt loading. Furthermore, theoretical calculations unveiled the feasibility of CO groups in the LUMO sites of FOO-COF as active electronic centers for anchoring the photodeposited Pt cocatalysts, due to its much lower Gibbs free energy of hydrogen adsorption and more negative charge density. We believe that the introduction of CO groups in the FOO-COF can act as both a dominant electronic collection center and partial active sites, which improve the D-A polarizability and boost the generation, separation, migration, and utilization of photogenerated carriers in the FOO-COF for achieving remarkable photocatalytic hydrogen evolution. The insights into the molecular-level mechanism in this work may guide the significance of the rational design of COF structures for advanced photocatalytic hydrogen evolution.